British Columbia Carpenter Apprenticeship Program

Transcription

1 British Columbia Carpenter Apprenticeship Program Level 3 Line H Competency H-7 Build Intersecting Roofs

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4 Competency H-7 Build Intersecting Roofs Contents Objectives...2 Learning Task 1: Describe an Equal-slope Intersecting Roof...3 Self Test Learning Task 2: Build an Equal-slope Intersecting Roof...11 Self Test Learning Task 3: Material Calculations for Ceilings and Roofs...39 Self Test BC Carpenter Apprenticeship Program Level 3 1

5 Competency H-7 Build Intersecting Roofs Roof construction is a major part of wood-frame construction. Whether the roof is a truss roof or a rafter-framed roof, knowledge of roof framing is an important skill for carpenters. Objectives When you have completed the Learning Tasks in this Competency, you will be able to: name the components of an equal-slope intersecting roof calculate the length of rafters for intersecting roofs determine the adjustments for valley and valley jack rafters lay out a valley rafter lay out a valley jack rafter lay out a valley cripple rafter lay out a hip valley cripple rafter calculate ceiling frame materials calculate roof framing materials Competencies Written: Frame equal-slope intersecting roofs You will be tested on your knowledge of intersecting roof terminology, layout methods, adjustments, calculations, and the use of rafter tables and framing squares. You will also be tested on your ability to calculate the materials required for roof construction. You must achieve at least 70% on this Written Competency. Practical: You will be required to calculate, lay out, cut, and erect an intersecting roof. You must achieve at least 70% on this Practical Competency. 2 BC Carpenter Apprenticeship Program Level 3

6 Competency H-7 Learning Task 1 Learning Task 1 Describe an Equal-slope Intersecting Roof An equal-slope intersecting roof has two or more roof lines meeting each other to form valleys between the roofs. The roofs have the same rise/run slope, and typically, intersect at 90 degrees. This style of roof also requires two or more ridge boards. The design of the roof may include hip or gable ends. Refer to Level 1 Learning Tasks H-4 and H-5 for more information regarding gable and hip ended roofs. Ridge board Line length Common rafter Overhang Rise Projection Run Span Figure 1. Roof framing terms (section view) Sizes of rafters for all roofs require reference to the BC Building Code. This is covered in Learning Task 2 Build an Equal-slope Intersecting Roof. A rafter-framed roof allows the interior roof space to be used as living space (as opposed to a truss roof which does not). The ceiling lines formed by the roof slope and (underside of) valleys add architectural interest. In these building designs, the rafters are tied together at the bottom by floor joists, instead of ceiling joists. The rafters become roof joists because they now carry the interior finish as well as the live and dead load. BC Carpenter Apprenticeship Program Level 3 3

7 Learning Task 1 Competency H-7 Ceiling or Floor Joists The ceiling is constructed before the roof framing is assembled. Hand-cut, or rafter-framed roofs require ceiling joists to tie the ends of the rafters together, laterally brace exterior walls, and support the ceiling finish. In roofs built without ridge support (unsupported ridge), the fastening of the ceiling joists to the rafters, and at interior bearing walls or beams, is extremely important. The ceiling joists are under tension. The fasteners that hold them are withstanding a shearing force. The fastener requirements for ceiling joists to top plates, and rafters to ceiling joists are in the BC Building Code tables and (6). Roofs built with a supported ridge, with a ridge beam, or bearing wall supporting the tops of the rafters, are referred to as vaulted ceilings or cathedral ceilings. They do not require ceiling joists. Table A-12 in the BC Building Code gives spans for ridge beams. Equal-slope, Intersecting Roof Framing Terms Common overhang (rafter tail) Major ridge Major end common Hip jack Major common Major hip Valley-valley cripple jack Shortest minor valley jack Minor ridge Supported (short) valley Shortest major valley jack Hip-valley cripple Longest major valley jack Supporting (long) valley Minor hip 1st minor hip jack Minor end common Hip overhang Longest minor valley jack (rafter tail) Minor common Figure 2. Roof framing terms (plan view) 4 BC Carpenter Apprenticeship Program Level 3

8 Competency H-7 Learning Task 1 Major Roof When two roofs intersect, the major roof is the roof with the wider span. The major roof is where the major ridge and the major hips are located. Minor Roof When two roofs intersect, the minor roof is the roof with the smaller span. The minor roof is where the minor ridge and minor hips are located. When both roofs have the same span, they will have the same total rise and the ridges for both roofs will intersect at the same height. In this case, the major roof is the larger of the two (covering more square footage). Some buildings will have more than one minor roof, in which case the minor roofs will be numbered. i.e., minor roof 1, minor roof 2, etc. Ridges The major ridge is the same as the ridge for a simple gable or hip roof. The minor ridge runs into the intersection of the valley rafters, with a double cheek cut. Hip The hip rafter extends from the ridge to the outside corner of the wall plate. In plan view, it runs at a 45 angle to the ridge, the common rafters, and the plates. In an intersecting roof, it s parallel to the valley rafter. Hip Jack The hip jack rafter is a shortened common rafter that extends from the hip to the plate. It s laid out using the same slope as the common rafter and has the same oncentre spacing, bird s mouth, and overhang as the common. It joins the hip with a single cheek cut. In plan view, it runs at 90 to the plate, 45 to the hip, and parallel to the common rafters. Supporting Valley The supporting valley rafter, sometimes referred to as the long valley, extends from the major ridge to the wall plate at the inside corner. In plan view, it runs at a 45 angle to the ridge. If the major roof has a hip end, the supporting valley rafter has the same line length as the major hip rafter. BC Carpenter Apprenticeship Program Level 3 5

