Special Roofs and Vertical Slating

Transcription

1 Special Roofs and Vertical Slating Roofs falling into the category of "special" roofs have the following characteristics: (a) any roof slope equal to or greater than the minimum pitch for the size of slate preferred that has a rafter length (horizontal projection ie, measured on plan, not along the slope, from ridge to eaves) greater than 9 metres (b) any roof slope less than, but not more than 5º less than, the minimum pitch for the size of slate preferred that has a rafter length (horizontal projection ie, measured on plan, not along the slope, from ridge to eaves) greater than 6 metres (c) any roof slope between 5 and 7 less than the minimum pitch for the size of slate preferred that has a rafter length (horizontal projection ie, measured on plan, not along the slope, from ridge to eaves) not exceeding 6 metres (d) any curved roof, including turrets and spires (e) traditional "open slating" or single-lap slating, such as diamond slating or "German" slating, etc. HOW THIS SECTION OF THE MANUAL IS STRUCTURED This section of Tegral s slating manual does not adhere to the pattern of previous sections. General information on some "special" roofs including curved slating and turrets and spires is provided, followed by more detailed information on aspects of vertical slating, a traditional method of providing a weatherproof and visually pleasing wall finish, common in many parts of Ireland. Detailed advice on designing and specifying "special" roofs is not provided in the manual nor, indeed, in ICP 2 as each design should be individually tailored to meet the specific conditions involved. For example, some long, low-pitch roofs may need to have a rigid underlay incorporated in the sub-structure. Or, for instance, the cutting schedule for slates on a curved roof cannot be prepared without knowing both the radius and the pitch of the slope. Tegral can call on unrivalled knowledge and more than 60 years experience of slating in Ireland when developing details and specifications for "special" roofs. Contact support@tegral.com to avail of this free service. The following information should be provided when seeking Tegral s advice: site location and exposure design drawings of the roof or details of the pitch, length of slope(s), radius, etc build-up of roof construction envisaged, including location of insulation. 55

2 Curved Slating Curved slating may be defined as slating on a roof that has both a shape that is curved on plan, and a length of eaves and ridge. Roofs which converge to a point at the ridge are classified separately, as turrets or spires. Although there is a long tradition of curved slating in Ireland and many other parts of Europe, this type of roofing is not covered by any code of practice. Tegral s recommendations are based on best traditional practice, appropriately updated to suit contemporary conditions. Careful detailing and workmanship, including good carpentry and slating skills, are called for. The provision of a "smooth" roof deck is critical to ensure that slates will sit neatly on the curve after precise cutting by an experienced slating contractor. Curved slating at eaves Tegral Thrutone slates. INTRODUCTION Curved slating requires that individual slates should be "taper-cut" along both sides. The final size of any particular slate will depend on the shape of the roof and position of the slate on that roof. All slates in a given course should have the same dimensions, but should be wider or narrower than the slates in contiguous courses. Trimming the sides of a slate requires nail holes to be positioned closer to the centre of the slate, reduces the side lap of the slating and can affect the weather protection afforded by the roof area in question. Thus, as a general rule, curved slating should be laid at the steepest possible pitch to compensate for the reduction in side-lap cover. Curved slating in exposed areas even where the pitch is steep and anywhere where the pitch does not exceed 30º, should always incorporate a carefully detailed, durable roofing underlay or substructure capable of adequately discharging into the gutter any rainwater which may penetrate the slating. 56

