Question 1

How does effective width differ from full width?

Accepted Answer

The full width of corrugated sheet is 1200 mm, but the effective width is 1100 mm (100 mm is lost to the side overlap). Always order by the effective width, or you will run short.

Question 2

What overlap should I use along the slope?

Accepted Answer

At least 150 mm when the pitch is above 15°. At 12–15° use 200 mm. Below 12° you need extra waterproofing. The overlap runs bottom-to-top, following the direction of water runoff.

Question 3

Is it worth ordering non-standard sheet lengths?

Accepted Answer

Yes — it saves material and fitting time. Sheets are cut to the exact slope length at the factory. The standard 2 m length often leaves offcuts, so ordering by the eaves-to-ridge length saves 10–20%.

Question 4

What is a valley and how does it affect quantity?

Accepted Answer

A valley is the internal angle where two slopes meet. Along it you get large triangular offcuts, with losses of up to 15–25% on the adjoining areas. Add extra material for complex roofs.

Question 5

How do I size the ridge and eaves flashings?

Accepted Answer

Ridge = house length + 2 × overlap (usually 10 cm per side). Eaves = total length of the overhangs. Gables = perimeter of the gable slopes. All trim metal is calculated separately from the sheets.

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