9 Learning Task 1 Competency H-7 Supported Valley The supported valley rafter, sometimes referred to as the short valley, extends from the supporting valley to the wall plate. In plan view, it runs at 90 to the supporting valley, and 45 to the plate. If the minor roof has a hip end, the supported valley rafter has the same line length as the minor hip rafter. Valley Jack Valley jack rafters are common rafters that are cut off to meet the valley rafter. They don t cross the wall plate. Jack rafters have the same slope and same ridge cut as the common rafter. The on-centre spacing is laid out with the common rafters. The valley jack rafter extends from the ridge to the valley. In plan view, it runs 90 to the ridge (like a common), and 45 to the valley. It has the same plumb cut as the common at the top and a single cheek cut at the bottom where it meets the valley. Installed, the top edge of the valley jack should be slightly above the edge of the valley to allow the roof sheathing to meet at the centre of the valley rafter. Hip-valley Cripple The hip-valley cripple extends from the hip rafter on the major roof to the valley. It touches neither the wall plate nor the ridge. It has a single cheek cut at each end. In plan view, it looks like a parallelogram. Valley-valley Cripple The valley-valley cripple extends from the supporting valley at the top to the supported valley at the bottom. It has a single cheek cut at each end. The long point on both ends is on the same side of the rafter. Cheek Cuts Cheek cuts are angled cuts used to fit hip, valley, jacks, and cripple rafters to other rafters. The supporting valley, jacks, and cripples have a single cheek cuts. The hip rafters, where they fit into the tripod, have double cheek cuts. Valley jack and hip jack rafters are often installed as pairs on either side of the valley or hip rafters and have opposing right- and left-hand single cheek cuts. 6 BC Carpenter Apprenticeship Program Level 3

10 Competency H-7 Learning Task 1 Single cheek cut Double cheek cut Figure 3. Cheek cuts Purlin Purlins are used to provide cross ventilation in vaulted ceilings. Purlins are also used for roof coverings that require support across the roof such as metal roofs or tile and slate roofs. Purlins are also referred to as strapping. Roof Joists The rafters in a vaulted ceiling are called roof joists. They support both the roof and the ceiling finish. In the BC Building Code, Span Tables A4 and A5 are used for these framing members. Working Points Working points are created by the intersections of the theory lines of the rafters. Often the working point for one rafter is the working point for other rafters. The working point is always located at the centre line of the rafter. Tripod The tripod connection method of connecting the common rafters to the hip, end common, and ridge is shown in Figure 4. The two commons and the end common form the three-legged tripod. Figure 4 shows the adjustments for the commons and the ridge. BC Carpenter Apprenticeship Program Level 3 7

11 Learning Task 1 Competency H-7 End common Half the thickness of the ridge Working point Half the thickness of the common Figure 4. Tripod connection method Now complete Self Test 1 and check your answers. 8 BC Carpenter Apprenticeship Program Level 3

12 Competency H-7 Learning Task 1 Self Test 1 1. Identify each roof component in Figure 1. a b c t s r d q p o n e f g h i j m k l Figure 1. BC Carpenter Apprenticeship Program Level 3 9

13 Learning Task 1 Competency H-7 Identify each roof component by writing its name beside the number: a. k. b. l. c. m. d. n. e. o. f. p. g. q. h. r. i. s. j. t. 2. Describe the difference between a major roof and a minor roof. 3. What type of cheek cut is used for a supporting valley rafter? 4. What type of cheek cut is used for a valley jack rafter? 5. Which rafter has a cheek cut at both ends? 6. What is the purpose of a purlin? 10 BC Carpenter Apprenticeship Program Level 3

14 Competency H-7 Learning Task 2 Learning Task 2 Build an Equal-slope Intersecting Roof An intersecting roof consists of two or more roofs meeting at an angle. In this Learning Task, you ll study roofs that intersect at right angles. On an equal-slope intersecting roof, the slope is the same for all roof surfaces. These can be hip roofs, gable roofs, or a combination. Major roof Minor roof Figure 1. Isometric drawing of an intersecting hip roof The hip and valley rafters support the join between the roof slopes. Planning for the construction of an intersecting roof begins with the blueprints. The rafter locations are not often shown on the blueprints, so the framer will draw the roof based on the floor plan. Supporting valley rafter Supported valley rafter The supporting and the supported valley rafters are exactly the same theory lengths and slope as the corresponding major and minor hip rafters. Figure 2. The angle formed by the hip and valley rafter (plan view) BC Carpenter Apprenticeship Program Level 3 11

15 Learning Task 2 Competency H-7 To draw the plan view location of a valley rafter on a rectangular building, draw a line from the inside corner of the building intersection toward the centre of the building at exactly 45 to the outside wall. Use a set square to lay out this angle. Draw the other valleys and hips towards the centre as well. Where these lines meet is the location of the ridge board. Even the most complicated equal slope intersecting roof can be drawn using this simple principle (Figure 3). Figure 3. Complicated intersecting hip roof The following principles will help when drawing rafter locations: All hips and valleys are at 45 to the wall plates and ridges. The widest span produces the highest ridge and is the major roof. The ridge boards are always centered between the walls. The ridge board is always parallel to the walls. 12 BC Carpenter Apprenticeship Program Level 3

