Dutch Gable or Gambrel Roof and The Scotch Valley The folowing pages are an extract from Carp 12 text TAFE NSW Construction and Transport Division 71
HIP and VALLEY ROOFING GAMBREL / DUTCH GABLE The effect of the gambrel ends is produced at the span ends by combining gables with a hipped roof. The gables are formed by extending the ridge board past the hip and ridge junction and then filling-in the extension like a gable. The small gables, or gablets, produced may be clad with a range of materials, such as flat fibre-cement sheet, timber weatherboards, Colorbond metal, Weathertex boards, Hardi-plank boards, etc. Isometric View Generally, a timber barge board is used to finish the junction between the roof tiles and the sheeting on the gable, with a lead flashing sealing the joint between the gable cladding and the tiled roof surface. Plan Fig. 117 Typical gambrel shape Where the verges are not pointed, a Colorbond barge and barge soaker/ gutter system may be used. Fig. 118 Colorbond barge and barge soaker/gutter 72 TAFE NSW Construction and Transport Division
Preparing The Gambrel End The lead flashing, usually 230 mm wide min., is laid across the width of the gambrel. It is turned up behind the cladding, minimum of 50 mm, and beaten over the profile of the roof tiles to form a waterproof seal. The lead flashing should be a minimum of 1.8 mm thick, also referred to as 20 kg lead, and run the full width of the gambrel plus a minimum of 300 mm either side of the hip centre. There must be an allowance for expansion of the lead due to heat absorption, therefore the maximum length of any one piece should be 2.0m lapped a minimum of 150 mm. 100 Fixing through sheet Walling plate 20kg Lead flashing Jack rafter Dutch-hip and rafters Fig. 119 Positioning the lead flashing Position of roof battens and tiles The lead should be coated with paint, plastic, rubber or bitumen to avoid galvanic action between dissimilar metals occurring. The bottom of the lead flashing should be set a minimum of 90 mm above the jack rafters to allow for tile battens and a course of roof tiles to fit under with ease. Note: Roofing lead exposed to temperatures of 40 C, or above, for prolonged periods will cause the lead to grow up to 2 mm in any 2.0 m length. This inevitably ends up causing stress splits in the width of the lead and leakage will occur. F.C. strip to Barge Lead flashing extended Sarki Hip tiles Fig. 120 Alternative gambrel end preparation and finish TAFE NSW Construction and Transport Division 73
HIP and VALLEY ROOFING Constructing The Gambrel STEP 1 Extend the ridge to the desired length in increments of rafter spacings. The most common method is to allow for a longer ridge during original construction, however the ridge may be extended later by strapping or fish-plating extensions to the end of the normal length hip roof ridge. Set out rafter positions on the plates Hip, line of intersection on plan Run of gambrel rafters Three additional pairs of common rafters required on this case. Fig. 121 Extending the ridge on centre line by rafter spacings. Galv. steel strap or nail plates Ridge extended as required Centering rafter Centering rafter Hip Creeper Extended hip Long hip Jack or crown-end rafter Long hip Jack or crown-end rafter fitted under ridge extension PLAN VIEW Fig. 122 Alternative method of extending the ridge 74 TAFE NSW Construction and Transport Division
STEP 2 Temporarily prop the end of the ridge to prevent sagging. Plumb up from the string line position, mark the end of the ridge and cut to length. Fit pairs of common rafters at the marked spacings to frame up the end. Brace Plumb line String line Prop Adjust ridge for height marked on prop. Centre up and plumb to line. Check for level and then secure brace. Fig. 123 Prop end of ridge STEP 3 Determine the position of the jack rafters and height of pitching plate by plumbing up the outside faces of the rafter plate positions onto the last pair of common rafters (Gambrel end rafters). This also determines the cutting length of the jack rafters. Gambrel rafter may be held against common rafter to obtain this point Locating position for pitching plate Fig. 124 Determine position and length of jack rafters. TAFE NSW Construction and Transport Division 75
HIP and VALLEY ROOFING STEP 4 Lay the pitching plate on the wall plate and transfer the rafter set outs onto it. Fit the inside edge of the pitching plate against the Gambrel rafters, measured the length of the plumb cut common rafter down. Align the outside of the first jack rafter with the set out mark on the gambrel rafters and bolt the pitching plate into place. Bolt to rafters Position & level the pitching plate Transfer rafter set-out from end-wall plate onto the pitching plate Place at bottom of plumb line Fig. 125 Set out pitching plate and fix into place STEP 5 Notch ends of jack rafters and fix into place using a string line to maintain a constant straight line. String line Pitching plate Rafters notched over pitching plate level with string line Gambrel rafters NOTE: In the case of tiled roofs, rafters must work full tile courses. Creeper Short hip Fig. 126 Fix jack rafters into place 76 TAFE NSW Construction and Transport Division
Note: Prepare ends of jack rafters, as shown below. Set out, cut and fit the short hips into place, as shown below. String line Gambrel rafter Pitching plate String line Plumb line Pitching plate Fig. 127 Notching jack rafters Fig. 128 Fitting string line Half mitre measured & marked Short hip measured from this point Fixing point marked on common rafter Short hip Fig. 129 Determine hip measuring point Fig. 130 Assemble hips STEP 6 Fit gambrel end trimming studs and ceiling trimmers. Fit collar ties as required. Fit gable trimming studs and fix lining. Fit barge boards and flashing. NOTE: Hanging and strutting beams have been omitted for clarity. Fig. 131 Complete the framing to the gambrel end TAFE NSW Construction and Transport Division 77
HIP and VALLEY ROOFING Alternative Gambrel Construction Method Fig. 132 Alternative method 78 TAFE NSW Construction and Transport Division
Methods Used for Small Spans Fig. 133 Use of ridge extension Fig. 134 Framing onto hip for small gambrel end TAFE NSW Construction and Transport Division 79
