Roof materials
Roof underlayment rolls
Whole underlayment rolls from entered roof area, roll coverage and allowance.
How to use this calculator
Prepare measurements and product data
Enter actual slope area and effective roll coverage after required laps. Roll length × width is geometric area and may exceed usable coverage. Establish effective coverage from membrane instructions and the planned laying arrangement; this calculator does not derive it.
Formula and rounding
Planned area = roof area × (1 + allowance / 100). Rolls = round up(planned area / effective roll coverage).
Worked example
Illustrative inputs: 80 m² roof area, 5% allowance and hypothetical effective coverage of 60 m² per roll. Coverage should already account for the selected product's required laps.
| Stage | Calculation |
|---|---|
| Area with allowance | 80 × 1.05 = 84 m² |
| Roll requirement | 84 / 60 = 1.4 → 2 rolls |
| Purchased effective coverage | 2 × 60 = 120 m² |
| Coverage above plan | 120 − 84 = 36 m² |
This model requires 2 rolls. The 36 m² of extra effective coverage does not mean every offcut can form any detail. Check strip lengths and layout before ordering.
Check before buying
- Coverage is effective rather than geometric.
- Required laps are not counted again as general allowance.
- Penetration details, tapes and other accessories are estimated separately.
Limitations
The tool does not plan strips, lap lengths, watertightness or penetration details, or select a membrane for roof pitch and construction. Percentage allowance does not replace instruction requirements; tapes and fixings are excluded.
Questions and answers
Does a 50 × 1.5 m roll always cover 75 m²?
75 m² is geometric area. Effective coverage after laps depends on instructions and laying arrangement. Do not use 75 m² without checking them.
Can surplus reduce the roll count?
Not when purchasing full rolls. One roll in the example covers 60 m² effectively, less than the planned 84 m². Reuse offcuts only after checking dimensions and suitability.
Sources and scope
- NIST — SI Units: Area
Square metres and area-unit notation; not an installation or cutting rule.
- Marley — Roofing Sitework Guide, underlays, pp. 12–17
Examples of underlay laps and laying details that depend on product, pitch and support. These UK system instructions do not supply universal effective roll coverage or establish compliance for a Polish roof.
- HSE — fragile roof surfaces
Avoid unnecessary access to fragile roofs and plan competent access. UK safety guidance is context, not a statement of Polish legal compliance.
The selected membrane instructions establish the laps and laying arrangement needed to determine usable coverage. Our assumed 60 m² per roll is illustrative, not a linked product yield. The 2-roll result assumes this coverage already accounts for laps, which are not added again. Unit and roof-access references do not establish membrane yield.