16 Competency H-7 Learning Task 2 Rafter Length Calculations Ridge Theory Length The theory length of the major ridge board for a hip roof is equal to the length of the building minus the run of the end common at each end, or the length of the building minus the span of the building. The hip roof shown in Figure 4 has a length of 40' and the run of each end common is 12'6". This means that the theory length of the major ridge board is 40' 12'6" 12'6" = 15'. The theory length of the major ridge board on a gable end roof is equal to the length of the building. In some designs, the ridge may project beyond the gable end to support the barge rafters. The theory length of the minor ridge on a hip roof is the same as the length of the minor wall plate it parallels. The length of this wall is referred to as the jog. The minor ridge forms a parallelogram with the wall. The theory length of the minor ridge on a gable end is equal to the jog plus the run of the minor common. Common Rafters The common rafter is the basis for much of the rest of the roof. The correct calculation and layout of this rafter is used in several ways: The roof slope given on the plans refers to the slope of the common rafter. The jacks and cripples have the same slope as the commons. The hip and valley rafters have the same number of units of run as the common. The hip and valley rafters use the same rise as the common (but a different run). The amount of the rafter that is left above the plate once the bird s mouth is cut is the same for all rafters on the roof. This is often referred to as the Wood Above Plate or WAP. The theory length of the common rafter for an intersecting roof is calculated in the same way as a common rafter on a gable end roof. (This is also described in Level 1, H-4 Learning Task 3.) Find the run of the common rafter by dividing the span (width) of the building by two. Expressed as a decimal, this will determine the number of units of run. BC Carpenter Apprenticeship Program Level 3 13

17 Learning Task 2 Competency H-7 2' o.c. 40' 1'8" TYP 25' 14' 8' 12' 18' Figure 4. Intersecting hip roof with Imperial dimensions 14 BC Carpenter Apprenticeship Program Level 3

18 Competency H-7 Learning Task 2 Name Run # Units Length/ Unit of Run Theory Length Major Common 12'6" " Common Overhang Adjustments Amount Cut Layout # thickness of ridge 1'8" " full fascia 1½" Major Ridge 40' 12'6" 12'6" = 15' 15' Major Hip X ½" Hip Overhang X ¼" Major 1st Hip Jack Common Diff in Jacks Supporting Valley + thickness of common at each end 45 thickness of common full 45 of fascia ¾" square/ plumb square/ plumb 1½" square 1 " 2 " 10'6" " 45 of hip 1 " 2' " X " 45 of ridge 1 " Minor Common 9' " Minor Hip X " Supported Valley X " Minor Ridge equal to jog; 12' Minor 1st Hip Jack Shortest Major Valley Jack Shortest Valley Valley Cripple Hip Valley Cripple Shortest Minor Valley Jack Longest Minor Valley Jack thickness of ridge 45 thickness of common thickness of valley ¾" 1 " " 45 of hip 1 " 1' " 1' " 8" " 2' " 8' " Table 1. Imperial measurements for intersecting hip roof of ridge; 45 of valley 45 of valley at each end 45 of hip; 45 valley of ridge; 45 of valley of ridge; 45 of valley ¾" 1 " 2 " 2 " 1 " 1 " double cheek double cheek single cheek single cheek square plumb double cheek square plumb single cheek single cheek single cheek single cheek single cheek single cheek 8 and 12 8 and 12 8 and 17 8 and 17 8 and 12 8 and 17 8 and 12 8 and 17 8 and 17 8 and 12 8 and 12 8 and 12 8 and 12 8 and 12 8 and 12 The information shown in Table 1 is presented in the typical order used to calculate the rafter lengths. This table is the basis both for estimating and building the roof. BC Carpenter Apprenticeship Program Level 3 15

19 Learning Task 2 Competency H-7 For example: 25' = 12. 5' 2 = units ofrun For a metric roof, the unit of run is typically 250 mm. Divide the span (width) of the building by two and then divide this value by 250 to determine the number of units of run: 7600 mm = 3800 mm mm = units ofrun 250 mm The concept of units of run is useful when calculating the hip and valley rafter lengths. The slope of the roof is shown on the building plans by a rise/run triangle and expressed as the rise in inches to the run in inches. (e.g., an 8 and 12 slope has a rise of 8 inches for each run of 12 inches.) The hypotenuse of this rise/run triangle is the Length of the Common Per Unit of Run (LCPUR). By using the Pythagorean Theorem (a 2 + b 2 = c 2 ) with a as the rise and b as the run, c is the hypotenuse or the LCPUR. Determine the LCPUR for a roof with an 8 and 12 slope. a + b = c = c = c c = " Determine the LCPUR for a metric roof with a rise of 150 mm and a run of 250 mm. a + b = c = c = c 2 c = mm 16 BC Carpenter Apprenticeship Program Level 3

