Roof materials
Roof covering packages
Whole roof-covering packages from entered measured area, coverage and allowance.
How to use this calculator
Prepare measurements and product data
Enter actual sloping roof area in m² rather than its horizontal projection. For a rectangle, multiply width by measured slope length. Copy effective coverage per package from the selected product documentation; gross sheet dimensions may omit laps. Use verified measurements or drawings without improvised roof access.
Formula and rounding
Planned area = roof area × (1 + allowance / 100). Packages = round up(planned area / package coverage).
Worked example
Illustrative inputs: 80 m² roof area, an assumed 5% allowance and hypothetical package coverage of 10 m². 5% is an example assumption, not a universal recommendation.
| Stage | Calculation |
|---|---|
| Area with allowance | 80 × 1.05 = 84 m² |
| Unrounded package requirement | 84 / 10 = 8.4 |
| Full-package purchase | 9 × 10 = 90 m² |
| Coverage above plan | 90 − 84 = 6 m² |
The model requires 9 packages. The 6 m² difference is measured against the plan including allowance; do not add it again as another allowance.
Check before buying
- Area includes the relevant slopes and overhangs.
- Package coverage belongs to the exact product and laying arrangement.
- Laps included in effective coverage have not been counted again as waste.
Limitations
The model divides total area by package coverage. It does not optimize sheet layout or cuts, or include flashings, fixings, ridges or valleys. It does not assess structure or permitted roof pitch.
Questions and answers
Can I use horizontal projected area?
Convert it to actual slope area first. For a single flat slope at constant angle α, area equals projected area / cos α. Do not apply this adjustment again to dimensions already measured along the slope.
What if the manufacturer only lists sheet dimensions?
Establish effective coverage and laying arrangement from that product's instructions. This calculator does not infer laps or turn gross dimensions into permitted coverage.
Sources and scope
- NIST — SI Units: Area
Square metres and area-unit notation; not an installation or cutting rule.
- Marley — Roofing Sitework Guide, batten gauge, pp. 60–61
Product-specific setting out between eaves/ridge battens and separate fixed points. The simple quantity calculator is not that setting-out method.
- 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 example assumes hypothetical coverage of 10 m² per package; this is not a specification of a linked product. Documentation helps distinguish slope area from effective coverage. Dividing planned area by coverage and rounding to full packages is our own quantity model. Obtain actual coverage from the selected roofing instructions.