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Onduline Roofing Calculator & Cut Planner

Calculate corrugated bitumen sheets with precise overlap adjustments based on roof pitch. Includes nail counts and waste minimization logic.

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About

Bitumen corrugated roofing requires precise calculation because the effective coverage area changes drastically with the roof pitch. Unlike flat sheet materials, corrugated sheets like Onduline rely on overlapping waves (corrugations) to create a watertight seal. If the pitch is shallow, water moves slower, requiring wider overlaps to prevent capillary action from drawing moisture underneath.

This tool adjusts the net usable width and length of standard 2000mm × 950mm sheets based on your specific pitch. It accounts for the manufacturer's strict fixing specifications—typically 20 nails per sheet—and helps estimate the specific cutting requirements to align corrugations correctly. Failure to calculate the side lap (1 vs 2 corrugations) correctly is a leading cause of leaks in low-pitch DIY roofing projects.

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Formulas

The calculation defines the net coverage per sheet based on the pitch constraints. The total sheet count N is derived from the roof area A and the net sheet area anet.

anet = (Wsheet − overlapside) × (Lsheet − overlapend)

Where Wsheet is 950mm and Lsheet is 2000mm. The number of fixings (nails) F follows the standard pattern:

F ≈ N × 20

Reference Data

Pitch (Degrees)End Lap (mm)Side Lap (Corrugations)Net Cover Width (mm)Net Cover Length (mm)
5−10°30027601700
10−15°20018551800
≥ 15°17018551830
Vertical Siding10018551900

Frequently Asked Questions

At lower pitches (below 15 degrees), gravity clears water more slowly. To prevent water from backing up through the overlaps (capillary action), manufacturers require a larger end lap (up to 300mm) and a double corrugation side lap. This reduces the usable surface area of every single sheet, increasing the total number required.
No. The 20-nail standard is designed to resist wind uplift. Bitumen sheets are lightweight; without the correct fixing density at the eaves, overlaps, and intermediate purlins, the roof may lift or rattle during storms. Omitting nails also voids most manufacturer warranties.
Bitumen melts when heated by friction. Use a coarse-toothed hand saw or a circular saw with a fitted coarse blade. Lubricating the blade with oil or water can prevent the bitumen from binding to the metal. Always cut on the upward slope of the corrugation to avoid crushing the profile.