20 1 2 3 Competency H-7 Learning Task 2 A framing square is stamped with tables. The rafter table is useful because it has the length per unit of run on the table for both common rafters and hip or valley rafters. Instead of calculating these numbers as described above, you can use the numbers on the framing square to calculate the theory lengths of all rafters COMMON RAFTER LENGTH PER FOOT OF RUN HIP OR VALLEY RAFTER LENGTH DIFF IN LE NGTH OF J ACK S 16 INCH CE NTR E S DIFF IN LE NGTH OF J ACK S 24 INCH CE NTR E S SIDE CUT OF JACKS SIDE CUT OF HIP OR VALLEY /8 43 1/4 6 11/16 8 1/ /4 24 5/ / /16 1 Figure 5. Framing square The theory length of the common rafter (also known as the Line Length) is the product of the number of units of run in the common rafter times the LCPUR. As per Table 1: Major common: = 180¼" Minor common: = 129¾" This is the theory length of the common rafter, down the slope, from the centre of the ridge to the outside of the plate. An intersecting roof will have major commons and minor commons. These rafters have the same slope, but because the buildings have different widths, the rafters have different runs and therefore have different line lengths. Both major and minor end common rafters are calculated the same way as the major and minor commons. The end commons differ from the commons by the adjustments discussed in the Rafter Layout section later in this Learning Task. Overhang of the Common Rafter The overhang of the common rafter is the length down the slope based on the projection. The projection is the horizontal distance that the roof extends beyond the side of the building. A 20" projection has a run of 1'8" = units of run. A 500 mm projection has 500/250 = 2 units of run. The overhang is calculated in the same way as the theory length of the common: units of run LCPUR: = 24 " BC Carpenter Apprenticeship Program Level 3 17

21 Learning Task 2 Competency H-7 This is the same overhang for all common and hip jack rafters over the entire roof. Hip and Valley Rafters The hip and valley rafters run at 45 to the ridge, plates, and commons. It is useful here to consider a 45 triangle. 45 Right-angle Triangle On an equal slope intersecting roof, the hip and valley rafters run (horizontally, in plan view) at 45 to all the other rafters. The 45 right angle triangle is a special triangle formed by drawing the diagonal across a square (Figure 6). The hypotenuse of the 45 triangle with sides of 12" (the unit of run in an imperial roof) is 16.97". Therefore, 17" is used as the run layout number on an imperial framing square for hips and valleys. Diagonal = 16.97" 12" 45º 12" Figure right angle triangle When framing in metric, the hypotenuse of the unit of run for the roof, typically 250 mm, results in a run of the hip and valley as mm. 354 mm is used as the run layout number on metric framing square for hips and valleys. 8" 12" 8" 17" Common Hip or valley Figure 7. Units of run for commons and hips or valleys 18 BC Carpenter Apprenticeship Program Level 3

22 Competency H-7 Learning Task 2 For each unit of run in the common rafter, there is a unit of run in the hip rafter. The size of the unit is larger, but there are an equal number of units of run in the hip rafter as there are in the common rafter. The run of the hip (or valley) rarely needs to be computed simply use the numbers of units from the common. Hip and Valley Line Length and Overhang Calculations The length of the hip (or valley) per unit of run (LHPUR) is the hypotenuse of the unit rise of the roof and the unit run of the hip. On an 8 and 12 slope roof, the LHPUR is 2 2 the product of = ". This is also found on the second line of the rafter table on the framing square. The theory length of the hip is the number of units of run in the common times the LHPUR: = 234½" This is the length, down the slope, of the hip from the working point at the tripod connection to the outside corner of the plates. The theory length of the minor hip is based on the number of units of run in the minor common: = 168 " The overhang of the hip or the valley is calculated the same way. Number of units of run in the common projection times the LHPUR: = 31¼" Valley Rafter Line Length Calculation The calculation for the theory length of the supporting (long) valley is the same as for the major hip rafter. In fact, because they form a parallelogram, the length does not need to be calculated, simply use the length of the major hip. The difference between these rafters is in the shortenings to their length and the layout of the cuts, both of which are described in the Rafter Layout section later in this Learning Task. On a gable end roof, the theory length of the valley is calculated in the same way as the major hip: the number of units of run in the major common times the LVPUR. The theory length of the supported (short) valley is the same as the theory length of the minor hip. These rafters form a parallelogram. On a gable end minor roof, the theory length of the supported valley is calculated by the number of units of run in the minor common times the LVPUR. BC Carpenter Apprenticeship Program Level 3 19

23 Learning Task 2 Competency H-7 Valley Jack Calculation Valley jack rafters are common rafters that are cut to meet the valley rafter. The length of the first valley jack rafter is often easy to measure. From the on-centre layout on the ridge board, use a framing square to measure the length of the first valley jack rafter (Figure 8). Ridge Length of the first valley jack Valley rafter Figure 8. Length of the first valley jack rafter Make the measurement to the long point of the valley jack. It s not difficult to calculate the length of this rafter. Notice that, in plan view, the run of the valley jack is equal to the horizontal distance between the on-centre layout of the jack and the working point where the valley meets the ridge. Multiply the number of units of run by the LCPUR to get the line length. As per Figure 4, the shortest valley jack rafter is one foot away from where the long valley hits the ridge. Find this by noting that the valley hits the ridge 8 feet over from the tripod (a parallelogram with the jog in the wall). The on-centre spacing of the commons and jacks, starting from the left end of the ridge, puts the valley jack 1' over from the valley. 1' = 1 unit = 14 " After the first valley jack is measured or calculated, the lengths of the next rafters are found by adding the difference in lengths of jacks (from the rafter table on the framing square) to the length of the first valley jack rafter (Figure 9). This is the difference in length on the slope and is referred to as the common difference. 20 BC Carpenter Apprenticeship Program Level 3

