Reviewed, sourced guidance
Roofing calculator method and assumptions
Direct answer
Footprint mode calculates plan length × plan width ÷ cos(uniform pitch) for a rectangular roof. Plane mode calculates each eaves length × horizontal run ÷ cos(pitch), then sums net areas. Only entered rates and allowances affect material quantities.
The versioned calculator uses either uniform-pitch footprint geometry or separately measured planes, plus four source-linked Marley tile configurations and user-entered rates for other materials.
Key takeaways
- Model version is 2026-09-25-v9.
- Footprint mode assumes a rectangular roof with one uniform pitch.
- No product preset or waste percentage is selected by default.
- The calculator runs in the browser and sends no form data.
Source review cadence
Every source cited on this site carries a dated check and a recheck trigger, so stale evidence is replaced rather than quietly relied on.
| Source | Last checked | Recheck | Triggers an early review |
|---|---|---|---|
| Cushty, Roof Area Calculator UK (observed 2026-09-25) | 2026-09-25 | every 180 days | Calculator feature or public guidance changes; A competitor comparison on this site changes |
| Cushty, Roof Tile Calculator UK (observed 2026-09-25) | 2026-09-25 | every 180 days | Calculator feature or public guidance changes; A competitor comparison on this site changes |
| Russell Roof Tiles, Tile Estimator (observed 2026-09-25) | 2026-09-25 | every 180 days | Calculator feature or public guidance changes; A competitor comparison on this site changes |
| BMI Redland, QuantMaster roof quantity estimating (observed 2026-09-25) | 2026-09-25 | every 90 days | Manufacturer, safety or technical guidance changes; A claim tied to this source changes |
| BMI Redland, FixMaster pitched roof fixing specification tool (observed 2026-09-25) | 2026-09-25 | every 90 days | Manufacturer, safety or technical guidance changes; A claim tied to this source changes |
| Marley, technical services for roof specification (observed 2026-09-25) | 2026-09-25 | every 90 days | Manufacturer, safety or technical guidance changes; A claim tied to this source changes |
| Marley, Duo Modern product data (observed 2026-09-25) | 2026-09-25 | every 90 days | Manufacturer, safety or technical guidance changes; A claim tied to this source changes |
| Marley, Universal HipFast system and box contents (observed 2026-09-25) | 2026-09-25 | every 90 days | Manufacturer, safety or technical guidance changes; A claim tied to this source changes |
| Marley, Universal RidgeFast system and box contents (observed 2026-09-25) | 2026-09-25 | every 90 days | Manufacturer, safety or technical guidance changes; A claim tied to this source changes |
| Marley, How to calculate roof pitch (observed 2026-09-25) | 2026-09-25 | every 90 days | Manufacturer, safety or technical guidance changes; A claim tied to this source changes |
| Health and Safety Executive, Roof work (page updated 2026-06-05; observed 2026-09-25) | 2026-09-25 | every 90 days | Manufacturer, safety or technical guidance changes; A claim tied to this source changes |
| DuPont Tyvek, Technical Manual - Roofs UK and Ireland (published 2025-04-11; observed 2026-09-25) | 2026-09-25 | every 90 days | Manufacturer, safety or technical guidance changes; A claim tied to this source changes |
| Which?, How much do roof repairs cost? (observed 2026-09-25) | 2026-09-25 | every 45 days | Survey date, cost range or inclusions change; Roof-cost claims on this site change |
Area model
Footprint mode multiplies the two outside-edge plan dimensions, then divides by cosine of one uniform pitch.
Plane mode divides each horizontal run by cosine of its pitch, multiplies by eaves length, then sums the areas.
- Use one footprint or one to four roof planes, not both methods together.
- Pitch input is from 0 to 89 degrees.
- Each measured length must be greater than zero and no more than 1,000 metres.
Sources for this section: Cushty, Roof Area Calculator UK (observed 2026-09-25)
Quantity and rounding model
Tile quantity is rounded up from area multiplied by tiles per square metre. An entered tile allowance is applied to that integer count and rounded up again.
Batten length uses the preset rate or an entered custom rate; membrane rolls use only the entered usable coverage. Pack count rounds the order quantity up to whole packs.
The optional Marley Universal HipFast and RidgeFast counts separately divide entered hip or ridge length by each published 6 m roll per box and round up; these are roll-length minima, not complete accessory schedules.
- The tile allowance is optional and defaults to none.
- Membrane coverage must represent usable area after overlaps.
- Net batten length excludes detailing and extra cuts.
Sources for this section: Marley, Duo Modern product data (observed 2026-09-25)Cushty, Roof Tile Calculator UK (observed 2026-09-25)Marley, Universal HipFast system and box contents (observed 2026-09-25)Marley, Universal RidgeFast system and box contents (observed 2026-09-25)
Omissions and review triggers
Footprint mode is limited to a rectangular outline with a uniform pitch; plane mode does not detect overlapping planes, roof openings, valleys or complex geometry.
Except for separate length-only HipFast and RidgeFast box minima, it does not count full fittings, fasteners or ventilation schedules. The RidgeFast count does not check tile profile or the manufacturer's tile-specific maximum rafter pitch. It does not assess structure, exposure, condition or safe access.
- Split and verify complex areas using a measured take-off.
- The optional preset covers only the named Marley product, finish and headlap values observed 25 September 2026; check current technical data for the exact roof conditions.
- Use a competent professional for inspection and specification.
Sources for this section: Health and Safety Executive, Roof work (page updated 2026-06-05; observed 2026-09-25)Marley, Duo Modern product data (observed 2026-09-25)Marley, Universal HipFast system and box contents (observed 2026-09-25)Marley, Universal RidgeFast system and box contents (observed 2026-09-25)
Sources
- Cushty, Roof Area Calculator UK (observed 2026-09-25)
- Cushty, Roof Tile Calculator UK (observed 2026-09-25)
- Marley, Duo Modern product data (observed 2026-09-25)
- Marley, Universal HipFast system and box contents (observed 2026-09-25)
- Marley, Universal RidgeFast system and box contents (observed 2026-09-25)
- Health and Safety Executive, Roof work (page updated 2026-06-05; observed 2026-09-25)