3 SPECIAL ROOFS & VERTICAL SLATING STRUCTURE Where an area of curved slating joins an area of "standard" slating in the same plane, the setting out of the structure or substructure should be adjusted to take account of the depth of counterbattens supporting the curved slating. The options are to counterbatten the entire roof or position the rafters on the curved roof lower by the depth of the counterbattens than the rafters on the standard roof. COUNTERBATTENS Counterbattens should be used on all curved roofs that are intended to receive slating battens. (See previous section of manual "non-standard" roofs.) They eliminate the possibility of moisture damming between the slating battens and the underlay whether flexible or rigid while also clamping flexible underlay and its overlaps in place. BATTENS The radius of the curve to be covered may limit the use of standard slating battens. Traditional methods of curving battens to fit a tight radius include soaking, to improve their flexibility, and shallow saw-cutting the backs of the battens to facilitate bending. The use of countersunk wood screws instead of nails can help restrain battens that have a tendency to "spring". SLATES Setting Out The widest possible slate that will "sit" around the eaves of a particular curve should always be used. The width of each slate should then be marked out along the length of the eaves. Chalk lines should be struck on to the battens at half-slate intervals from the radial point. This will allow for the correct tapering of individual slate courses and assist the slater in maintaining the correct slating bond. Setting out of cone. Variation Courses In the case of short rafter lengths and large radii, it may be possible to taper the slating regularly from eaves to ridge. Often, however, it will be necessary to introduce "variation courses" or "break-courses". These are required where the width of the head of a trimmed slate would result in a side lap smaller than the specified headlap. Variation courses involve the introduction of a slate course either one-and-a-half times or twice as wide as the slate course immediately below it. Hidden visqueen soakers (1000 gauge), resistant to UV light, should be laid between the narrow course of slates and the wider variation course, to provide additional weather protection. Cutting and Holing Slates The taper-cutting and reholing of Thrutone slates should be carried out using a circular bench saw with a diamond-tipped blade or a slater s guillotine or snips, and a holing machine. Experienced slaters can both shape and hole natural slates using a slater s axe. Where it is not possible to use battens, the roof should be boarded out, as with a turret or spire. Curved slating at variation course Tegral Classic Natural slates. 57

4 Turrets and Spires Although turrets and spires can be cone-shaped, multi-sided or formed like a pyramid, the following advice relates to cones and parts of cones where the diameter of the area to be slated is approximately 3 m or less. All turrets and spires terminate at a point apex, which is usually capped in metal. The same general advice applies as for curved slating. Boarding out of cone. BOARDING OUT Many "special" roofs may be boarded out to provide additional protection through a rigid, or fully supported flexible, underlay. Counterbattens should be used on all boarded roofs that are intended to receive slating battens. Where it is not possible to use battens for example, where the radius of a spire is too tight or where the slates used on a turret are too small the roof should be boarded out and the slates fixed directly to the boarding. This is common practice in Scotland where the boarding is called "sarking" and in North America. The usual method of boarding out a curved roof slope is to lay trapezoidal strips of WBP plywood or T & G boarding "diagonally", spiralling around the curve. Sufficient thickness of boarding at least 25 mm should be provided to allow for nailing the slates. In some cases, it may be advantageous to build up plywood boarding in layers of thin strips, with each alternate layer of plywood laid to spiral in the opposing direction. 58