24 1 2 3 Competency H-7 Learning Task COMMON RAFTER LENGTH PER FOOT OF RUN HIP OR VALLEY RAFTER LENGTH DIFF IN LE NGTH OF J ACK S 16 INCH CE NTR E S DIFF IN LE NGTH OF J ACK S 24 INCH CE NTR E S SIDE CUT OF JACKS SIDE CUT OF HIP OR VALLEY /8 43 1/4 6 11/16 8 1/ /4 24 5/ / /16 1 Figure 9. The difference in length of jack rafters The common difference in all the jacks and cripples is based on the rafter spacing. Rafters spaced 2 feet apart run 2 feet less (or more) than the next rafter. On an 8 and 12 roof, 2 feet = 2 units = 28 " = common difference. Valley Valley Cripple Rafter Calculation The valley valley cripple is often easy to measure. Measure the length of the shortest valley cripple from the layout of the corresponding valley jack. It s worth setting a quick string line to line up the valley jack above the cripple. Measure on the same side of the rafter, from long point to long point. The run of the valley valley cripple is twice the distance it is from the intersection of the valleys. From the run, get the number of units LCPUR = line length of valley cripple. Subsequent valley cripple rafters are longer by twice the common difference. BC Carpenter Apprenticeship Program Level 3 21

25 Learning Task 2 Competency H-7 Hip Valley Cripple Calculation The hip valley cripple rafter runs between the valley rafter and the hip rafter (Figure 10). Hip valley cripple Valley jack Hip jack Jog in the building Figure 10. Plan view of a hip valley cripple jack rafter The calculation of the hip valley cripple is simple. The wall plate on the major roof forms a 45 right angle triangle with the run of the hip valley cripple. The jog in the building is the same as the run of the rafter. On the roof from Figure 4, the jog is 8 feet: 8' = 8 units = 115 " (line length of hip valley cripple) Rafter Layout The theory lengths are calculated and measured from working point to working point. These working points are theoretical points. If a fishing line were stretched from point to point, the actual lengths would be the same as the theory lengths. However, since the roof is built with lumber, the lengths must be adjusted to make room for the other rafters. These are called shortenings or adjustments. Building is typically done with 2 dimension lumber (e.g., 2 4, 2 6, etc.) which has an actual thickness of 1½". Larger dimension lumber may also be used, such as in timberframe construction. 22 BC Carpenter Apprenticeship Program Level 3

26 Competency H-7 Learning Task 2 All these shortenings and the layout of the cut lines are based on the plan view. Fullscale drawings of the connections between rafters assist in visualizing the adjustments. The adjustment and layout line measurements are always made square from the plumb lines. Ridge Layout Looking at the tripod connection, notice that if the ridge is cut to its working point theory length, it would only support half the thickness of the common. Half the thickness of the common needs to be added to the length at each end of the ridge. Because the ridge is level, it gets square cuts at each end. Working point End common ½ the thickness of the ridge ½ the thickness of the common Figure 11. Tripod connection The minor ridge gets an adjustment of plus half the thickness of the common at the tripod and minus half the diagonal thickness of the valley at the other end. Because this is a double cheek cut, lay out for 45 cut lines off the shortening line. Since the lines are laid out on the side of the rafter, rather than in the centre where the theory points are, lay out and cut on lines half the thickness of the ridge on either side of the shortening. Cut the long point first. Layout of Rafters Installed into the Ridge For all roof framing members other than the ridge, the plumb lines at the working points are laid out using the framing square and roof slope numbers and marking on the rise. BC Carpenter Apprenticeship Program Level 3 23

27 Learning Task 2 Competency H-7 After laying out the plumb lines at the working points, lay out the shortenings for length. It s important that the measurements for the shortenings are made square to the plumb lines. For the common rafters and valley jacks that hit the ridge at the top, the adjustment at the ridge is minus half the thickness of the ridge. If the ridge is 1½", this shortening is ¾". At the tripod, the shortening for the end common is minus half the thickness of the common. Bird s Mouth The bird s mouth is the shaping of the rafter to fit tightly on the wall plate. It s formed by a plumb (vertical) cut and a seat (horizontal) cut. The BC Building Code requires a minimum bearing (seat cut) dimension of 38 mm (1½"). This is a minimum and is rarely used as it does not allow for adequate nailing. A rule of thumb for laying out the bird s mouth is to lay out the plumb line across the whole rafter and mark the seat cut so that ⅔ of the plumb cut is above the plate (sometimes referred to as wood above plate or WAP) and ⅓ is below the plate. This typically ensures a bearing dimension larger than the Code requirement and is adequate for nailing. Depending on the climate zone, the wood above plate may not be large enough to meet the Code requirement for insulation and ventilation of the attic space. In this case, it s possible to sit the rafter on a rafter plate fastened to the top of the ceiling joists and run the ceiling joist into the soffit space where the rafter connects to the ceiling joist. To lay out the bird s mouth, use the roof slope numbers, mark on the rise for the plumb cut, and the run for the seat cut. The WAP for every rafter on the roof comes from the WAP on the common rafter. Overhang The common and hip jacks get shortenings at the overhang. Starting from the plumb line at the working point, measure square back the full thickness of the sub fascia to the cut line. Hip Rafter Layout Hip and valley rafter layout uses the same rise as the common, but 17" or 354 mm as the run. 24 BC Carpenter Apprenticeship Program Level 3

28 Competency H-7 Learning Task 2 Hip Half of the diagonal of the common rafter End common Working point Half the thickness of the common Figure 12. Tripod connection showing shortening for the hip At the tripod, the hip gets shortened by ½ the diagonal thickness of the common. To get the diagonal thickness of any board, lay out a 45 angle on the edge of stock and measure the length of the line. For 2 dimension lumber, the full diagonal will be 2 " (54 mm), and half that diagonal thickness is 1 " (27 mm). For timbers of any stock other than dimension lumber, it s most accurate to lay out the 45 line and measure the length of the line. An alternate to measuring is to calculate the thickness mathematically: = " = = 1 " " Working point 4" 17" Figure 13. Shortening the hip for half the diagonal thickness of the common Lay out the working point plumb line of the hip, measure square back half the diagonal thickness, and mark a second line. This is the shortening for length. BC Carpenter Apprenticeship Program Level 3 25

