Roofing Terms Explained

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2 FEATURES

3 Roofing Terms Explained Rafter: The roofing members that form the slopes which basically support the external cladding of the roof. Ridge Beam: The horizontal member to which the top ends of the rafters are fixed. Wall Top Plate: The uppermost framing member running along the top of the wall and that which all the rafters rest upon and fix to at the lower end. Common Rafter: The main sloping rafter in a roof spanning between the top plates of the wall and the ridge. Hip Rafter: The rafter that forms the intersection between two slopes of a roof making an external corner and running from the ridge to the eaves. Valley Rafter: The rafter that forms the intersection between two slopes of a roof making an internal corner and running from the ridge to the eaves. Hip Jacks: Rafters that span between the Hip Rafter and the eaves. They are installed in opposing pairs resting on the wall top plate. Valley Jacks: Rafters that span between the Ridge at their top and the Valley Rafter at the bottom end instead of the eaves. They are also installed in opposed pairs. End Jack: the rafter that runs from the end of the Ridge Beam to the Wall Top Plate in a Hip and Valley roof. In most cases it will have the same dimensions as a Common Rafter. Eaves: The overhanging portion of a roof often ending in a Fascia board to which gutters may be attached. Gutter: The trough that channels water from the eaves to the downspouts. Purlins: A horizontal structural member that supports roof covering and carries loads to the primary framing members. They run parallel with the ridge underneath and supporting the rafters. Joists: Any of the small timbers or metal beams ranged parallel from wall to wall in a structure to support a floor or ceiling. Span: The overall width across the roof measured from the outside edges of the Wall Top Plates. Building Codes will specify maximum allowable spans for different grades and sizes of timber. Run: The baseline horizontal measurement of the slope of a roof. For most roofs with the same slope on both sides the Run with equal half the Span distance. Rise: The height which the slope attains over the distance between the Wall top Plate and the Ridge, i.e. the Run. The Rise can be measured from the outside edge of the Top Wall Plate to the centreline of the ridge along a theoretical Backing Line. These measurements are transferred to the top edge of the Rafter to greatly simplify matters when using the Adjustable Roofing Protractor. Slope or Pitch: The Angle of the roof forms a triangle of which the rafter forms the longest side according to the relationship between its other two sides Rise and Run. The Slope or Pitch is commonly expressed as an Angle or simply according to the ratio of Rise over Run. The attached tables support either form, because it forms the basis of all rafter dimensions. Bird s Mouth: Refers to the removal of a triangular piece from the underside of the Rafter to provide a seat locating the lower end resting upon the Top Wall Plate. Backing Line: A line taken from the Top Outer point of the Wall Top Plate to the centre of the Ridge parallel to the top and bottom edges of the rafter. The depth of the backing line is governed by the size of the Bird s Mouth Cut according to the One Third or Hap Rule. The Hap Rule: The depth of the Bird s Mouth and hence the position of the Backing Line should be no greater than one third the depth of the Rafter. Also the horizontal length of the seat need not be wider than the breadth of the Wall Top Plate. This is to maintain the maximum strength of the Rafter by avoiding cutting too much out of the Bird s Mouth. On shallow pitched roofs the backing line may therefore be less than one third deep. Tail Cut: A horizontal cut under the eaves to trim the lower edge of the Rafters so that they don t protrude below the Fascia Boards. Plumb Cut: The cut made in the vertical Plane where the Rafter meets the Ridge Beam. Any line marked in the same vertical is known as Plumb Line.

4 Roof Design Basics Gable: The upper portion of a sidewall that comes to a triangular point at the ridge of a sloping roof. Hip roof: A type of roof containing sloping planes of the same pitch on each of four sides. A hip roof contains no gables. Gable roof: A type of roof containing sloping planes of the same pitch on each side of the ridge. A gable roof typically contains a gable at each end. Skillion Roof: A Skillion Roof slopes from one side of the building to the other in a single span and no ridge. As a result the external walls are of varying heights. Lean-to roof: A roof with one slope only that is built against a higher wall. Dutch Gable: Is a combination of a Hip roof with a small Gable at the top and a sloping roof below. Gambrel roof: A type of roof containing two sloping planes of different pitch on each side of the ridge. The lower plane has a steeper slope than the upper. A gambrel roof usually contains a gable at each end, just like a standard gable roof. Mansard roof: A type of roof containing two sloping planes of different pitch on each of four sides. The lower plane has a much steeper pitch than the upper, often approaching vertical. It contains no Gables. Gazebo Roof: Often Hexagonal or Octagonal but usually symmetrical in their design. Jerkinhead Roof: Consists of a Gable Roof with a truncated Hip Roof section at the top to reduce the height of the Gable.

5 ROOF CUTTING BENCH The underside of the plumb cut can be marked using a chalk line. Jack and Creeper Rafters can also be laid out easily on this bench. For a timber Fascia the tail cut should be done after the roof is built using string lines. Using a metal Fascia the tail cut can be more easily made on the roof bench since the brackets are adjustable.

