16 Steps to Self-build: Constructing the Roof
STEP 5: CONSTRUCTING THE ROOF
This month David Snell explains the different methods for building the roof structure of your new home
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Every month, self-build expert David Snell will break down the stages from our project planner in more detail, giving you a comprehensive timeline for your self-build. This month, work starts on the roof
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A cut and pitch roof can provide clear space within the roof and it is much easier to maintain this space through ‘L’ shaped sections and projections. A cut and pitch roof will, almost certainly, require additional support from the superstructure and the provision of purlins, supporting beams built into the superstructure walling.
Installing Prefabricated Trussed Roofs
If you choose a trussed roof for your selfbuild, the first step will be to sort the roof materials into their various sizes in accordance with the supplied plans of the supplier or manufacturer. The golden rule is to never cut a truss unless under specific instruction and with approval of the manufacturer. Once the trusses are organised, mark the wallplate with the correct spacings, at which point the carpenters can stack all the trusses at one end of the wallplate. If the materials are particularly heavy – likely if attic trusses are being used – you’ll need to use a crane to lift them up (and ensure that you have access for the plant).
of 50mm Starting materials, from at the the the opposite gable end truss wall end and can to secured your be raised, stack to the wallplate using proprietary truss clips. Each truss is then raised in succession, fixed to the wallplate and then secured, upright and vertical, to the already standing trusses by a rough batten that runs approximately 45° up or down each plane of the roof. If any large tanks are to be installed in the roof, they need to be positioned at this stage, otherwise, when all of the trusses are fixed, there may not be enough room between the timbers to get them in. Double or treble trusses may be needed either side of any proposed openings in the roof — for dormers, rooflights or chimneys for example. Speaking of chimneys, rafters and roof timbers need to be kept clear of any chimney blockwork by 50mm. This may require trimmers (timbers) if the chimney width is greater than the span between the timbers. Any infilling and trimmers that are fixed between the double or treble trusses need to be cut and fixed in accordance with the roof manufacturer’s instructions. If your design calls for a run of opposing trusses (for L-shaped designs, for example), you should fix the necessary girder trusses at this point and fix with truss hangers. Any loose infill or reducing trusses that will fill in the gap between the two meeting roof panes are also constructed or fixed here. You should look to construct any necessary dormers at this point. These can be supplied prefabricated, but it’s more common to construct these out of sawn timber. Rooflights should also be fixed at this stage. All binders can be fixed within the web of the trusses from node point to node point, at ceiling level and at ridge level. This is followed by fixing the diagonal bracings. Metal bracings, built into or screw fixed to the walls, should be used at ceiling tie level and up the run of the rafters at 2m intervals. These must cross and be fixed to at least three rafters/ceiling ties, with noggins beneath. If sarking boarding is required then this should be fixed to the slope of the roof planes at this point. The fascias and soffits are then fixed together, along with any necessary ventilation grilles, before the gable ladders and bargeboards are affixed. At this point, most warranty companies and some building inspectors will want to inspect the work.
Constructing a Cut and Pitch Roof
Should you opt for a cut and pitch roof, you’ll start with a delivery of lumber, which will need to be sorted on site by size and intended use. Timber purlins and supporting steels should have been built in as the external walling was raised. Any steel purlins used will need a wallplate bolted to their top surface to carry and affix the rafters. Unlike when constructing the roof with trusses, where the main seating to wallplate is via the ceiling ties, cut rafters are ‘ birdmouthed’ over the plate (a cut in the rafter that resembles a bird’s mouth). This is partly because the cut rafters have a thicker profile than trusses, but it also provides a fixing. The end rafters should be fixed to the plate at each end of the run of the roof, or at a convenient distance, with the ridge board supported by and spanning between
them. the posing ping wallplate the The each ridge infill other to board. rafters the at ridge the can top, board, then effectively directly be set from trap- opcut There and pitch are common roof and points truss roof between construc- the tion: keep the rafters and roof timbers clear of any chimney blockwork by 50mm, which may require trimmers, while any tanks or other equipment should be up there before the roofwork is finished. The ceiling ties are then cut and fixed from plate to plate and bolted to the sides of the rafters at each end, with collar ties to be cut and fixed to rafters. Metal bracings, built into or screw fixed to the walls, should be employed at ceiling tie level and up the run of the rafters at 2m intervals. These must cross and be fixed to at least three rafters, with noggins beneath. Eaves can be created with queen posts, which also provide additional support. If constructing a hipped roof, you will need to construct the hips with a hip board or blade from wallplate to ridge. Infill with hip or Jack rafters cut in increasing sizes to span from the wallplate to the hip blade. Where necessary, employ a dragon tie, which spans across the corner of the hip. Construct any valleys with a valley blade from ridge to wallplate, and infill with valley rafters cut in reducing sizes to span from valley blade to ridge board. Then follow the same steps as with a prefabricated roof at this point: construct any dormers and fix any rooflights; construct lay boards and fix down any valleys; add sarking boarding (if required); fix together the fascias and soffits with any necessary ventilation grilles or strips; and fix any gable ladders or bargeboards. Your warranty company, and some building inspectors, will want to visit to check the work done now.
Other Roof-level Tasks
While the roof is being constructed there are other jobs that can be done. Your plumber will be on site to set the vent pipes from the drainage points up and through the roof, with fixing skirts dressed down once the roof is covered. Any flue pipes can also be raised through the roof, with any necessary flashings or collars left to dress down after the roof covering is laid. Any flashings (including for rooflights), stepped flashings or soakers are also prepared. If the house will feature mechanical ventilation, the insulated ducting should be installed. If not already done, all midspan strutting to floor joists or ceiling ties should be completed and all timbers blocked out to the end walls to maintain rigidity. Tank stands should be constructed by the carpenter, while any boarding that will go in the loft should be fed into the roof void — if it’s not moisture resistant, it should be protected until the roof covering is complete.