Through-fastened (also called exposed-fastener or pierce-fixed) metal roofing is fixed by screws or nails that pass through the sheet into the supporting purlins, battens or deck. Corrugated sheet and trapezoidal (ribbed) profiles are the most common forms. It is the dominant roof type for houses in Australia and New Zealand and is widely used on agricultural, industrial and residential buildings around the world.
Fasteners and washers
Each fastener needs a sealing washer to keep water out of the hole. The New Zealand metal roofing code of practice recommends washers of EPDM rubber and says fasteners should be tightened only enough to form a weatherproof seal, since overtightening dents the sheet and lets water pond around the seal. Metal swarf (drilling filings) should be cleared from the roof surface and from under washers, because it rusts and stains the coating.
Manufacturers specify whether screws go through the raised crests (ribs) or the flat pans of a given profile, and set screw spacing from wind loads, with closer spacing at edges and corners.
Side laps
Sheets overlap along their sides by a set amount, often one or more ribs or corrugations; corrugated sheet commonly laps by one and a half corrugations. Laps are normally laid so that the exposed edge faces away from the prevailing wind.
End laps
The New Zealand code recommends avoiding end laps where possible. Where they are needed, it gives a minimum lap of 150 mm on pitches of 10 degrees or more and recommends 200 mm on lower pitches, sealed at both ends of the lap, with the upper seal being critical.
Thermal movement
Long pierce-fixed sheets expand and contract and can elongate fastener holes or shear screws. The New Zealand code requires expansion provisions for run lengths over 20 m when fixing into timber or hot-rolled steel, using measures such as oversized holes or a stepped joint in the roof.
Installation outline
- Check purlin or batten spacing and alignment.
- Start laying from the end of the roof opposite the prevailing wind, square to the eave.
- Fix each sheet with screws in the specified positions, lapping and stitching side laps as required.
- Turn up the sheet pans at the ridge (stop ends) and turn down at the gutter (drip edge) where specified.
- Install ridge, barge and wall flashings and clear all swarf.
Sources
- NZ Metal Roof and Wall Cladding Code of Practice: Fastening RoofingNZ Metal Roofing Manufacturers Inc. · metalroofing.org.nz
- NZ Metal Roof and Wall Cladding Code of Practice: End LapsNZ Metal Roofing Manufacturers Inc. · metalroofing.org.nz
- NZ Metal Roof and Wall Cladding Code of Practice: Thermal Expansion and ContractionNZ Metal Roofing Manufacturers Inc. · metalroofing.org.nz
- Corrugated galvanised ironWikipedia · en.wikipedia.org
- Metal roofWikipedia · en.wikipedia.org