Tips on replacing a roof
TIPS
- Do not drill and fix a bitumenised corrugated sheet in the valley of the sheet’s corrugated design – It will invariably leak when it rains. Nail through the top (crown) of each convex curved corrugated ripple using a wooden support beneath the crown when you commence nailing.
- Do not cut corrugated sheets in the extreme cold – they will split or shatter.
- If you are erecting a roof cover on a shallow garage roof pitch, be sure it is strong enough to be able to withstand a heavy snowfall in winter. Place and space purlins at 450mm centres.
- Fix all roof sheets loosely at first and, when happy with their positioning, overlaps and verge overhangs, tighten the fixings but don’t over-tighten them. Corrugated, bitumenised roof sheets do not need holes drilling at first. The nails are driven straight through each bitumenised sheet’s corrugated crown and each hole is sealed by the nail’s PVC head cover.
- Roof pitches 10 to 15 Degrees
- All roof support purlins must be positioned at 450 mm centres (or closer) for added strength.
- Side overlaps of roofing sheets must be 1 full corrugation. Use two corrugations on shallow roof pitches of less than 10 degrees.
- End overlaps of two roof sheets must be at least 200mm. Use 260mm overlaps on shallow roof pitches of less than 10 degrees.
- Overlap roof ridge pieces by a minimum of 110mm as you nail each piece to the roof apex purlins and corrugated sheeting already in position beneath.
- When draining corrugated roof sheets into a rainwater gutter facility, allow at least a 65mm roofing overhang. Do not extend beyond 70mm as strong winds could easily catch under the roof and cause untold damage very quickly.
In buildings where there is the possibility of high humidity (eg. with animals inside plus a concrete floor base), I strongly recommend you put a vapour barrier of bitumen felt beneath the purlins during construction. Make sure it is taut by stapling it tight. Overlaps must be a minimum of 120mm.
Do not put in bitumen felt where livestock can reach and chew it. Alternatively, cut and fix exterior ply beneath the purlins to prevent any condensation penetration.
Cut and place a barge board verge beneath the corrugated dallas roofing sheets at each side of the roof where it has a deliberate corrugated overhang. Start the sheet fixing by nailing through the corrugated crowns into the purlins at the edge of the roof and then fold down the corrugation edge onto the barge board along the roof overhang and nail down into position – it will make a perfect finish.
Nail down every crown at the bottom edge of each corrugated sheet leaving a maximum overhang of 70mm for the rainwater gutter.
At each other purlin you only need nail through every other crown point, except also along the roof apex purlin, where you must nail down through each crown point. Be sure to use a taut string nailing guide along every purlin, especially along the roof apex.
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