A roofing sheet is wider than the area it actually covers, because each sheet overlaps the next. A box profile sheet covers about 0.80 m of roof even though it measures more across. Ordering by full sheet width leaves you short, which is why the calculator uses effective cover.

The higher the gauge number, the thinner the steel. Gauge 30 is lighter and cheaper and is fine for stores and sheds. For a house, a long span between purlins, or a roof people will walk on during maintenance, gauge 28 is the sturdier choice.

Around 5 degrees is the practical minimum for box profile and polycarbonate sheets. Below that, rainwater sits at the laps and works its way through. Stone-coated tiles need a noticeably steeper pitch — check the manufacturer’s figure for the tile you choose.

Where one sheet meets the next down the slope, an end lap of about 150 mm is standard, increased on shallow pitches. On a short roof, a single sheet cut to the full rafter length avoids an end lap altogether — the most common place for a leak.