29 Learning Task 2 Competency H-7 From this shortening line, measure and lay out half the thickness of the hip on either side. Once again, these cut lines for a double cheek cut come from the fact that the layout is on the side of the rafter, which is ¾" (19 mm) from the centre of 2 stock. Set the saw to 45 and cut the long point first. Be sure to cut on the waste side of the line " Working point ¾" 2 6 hip rafter Figure 14. Lay out the double cheek cut using half the thickness of 2 stock The bird s mouth of the hip rafter is a tricky layout. Looking at a plan view drawing of the hip at the corner of the plates, notice that the hip has to be cut at the working point line for the rafter to clear the corner of the building. However, to get the tops of the rafters to line up and ensure the sheathing lays flat, the Wood Above Plate (WAP) needs to be the same as the common rafter. This cut needs to be made at the place where these plumb WAP lines line up: at the edge of the plate. For the hip rafter, the side of the hip (where the layout is made) crosses the plate half the thickness of the hip towards the ridge from the working point line. Using 2 dimension lumber, this is ¾" (19 mm) up the hip. The WAP is marked on the line closest to the ridge and the cut is made on the line closest to the fascia. Half of the thickness of the hip Building line Working point Working point Point where the hip meets the building line Figure 15. Bird s mouth on the hip rafter (plan view) 26 BC Carpenter Apprenticeship Program Level 3

30 Competency H-7 Learning Task 2 Depth of the common rafter s bird s mouth Half of the thickness of the valley Working point Figure 16. Bird s mouth on the hip rafter (elevation view) The shortening at the overhang of the hip is the full diagonal thickness of the fascia, 2 " (54 mm) when using 2 dimension lumber. From the working point line, measure up for the shortening. Then lay out a double cheek cut similar to the one at the tripod. Hip rafter Hip jack rafter 3" 3" Figure 17. Wood Above Plate (WAP) BC Carpenter Apprenticeship Program Level 3 27

31 Learning Task 2 Competency H-7 Projection Building line Full diagonal thickness of the rough fascia Outside of rough fascia Working point Figure 18. Double cheek cut at the overhang of the hip rafter Valley Rafter Layout The supporting valley connects to the ridge at a point along the ridge equal to the jog in the building. This point in the centre of the ridge will align with the centre line of the valley. These are the working points. The shortening of the valley is half the diagonal thickness of the ridge. Lay out the long point cut line of the single cheek cut half the thickness of the valley towards the ridge, set the saw to 45 and cut on the long point on the waste side of the line. Install this rafter so that the centre of the valley is flush with the top of the ridge. The corner of the valley that sticks above the ridge is trimmed off later. Half the diagonal thickness of the ridge board Working point Ridge Supporting valley Figure 19. Plan view of long valley at ridge 28 BC Carpenter Apprenticeship Program Level 3

32 Competency H-7 Learning Task 2 ½" Working point 4" 17" Figure 20. Lay out single cheek cut of valley At the plate, the sides of the valley cross the plate ¾" (19 mm) down the valley from the working point. The wood above plate is laid out at the working point line (the line closest to the ridge), but the cut is on the line closest to the fascia. Rough fascia Valley rafter Building line Working point Figure 21. The centre of the valley meets the building line at the working point Depth of the common rafter s bird s mouth Half of the thickness of the valley Working point Figure 22. Lay out the valley bird s mouth The overhang of the valley rafter gets an inside double cheek cut. The shortening is the full diagonal thickness of the rough fascia, with cut lines half the thickness of the BC Carpenter Apprenticeship Program Level 3 29

33 Learning Task 2 Competency H-7 valley away from the building. In this case, lay out on both sides of the rafter. Set the saw at 45 and set the depth to half the diagonal thickness of the valley. Cut half-way through on one side, flip the board and cut half-way through. In a case where the soffit covers the end cuts of the rafters, a single cheek cut on the valley will be adequate. Working point Full diagonal of the rough fascia board Rough fascia Valley rafter Building line Working point Figure 23. Inside double cheek cut at valley tail One way to remember the differences and similarities of the hip and valley rafters is: the WAP is on the line closest to the ridge on both rafters, and the cut line is on the line closest to the fascia on both rafters. This is true even though the WAP is laid out up the hip, and the cut line is laid out down the valley. Supported Valley Layout On a building where the minor roof has a smaller span than the major, the supported valley rafter hits the supporting valley at a 90 angle part-way down the supporting valley. The line length of the supported valley is measured to the working point, which is at the centre of the supporting valley. The supported valley must be shortened by half the thickness of the supporting valley (Figure 24). 30 BC Carpenter Apprenticeship Program Level 3

34 Competency H-7 Learning Task 2 Ridge Half of the thickness of the supporting valley Supporting valley Supported valley Figure 24. Shortening of the supported valley The measurement of the shortening is made square to the plumb cut. The saw is set square. Jack and Cripple Rafter Layout Hip jacks, valley jacks, and cripples all get single cheek cuts. The shortening adjustments for these rafters are half the diagonal thickness of the hip or valley they connect with. Working point Half of the diagonal thickness of the hip rafter (1 1 16" for a 2 6 hip) Hip Hip jack Figure 25. Shortening of the hip jack It s usually easiest to cut on the long point line of the rafter (a typical right-handed circular saw will have the large side of its base supported by the board). Lay out the long point by measuring square to the shortening adjustment line half the thickness of the rafter. BC Carpenter Apprenticeship Program Level 3 31