5 SPECIAL ROOFS & VERTICAL SLATING Other Special Details COURSING THROUGH Coursing through refers to the design requirement of maintaining the horizontal slate course lines at intersections between roofs having different pitches.this can be achieved by setting out and securing the slate battens on the shallowest pitched roof to the appropriate batten gauge for the size of slate recommended fo the roof pitch involved. The battens on the steeper pitched roof should be lined up to meet with the battens on the shallower pitched roof. This will tighten the gauge on the steeper pitched roof, making it necessary to cut and re-drill on site the slates used on the steeper pitch. EYEBROW WINDOWS Eyebrow dormer windows are among the most complicated roof features to slate. Accurate setting out and carpentry are critical for success. The pitch of the top of an eyebrow should not be less than the minimum recommended pitch for the chosen slate. The curved edges of an eyebrow window form concave and convex curves, and should be boarded out in solid carcassing and slated in a manner similar to turrets and spires. Every slate around an eyebrow should be soakered. LACED AND SWEPT VALLEYS The construction of a laced or swept valley is complicated and should only be undertaken by an experienced slater. Laced and swept valleys are not recommended where the roof pitch is less than 45. MITRED VALLEYS Introduction Mitred valleys are not recommended where the roof pitch is less than 35 or the valley length is greater than 6 m or is in an exposed location. Underlay and battens A continuous length of high-performance underlay, at least 600 mm wide, should be laid along the length of the valley. The underlay on both roof slopes should be carefully cut to a close mitre at the centre line of the valley, overlapping the continuous length of underlay. The battens on both slopes should be mitre-cut to meet at the centre line of the valley. Slates Slates should be cut to a close-fitting rake at the valley line, ensuring that the tail of no slate is less than 50 mm wide. Each course of slates should be soakered. Width of slate Chevron soaker. Roof pitch Chevron-shaped soakers, which are partially visible, are suitable for use on pitches greater than 35. Width of slate 150mm Roof pitch Length of slate at mitre + 25mm Gauge at valley + lap + 25mm Butterfly soaker. Butterfly-shaped soakers, which are fully concealed from view, are suitable for use only where pitches are greater than

6 Vertical Slating The principles of vertical slating are similar to those of roof slating generally, although substantially fewer slates are required per square metre, as headlaps can be reduced to 50 mm and sidelaps can also be reduced. Installation is made quick and easy by using stainless steel nail hooks, rather than nails, to fix the slates. By using shaped slates or slates having different colours or textures, striking decorative finishes can be created. Each of the following details is briefly discussed in the next few pages: The Bottom Edge The Top Edge Corners The Roof Verge Openings Basket weave 600 x 300 mm Tegral Thrutone slates. UNDERLAY There is no need to use an underlay when slating onto a masonry cavity wall. In timber frame construction, a vapour-permeable membrane is recommended. COUNTERBATTENS When fixing slates to a masonry wall the introduction of vertical counterbattens at 400 or 600 mm centres will reduce the number of masonry fixings required to support battens. In the case of timber frame construction, where a ventilated cavity is recommended, the use of counterbattens will assist in ensuring free movement of air behind the slating. SLATES When slating into the apex of a gable, the use of small, triangular pieces of slate should be avoided. 60

7 SPECIAL ROOFS & VERTICAL SLATING Decorative banding 500 x 250 mm Tegral Thrutone slates. Diamond pattern 400 x 400 mm Tegral Thrutone slates. 61

8 The Bottom Edge The bottom edge should be constructed generally along the lines of a roof eaves. The bottom slates should tilt approximately 8 mm to 15 mm out beyond the plane of the faces of the battens. Bottom edge Tegral Thrutone slates ADVISORY NOTE The exposed underside of the tilting fillet should be painted if it is visible from beneath. 62

9 SPECIAL ROOFS & VERTICAL SLATING The Top Edge The top course should comprise short slates, cut to maintain the margin. An apron flashing piece should be dressed down over the top slates and may be cut to a decorative finish, if desired. Top edge to vertical slating Tegral Thrutone slates 63

10 Corners A series of coursed soakers should be used to weather internal and external corners. One-and-a-half-width slates should be used to start alternate courses. The edge of a slate at a corner should be rake-cut slightly, to form a neat junction. Alternatively, a Tegral cover-trim can be used. Corner details Tegral Thrutone slates COUNTERBATTENS At external corners the use of extra-wide counterbattens is recommended to facilitate secure fixing to the structure. 64

11 SPECIAL ROOFS & VERTICAL SLATING The Roof Verge A raking batten should be fixed to the wall. Slates should be splay-cut and fixed to the raking batten. A series of coursed soakers should be used at the junction between the vertical slating and roof slating. Alternatively, a Tegral trim can be used to weather the junction. Roof verge Tegral Thrutone slates UNDERLAY The roofing underlay should be carried out over the wall and turned down. 65

12 Openings Details should be formed as previously discussed for bottom edges, top edges and corners. 66