6 COMMON RAFTER SETOUT The diagram shows a common rafter spanning half the width of a standard roof. Hence the distance H will equal half the width of the building. The distance X is calculated according to the formula; V = H T/2. The H and V value can be used in conjunction with the tables provided to calculate other lengths. The Rise R is calculated using the rise height factor according to the formula; R = V x Rise Height Factor. The lengths L1 and L2 for the positions of the Tail and Bird s Mouth cuts can be calculated using the common rafter factor according to the formula; L1 = V x Common Rafter Factor.

7 COMMON RAFTER EXAMPLE The following is a worked example for a 6/12 or 1 in 2 roof pitch, giving an angle of with a 10 foot or 120 inch half span or run. For clarity sake the example uses decimal inches since the mathematics is straightforward if fractional dimensions are avoided. Some builders prefer to work to the centreline of the roof in the middle of the ridge and make an allowance for it s width at the end, but it is more accurate to apply the allowance prior to calculating the lengths that you need to mark your cuts with the tool. Hence for a run of 120 with a 2 thick ridge beam; V = 120 2/2 V = 119 The Common Rafter Length Factor would be taken from the tables and is and the Rise Height Factor is 0.5 Hence the following calculations R = 119 x 0.5 R = 59.5 L = 119 x L = By varying the inputs according to the desired outcome any of the other dimensions that are shown may also be obtained.

8 HIP AND VALLEY RAFTERS This example shows how Hip or Valley rafters are set out. Allowances A1 and A2 are usually applied to allow for the unique geometry of the Hip and Valley. Since they complicate the geometric calculations most builders prefer to measure the length to the Bird s Mouth in a direct diagonal line after the roof is pitched (built). For a 6/12 roof pitch the Hip or Valley angle of can be read directly from the tables. The angle of can also be found under the Hip or Valley Side Cut Angle column. Mitres in the Hip or Valley seat will also be at this angle. The Hip or Valley Length Factor can be readily found in tables under Length Factors. The lengths L or L1 can be calculated according to the formulae; L = V x Hip or Valley Length Factor. L1 = (V-A2) x Hip or Valley Length Factor. If you wish to measure, calculate or approximate the allowances A1 and A2 then by following the diagram you may calculate the Length L1. This in turn would enable you to cut Hip and Valley rafters directly from the work bench. Valley seats are almost always mitred as shown, but Hip seats are difficult to cut with the mitre facing inwards so the simplified version is usually used. To do this the corner of the Top plates is usually mitred instead.

9 CREEPER or JACK RAFTERS The length of Creeper or Jack Rafters is reduced according to what is known as the common diminishment. Using the tables provided this length can be calculated as using the Common Rafter Factor. The example which follows is for a 6/12 pitched roof, the common rafter factor from the tables being Therefore; 18 x = Note: All lengths are measured to the point of cut.

10 This picture shows the correct use of the side cut marker set to the common side cut angle taken from the tables for 6/12. CREEPER or JACK RAFTER - SIDE CUT

11 ROOF CONSTRUCTION All Purlins are measured for length after the roof is pitched (built). A simple Purlin prop gauge can be used to determine the Prop lengths and cutouts.

12 PURLIN ANGLES SETOUT The tool can also be applied simply as a protractor to set out the Purlin Cuts. The angles are taken directly from the tables provided. The diagram shown refers to a 24 roof pitch. Having two lockable protractors is invaluable in providing the ability to reliably transfer the correct settings to any number of Purlins.

13 EXPOSED BEAM RAFTERS

14 General Information The manufacturer warrants this hand tool free from defects and suitable for its intended use as outlined in the accompanying instructions. Nedful Products Taiwan accepts no responsibility for loss or damage accidental or otherwise arising from the improper use of this product. All building work should be carried out strictly in accordance with the relevant local building codes. The services building engineer may be required to be engaged under some circumstances. Under prevailing conditions certain extra precautions may be required if you are constructing in designated Cyclonic or Termite controlled areas. The Roofing Protractor is a layout or set out tool only and is not for use as a power saw guide, you must wear safety glasses and safety equipment when using this tool. Great Website s for Rafter Mathematics Great Books for Roofing Roof Building Manual Lloyd Hiddle & Allan Staines ISBN Roof Framer s Bible Barry D. Mussel ISBN Roof Cuts & Rafter Lengths Hancock s Roof Books ISBN For more information on advanced roofing via the internet try a Google Search using; Rafter Roof Book Trig, and read from the first 50 results Website For more information, downloads and more go to

15 Roofing Calculations - American Imperial System Rise Ratio Angles Degrees Factors Side Cut Angles Degrees Run Common Rafter Angle Hip or Valley Rafter Angle Purlin Plumb Cut Common Rafter Hip or Valley Rafter Rise Height Common Side Cut Hip or Valley Side Cut Purlin Side Cut / / / / / / / / / /

16 Inch/64 Fractional Decimal Fractional Decimal mm Inch/64 inch inch inch inch mm 1 1/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

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