35 LEARnIng TASk 2 CoMPETEnCy H-7 The valley jack has a common plumb cut layout at the ridge and a single cheek cut at the lower end at the valley. The hip jack has a single cheek cut at the hip at the top and a common bird s mouth and overhang. The valley valley cripple has a single cheek cut at both ends, long point on the same side. The hip valley cripple has a single cheek cut at both ends, long point on opposite sides. When cutting jacks in pairs, be sure to lay out the long point cut on the opposite sides of each rafter. Ridge board Valley jack rafters Working point Back off this portion of long valley rafter Working point Working point Supporting or long valley rafter Centre line of long valley rafter to upper edge of ridge Valley cripple jack rafter Working point Position of end of ridge against valleys Supported or short valley rafter Figure 26. Valley, valley jack, and valley cripple jack vaulted Ceilings Vaulted ceilings often use heavy valley rafters to carry the load of the valley jacks. The layout and cutting of the laminated veneer beams shown in Figure 27 is critical. If they re cut short, new beams have to be ordered, delaying the construction schedule. 32 BC CARPEnTER APPREnTICESHIP PRogRAM LEvEL 3

36 Competency H-7 Learning Task 2 Figure 27. Valley construction of a vaulted ceiling The valley jack rafters shown in Figure 27 are nailed to the side of the valley beam with seven 3¼" nails. The structural engineer often requires angled joist hangers at this location. Tail Cut The inside double cheek cut is made by cutting each lamination at a 45 bevel (Figure 28). Figure 28. Valley beam tail cut BC Carpenter Apprenticeship Program Level 3 33

37 Learning Task 2 Competency H-7 Figure 29 and Table 2 are an example of a similar roof with metric dimensions. 600 o.c TYP Figure 29. Intersecting hip roof with metric dimensions 34 BC Carpenter Apprenticeship Program Level 3

38 Competency H-7 Learning Task 2 Name Run # Units Length/ Unit of Run Theory Length Major Common Common Overhang Adjustments Amount Cut Layout # thickness of ridge 10 square/ plumb full fascia 38 square/ plumb Major Ridge = Major Hip X Hip Overhang X Major 1st Hip Jack Common Diff in Jacks Supporting Valley + thickness of each end 45 thickness of common full 45 of fascia +38 square/ plumb 54 double cheek 38 double cheek of hip 27 single cheek X Minor Common Minor Hip X Supported Valley Minor 1st Hip Jack Shortest Major Valley Jack Shortest Valley Valley Cripple Hip-Valley Cripple Shortest Minor Valley Jack Longest Minor Valley Jack X of ridge thickness of ridge 45 thickness of common thickness of valley 13 square plumb 10 square/ plumb 27 double cheek 19 square/ plumb of hip 27 single cheek Table 2. Metric measurements for intersecting hip roof of ridge;- 45 of valley 45 of valley at each end 45 of hip; 45 valley of ridge; 45 of valley of ridge; 45 of valley 10, 27 single cheek 27, 27 single cheek 27, 27 single cheek 10, 27 single cheek 10, 27 single cheek square BC Carpenter Apprenticeship Program Level 3 35

39 Learning Task 2 Competency H-7 Putting it All Together Once the ceiling joists are in place, the construction of the roof usually starts with making a walk-way on top of the joists directly under the ridges and scaffolding at the plate, either inside or outside the building. Typically the major ridge goes up first, supported by pairs of major common rafters and braced by the end commons on a hip roof or by a sway brace on a gable end roof. Major hips and valleys follow, with jacks and cripples filled in. It s important to keep the ridge, hips, and valleys straight by installing commons and jacks in opposite pairs. It s good practice to place a string line along these rafters to ensure their straightness. The minor roof follows with the short valley first and the minor ridge, with its double cheek cut at the intersection of the two valleys, supported by a pair of minor commons. Sheathing or strapping the roof typically begins at the bottom of the roof. The fascia has been straightened and the plywood lays out much as a floor system, keeping an eye on the rafter spacing as you nail off. Where sheathing runs into the valley, making long point and short point measurements from the leading edge of plywood ensures a good fit. This is not a 45 cut. It s not difficult to calculate this cut. The offset from the long point to the short point is the run of a sheet of plywood with 48" on the slope. It s a ratio equation where, on an 8 and 12 roof: Offset 12 = 48" = It s usually easier to cut this sheet on the ground than on the roof. Now complete Self Test 2 and check your answers. 36 BC Carpenter Apprenticeship Program Level 3

40 Competency H-7 Learning Task 2 Self Test 2 1. Explain how the layout numbers for a valley rafter is derived. 2. What figures on the framing square are used to lay out the seat cut at the bird s mouth for a valley rafter? 3. Calculate the line length of a supporting valley rafter for a building that is 26' wide with a 5 in 12 slope. (Use Figure 1.) / / / /8 25 5/ / / /2 24 3/4 11 5/ /16 Figure Calculate the overhang length for a valley rafter if the projection is 16" and the roof slope is 5 in 12. (Use Figure 1.) BC Carpenter Apprenticeship Program Level 3 37