HIP and VALLEY ROOFING Determining the Length of the Ridge The ridge may be extended by any length, but for convenience of construction it is usually extended by rafter spacings. Once the length has been determined the ridge is set out as for gable and hip roofs, i.e. laying the ridge along the top wall plate and transferring the rafter set out positions directly onto it. STEP 1 Determine the length of the ridge between apex points: Length of roof - Span of roof 7.200-4.500 2.700 m STEP 2 Determine the length of the ridge extension at each end, based on rafter spacings: (3 Rafter spacings) x 2 (3 x 0.450) x 2 1.350 x 2 2.700 m STEP 3 Add the lengths together to find the total length: 2.700 + 2.700 5.400 m Fig. 135 Typical Gambrel roof plan STEP 4 As the length at this point is based on centre line length, half a rafter thickness must be added to each end to obtain the cutting length of the ridge. For this example the rafters are 35 mm thick: 5.400 + 35 5.435 m Order: 1/ 5.7 80 TAFE NSW Construction and Transport Division
SCOTCH VALLEY CONSTRUCTION This method is used as an alternative to conventional construction to form a valley. The valley board is replaced with a Lay board, which is laid on top of the rafters to form a fixing platform for the feet of the minor span valley creepers. The scotch valley is mainly used where an addition is added to an existing building. It provides a quick and very efficient method of construction, requiring little or no disturbance to the existing roof. The existing rafters remain in place to support the lay board, which in turn supports the feet of the valley creepers. This system may also be adapted for use where dormer windows are built into existing roofs. Fig. 140 Application of the scotch valley Fig. 141 Scotch valley construction TAFE NSW Construction and Transport Division 81
HIP and VALLEY ROOFING Method of Construction The following method is applied to roofs having the same pitch. STEP 1 Locate the height of the minor span ridge on the common rafters of the existing main roof. Fig. 142 Obtaining ridge height STEP 2 Position the ridge and rafters to form the minor roof. Fig. 143 Framing up the minor span 82 TAFE NSW Construction and Transport Division
STEP 3 Measure the length of the lay board, then cut and fix into position. Fig. 144 Measuring the lay board STEP 4 Measure the length of the longest creeper. Use plumb bevel common rafter for the top of the creeper and level bevel common rafter and edge bevel creeper for the compound foot cut. The remainder of the creepers can be set out using the steel square or by calculating a deduction from long point to long point. Fig. 145 Cutting and fixing the valley creepers TAFE NSW Construction and Transport Division 83
HIP and VALLEY ROOFING GLOSSARY OF TERMS Adjacent - Apex - Bisect - Circa - Cluster - Complimentary - Constant - Corbelled - Dihedral - F.C. - Hypotenuse - Inclined - In-situ - In-to-over - Lined on-the-rake - Parapet - Patent - Means it is placed next to or found beside something. This is the very top or point of something, like the apex of a roof, meaning where members come together at a common top position. This means to cut, separate or divide something exactly in half, such as when a 90 angle is bisected it becomes two 45 angles. This means around, approximately, round about, etc. Usually refers to dates when estimating the age of a building or structure such as circa 1854, also written as (c 1854). This is a term, which refers to a group or gathering of a number of members in a frame, such as a roof cluster, which consists of the end of the ridge, two centring rafters, a crown end rafter and two hip rafters. In this case it means any two angles, which make up a right angle. In this case it means a number, quantity or amount, which is used as the basis for several calculations. For example the length of 1.414m is a constant, which may be used to calculate the 45 hypotenuse length of a right-angled triangle, once the length of one side is known. Refers to stepped out brickwork used to support other members, such as corbelled eaves. This is the angle formed between any two surfaces where they meet along a common length, such as a ridge in a roof. A dihedral angle is formed between the underside of the two roof surfaces or where a roof surface meets a parapet wall, etc. This is an abbreviation for fibre cement, similar to the product Hardiflex. This is the angled side of a right-angled triangle. Means to be at an angle to something, such as an inclined strut or brace. This is an abbreviated version meaning in situation or position, such as pouring concrete in-situ, meaning to pour in place into the formwork. This is a method used for marking the spacings of members. It literally means marking from the inside face or edge of one member to the outside face or edge of the next member. It is equal to working centre-to-centre but is more practical for fixing purposes as it allows one edge to be lined up with the mark, so it is easily seen, ready for fixing. This is a term used to describe the ceiling lining of a pitched roof, which is fixed to the underside of the rafters. There is no access to the roof structure as there is no roof space formed. This is a vertical wall or gable, which extends past the line of the roof to enclose the roof from view. The parapet is usually constructed of brickwork or timber framing and clad with sheet material. This is a term used to describe a product which has had its design registered with the Patents office. It is the original idea of a person or persons, which cannot be copied without consent. 84 TAFE NSW Construction and Transport Division
Primed - Scribe - Slat - Soffit- This is a protective white or pink paint coating applied to timber before it is fixed into place. It seals the timber and provides a surface ready to take undercoat paint prior to the finishing coats. It helps to prevent timber decay from occurring. This is where the shape of one piece is to be fitted to the surface shape of another. It may also refer to anything placed in-situ and marked to suit the final resting position of that member, such as scribing a hip in position. Refers to a thin narrow piece of timber or several thin arrow pieces put together to make something else, such as slats of timber used to make a sheet of lattice. This is the horizontal under face of a structure or lining. The eaves soffit is the underside face of the eaves sheeting. Trapezoid - This is an irregular quadrilateral with only two parallel sides. The shape may be found on the side roof surface of a hip roof made up of the fascia, ridge and two hips TAFE NSW Construction and Transport Division 85