Guides
Roof measurement and coverage
Measure roof areas and use product coverage without turning a quantity estimate into a design.
Establish whether roof dimensions describe a horizontal projection or the actual slope. Purchasing uses covered area and effective product coverage. Obtaining measurements should not involve improvised roof access.
Key points
- Horizontal projection and sloping area differ.
- Effective coverage reflects the product's laying arrangement.
- Openings, overlaps and accessories need separate scope.
In this guide
Prepare your inputs
- Verified drawings or competent measurements with date and units.
- Each slope's dimensions, pitch and projection/slope notation.
- Covering instructions, pack data, overlaps and accessories.
1. Plan the measurement source
Use verified documentation or measurement with appropriately planned access. A ground view alone establishes neither dimensions nor roof condition. Check that drawings match the current building and include overhangs.
Do not assume rooflights, sheets or old coverings are safe to stand on. HSE guidance on fragile roofs emphasizes planning and avoiding unnecessary access. It supplies safety context rather than a statement of Polish legal compliance; a competent contractor should establish access arrangements.
Source: [1] HSE — fragile roof surfaces
2. Separate projection from slope area
For a planar slope with constant angle α, sloping area equals horizontal projected area divided by cos α. Use degrees when that is how the drawing records pitch. Do not apply this factor again to a length already measured along the slope.
Divide the roof into separate slopes. A rectangle uses slope length × width. Triangles and other shapes need their own geometry rather than a bounding rectangle. Record chimney and opening deductions and add flashings separately.
Slope area = projected area / cos α
Source: [2] NIST — SI Units: Area
3. Convert area to covering
Use effective coverage for the exact pack or tile consumption appropriate to gauge and layout. Overall element width may differ from cover width. Instructions establish overlaps rather than gross sheet dimensions alone.
Cutting allowance is separate from overlaps already included in effective coverage. Ridge, eaves, valleys, hips, ventilation and flashings are not automatically part of an area result. Record them separately in pieces or metres.
Source: [3] Marley — Roofing Sitework Guide, batten gauge, pp. 60–61
40 m² projection at 30°
One planar slope: 40 m² projection, 30° pitch, illustrative 5% allowance and hypothetical 3 m² effective pack coverage.
- 1Horizontal projection area
- 40 m²
- 2Slope area at 30°
- 40 / cos 30° ≈ 46.188 m²
| Stage | Result |
|---|---|
| Slope area: 40 / cos 30° | ≈ 46.188 m² |
| Allowance: × 1.05 | ≈ 48.497 m² |
| Packs: ceil(48.497 / 3) | 17 |
| Purchased coverage | 51 m² |
| Surplus over plan | ≈ 2.503 m² |
The model requires 17 packs. Using only the 40 m² projection would understate slope area.
This does not specify access, product selection, permitted pitch or fixing.
Common mistakes
| Mistake | How to avoid it |
|---|---|
| Applying pitch again to slope length. | Mark each dimension as projection or slope. |
| Treating sheet width as cover width. | Use effective coverage from instructions. |
| Adding overlaps twice. | Check what coverage already includes. |
Check before using the result
- Measurements have a verified, safely obtained source.
- Each slope uses the right geometry.
- Coverage, allowance and accessories are separate.
Method limits
- Quantity tools do not assess structure, loading or fixings.
- The model does not optimize cutting or produce a complete roof specification.
Questions and answers
Is a satellite image sufficient?
It can help a sketch but does not establish slope length, pitch, overhangs or details. Verify inputs before ordering.
Should chimneys be deducted?
Record actual areas and check cutting and flashings. Deducted area does not proportionally reduce every material.
Sources and verification scope
- 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.
Access checked: - NIST — SI Units: Area
Square metres and area-unit notation; not an installation or cutting rule.
Access checked: - 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.
Access checked: