Roof Size Calculator

You don't need one. Address-based tools use paid satellite/aerial imagery APIs to estimate your roof from above, which is why they ask for contact details in exchange for the result. This calculator uses the same underlying maths a roofer uses on-site: your building's length and width (footprint), plus the roof pitch and eaves overhang, are enough to calculate the actual sloped covering area precisely.

More than most people expect. A typical 300mm overhang on all sides of a 10m x 6m building adds 0.6m to each dimension (10.6m x 6.6m), taking the plan area from 60m² to 69.96m² — a 16.6% increase — before the pitch multiplier is even applied. Skipping the overhang is one of the most common reasons a rough estimate under-orders material.

The steeper the pitch, the more surface area covers the same footprint, because the roof surface is longer than its flat projection. At 35° the multiplier is about 1.22 (22% more area than the footprint); at 45° it rises to about 1.41 (41% more). This calculator applies the exact trigonometric multiplier (1 / cos(pitch)) rather than a rounded rule-of-thumb figure.

The footprint is the building's plan (bird's-eye) size — what an address or satellite-imagery tool typically measures. The roof area is the actual sloped surface you cover with tiles, slates or felt, which is always equal to or larger than the footprint once the pitch and eaves overhang are accounted for. Ordering materials against the footprint instead of the true roof area is a common and expensive mistake.

Standard UK concrete interlocking tiles cover at roughly 10 tiles per square metre, though the exact figure depends on the tile profile and batten gauge specified by the manufacturer. This calculator uses that planning figure and adds your chosen waste allowance for cuts, breakages and complex roof shapes.

A standard 500mm x 250mm natural slate at typical UK headlap covers at roughly 20 slates per square metre. Larger slates need fewer per m² but cost more each; smaller slates need more but are often cheaper — the Covering Comparison table shows the trade-off in both tile count and total cost for your specific roof size.

A "square" is a trade unit equal to 100 square feet (about 9.29m²). UK and international roofers still quote labour, underlay and felt in squares because it's a convenient round number for large areas. This calculator shows both m² and squares so the figure is useful whether you're comparing your own numbers or a roofer's quote.

As a planning guide: large-format concrete or clay interlocking tiles can go down to around 12.5°, standard double-lap plain clay or concrete tiles typically need at least 35-40°, natural slate is commonly laid down to around 20° depending on size and headlap, and roofing felt/membrane systems are designed for flat or near-flat roofs (this calculator treats felt as a 0° default). If your entered pitch is below your chosen covering's typical minimum, the Assessment panel will flag it.

A simple gable roof with few cuts typically needs around 5% extra, a standard roof with some hips or valleys around 10%, and a complex roof shape with lots of cuts, dormers or valleys can need 15% or more. This calculator pre-selects a sensible default from your Roof Type but you can always override it.

For a gable or hip roof, this calculator estimates ridge tiles using a dry-fix system at roughly 3 tiles per linear metre of ridge (a 450mm tile with a small overlap). Lean-to/mono-pitch and flat roofs have no ridge, so this shows as "N/A" for those roof types.

Both, plus lean-to/mono-pitch and flat roofs. The total roof area formula is the same for gable and hip roofs of the same footprint and pitch — what differs is the ridge length, which this calculator estimates separately using an approximation for equal-pitch hips (Length - Width, floored at zero). Choose your roof shape from the Roof Type field and the calculator adjusts the ridge and default waste allowance accordingly.

Materials-only prices vary widely by covering: this calculator estimates roughly £1.00-£1.75 per concrete tile, £0.90-£1.30 per clay plain tile, £1.50-£2.50 per natural slate, £27-£39 per 3m² metal sheet, and £20-£35 per 10m² felt roll (2026 UK guide prices). These exclude labour, scaffolding, underlay, battens and flashing — get a like-for-like quote from a roofer for the full installed cost.

That means your building dimensions or pitch are outside the plausible domestic/light-commercial range this calculator is built for (very small, very large, or an unusual pitch), or your inputs otherwise don't produce a sensible roof area. Re-check that length and width are in metres (not feet) and that pitch is in degrees (not a ratio like "6:12").

In most cases, simply replacing an existing roof covering on a house is permitted development and doesn't need planning permission, provided the new roof doesn't increase the height beyond the existing roof plane and you're not in certain protected areas or dealing with a listed building. Always check current guidance on gov.uk or with your local planning authority before starting work, since rules vary for extensions, dormers and conservation areas.

Measure the external wall-to-wall dimensions at ground or eaves level along both the longer and shorter sides of the building, in metres. If your building isn't a simple rectangle (an L-shape, or with a wing/extension), measure and calculate each roughly-rectangular section separately and add the resulting roof areas together, rather than trying to average one overall length and width.