41 Learning Task 2 Competency H-7 5. Draw a sketch showing the layout of the supporting valley rafter. Show in detail, including measurements, adjustments, and cut lines. Use a roof slope of 5 in 12, a 1 8 ridge board, 2 4 common rafter, and 2 6 valley rafter. 38 BC Carpenter Apprenticeship Program Level 3

42 Competency H-7 Learning Task 3 Learning Task 3 Material Calculations for Ceilings and Roofs The ceiling is constructed before the roof framing is assembled. The material calculations include the ceiling joist and blocking quantities. Ceiling Joists Rafter-framed roofs use ceiling joists to tie the ends of the rafters together. The ceiling joists also support the ceiling finish. In roofs built without ridge support, the fastening of the ceiling joists to the rafters is extremely important. The fastener requirements for ceiling joists to top plates, and rafters to ceiling joists, are in BCBC Table and (6). The design of the building may require that the underside of the roof is finished (as opposed to being attic space). This provides interesting architectural space, with sloped ceilings and the underside of the valley visible from below. In these designs, the rafters become roof joists and the dead load of the ceiling finish and insulation is additional to the load of the roof itself. Use BCBC Tables A-4 and A-5. These roof designs may require a supported ridge and are referred to as a vaulted or cathedral ceiling. The ridge could be supported by a ridge beam designed from BCBC Table A-12 or a load-bearing wall. The design may also specify living space in what is often considered attic space. In this case, the ceiling joists become floor joists, subject to all floor loads, as well as tying the roof joists together. Size and Spacing The size and spacing of ceiling joists is designed using the tables in Part 9 of the Building Code. The design process is almost exactly the same as the design of floor joists. Refer to Competency H-2: Build Foundations and Floors for instructions in designing ceiling joists. Ceiling Joist Length The length of the ceiling joist is based on the design of the building. For small structures, a single joist may span the entire building so its length will be equal to the building s width. For wider buildings, the ceiling joists are lapped over interior bearing walls or beams. BC Carpenter Apprenticeship Program Level 3 39

43 Learning Task 3 Competency H-7 Load-bearing Wall When lapped over a bearing wall, the joists must be nailed to each other with one more nail than required to nail the ceiling joist to the rafter (Figure 1). 7 nails 6 nails Figure 1. Joining ceiling joists over a bearing wall The minimum amount of lap is not clearly specified in the Building Code. When the ceiling joists provide a tie between the ends of the roof rafters, allow for a minimum lap of 12" or butt the joists and cleat the joint with a 24" block of joist material. Whenever possible, lapping the joists is preferred. This is so that the number of nails required does not split the wood. The nailing shown in Figure 1 is only required if the rafters are not supported at the ridge. If a ridge beam or load-bearing wall supports the ridge, only two nails are required. Supported by a Beam If the beam is under the ceiling joists, often referred to as a drop beam, the connection of the joists is the same as shown in Figure 1. If the beam is made flush with the ceiling joists, often referred to as a flush beam, the joists will be attached to the beam with joist hangers. A metal strap, or tie, should be used over the beam to tie the opposing ceiling joists together (Figure 2). If the joists support a floor, the floor sheathing will tie the joists together. 40 BC Carpenter Apprenticeship Program Level 3

44 Competency H-7 Learning Task 3 Metal strap tie Figure 2. Ceiling joists butting into a flush beam The materials for the flush beam are considered as part of the ceiling framing. The amount of material is equal to the length of the beam multiplied by the number of plies in the beam. Number of Ceiling Joists The number of ceiling joists depends upon the spacing of the joists and the size of the building. For gable roofs, the first ceiling joist is located beside the first rafter away from the gable end (Figure 3). Backing is installed at the gable wall to support the ceiling finish. This backing is often 2 4s laid on flat. BC Carpenter Apprenticeship Program Level 3 41

45 Learning Task 3 Competency H-7 Figure 3. Ceiling joist located beside rafter Figure 3 shows ceiling joists located beside every rafter. This arrangement requires less nailing at the connection. Figure 4 shows rafters spaced at 24" o.c. while the ceiling joists are at 16" o.c. In this case, the rafters are only tied to every third ceiling joist. Figure 4. Rafters tied to every third ceiling joist The arrangement shown in Figure 4 is common because the span of the ceiling joist usually requires the ceiling joists to have a closer spacing than the rafters. 42 BC Carpenter Apprenticeship Program Level 3

46 Competency H-7 Learning Task 3 Calculate the number of ceiling joists by dividing the length of the building by the spacing of the ceiling joists. Round the result down to the nearest whole number. Blocking Blocking at Openings Openings are required in the ceiling framing to accommodate attic access holes and chimneys. The framing for the openings is identical to the framing for openings in floors (Figure 5). Figure 5. Opening in ceiling joists For openings that do not require more than one ceiling joist to be cut, the construction shown in Figure 5 is adequate. If the ceiling joists are required to tie the ends of the rafters together and the opening is large, an engineer must design the construction. Special framing is required to transfer the tension loads across the wide opening. If the rafters are supported at the ridge by a ridge beam or bearing wall, large openings in ceiling joists can be designed and built following the Building Code regulations for floor openings. The length of the headers for the opening is included in the blocking requirements. Blocking Between Joists Although not specifically required by the Building Code, blocking should be installed at the middle of the span of the ceiling joists. The blocks should be the full depth of the ceiling joists. This blocking stiffens the ceiling framing and prevents the ceiling joists from twisting. BC Carpenter Apprenticeship Program Level 3 43