A good sheet on a badly fitted roof will still leak, rattle or rust early. Most of these failures come down to a handful of preventable mistakes. If you're supervising a build, or hiring a fundi, these are the five worth checking before the first sheet goes up.
1. Purlins spaced too wide for the gauge
Sheets sag and dent between supports that are too far apart, and thin sheet is affected first. The wider the spacing, the lower the gauge (thicker sheet) you need. Decide the purlin spacing and the gauge together, not one after the other. Our roofing gauge guide explains how they relate.
2. Too little overlap, or overlap facing the wrong way
Water finds its way through short overlaps, especially in driving rain. As a rule of thumb, allow around 150 mm of end lap, more on a low pitch, and lay the side laps away from the prevailing wind so gusts don't push rain under the edge. Follow the fixing guidance for your particular profile.
3. Wrong fasteners, or fixing them wrong
Use fasteners with sealing washers made for roofing, and use enough of them. Tighten until the washer seats and stop: over-tightening crushes the washer and dimples the sheet, and it leaks. Under-tightening lets the sheet rattle and lift in the wind. Drive fasteners straight, and keep them in the correct part of the profile for the sheet you're using.
4. Cutting badly and leaving the mess behind
An abrasive angle grinder throws hot sparks that burn the coating around the cut, and the burnt edge rusts first. Use tin snips, a nibbler or a shear where you can. Whatever you use, sweep off the metal filings, because loose steel particles left on the roof rust and stain the sheet around them. If you order custom lengths from us, most of the cutting has already been done.
5. Skipping the details at the edges
Most leaks start at junctions, not in the middle of a sheet. Ridge caps, flashings against walls, valley gutters and eaves need proper detailing and sealing, and they should be planned before the roof goes on. For roofs over habitable rooms, ask about an underlay beneath the sheets to control condensation and reduce noise.
Ask before you start, not after
Fixing patterns, overlap allowances, gutter sizing and ridge selection are all things our team can advise on for your roof. We don't fit roofs directly, but we can recommend vetted contractors and fundis in your area if you want one.