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Roof Pitch Calculator — Degrees, Ratio & Percentage
Calculate roof pitch angle from rise and run measurements. Shows degrees, ratio and what tile types are suitable for your pitch.
Vertical height of the roof slope
Horizontal distance (eaves to ridge, plan)
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How We Calculate This
Our roof pitch calculator determines the angle of your roof slope from either rise and run measurements, or from the total span and ridge height. It displays the result in degrees, as a ratio and as a percentage gradient.
The formula
Pitch (degrees) = arctan(Rise ÷ Run)
Pitch (%) = (Rise ÷ Run) × 100
Pitch ratio = (Rise ÷ Run) × 12 : 12
If using span and ridge height: Run = Span ÷ 2, then apply the same formula.
UK roofing normally states pitch in degrees. The X:12 ratio (rise per 12 units of run) is a US/imperial convention — we show it for cross-reference.
Common UK pitch ranges
- Under 10°: Very low pitch — flat roof membrane required (EPDM, GRP, built-up felt)
- 10-15°: Low pitch — specialist low-pitch tiles or metal sheeting only
- 15-22.5°: Low-moderate pitch — interlocking concrete tiles, some large-format slates
- 22.5-30°: Moderate pitch — most interlocking tiles and slates
- 30-45°: Standard UK pitch — all tile types suitable
- 45-60°: Steep pitch — traditional look, excellent weather protection
The calculator also shows the rafter length (the sloping distance from eaves to ridge), which is useful for ordering materials like battens and felt.
Frequently Asked Questions
The most common roof pitch for UK houses is between 30° and 45°. New-build houses typically have a pitch of around 35-40°. This range works well with most tile types and provides good weather resistance. Older Victorian and Edwardian properties often have steeper pitches of 40-50°. Minimum pitch for most tiles is around 22.5° but always check the tile manufacturer's specifications. Once you know the pitch, use our Roof Area Calculator to convert plan area to actual sloping roof area.
From inside the loft: hold a spirit level horizontally from a rafter and measure 1 metre along it. Then measure vertically from that point down to the rafter — this is the rise per metre. Use tan(angle) = rise/run to calculate degrees. From outside: measure the vertical height from the top of the wall plate to the ridge, and the horizontal distance from the wall to directly below the ridge (half the span).
Minimum pitches vary by tile type. Concrete interlocking tiles: typically 15-22.5° minimum. Plain clay tiles: 35° minimum. Natural slate: 20-25° minimum (depends on slate size and headlap). Large format tiles: 15-17.5° minimum. For pitches below 10°, you will need a flat roof membrane — see our Flat Roof Membrane Calculator. Use our Roof Tile Calculator to work out tile quantities once you know the pitch is suitable. For snow loading on steeper pitches, try our Roof Snow Load Calculator.
A 4:12 pitch means the roof rises 4 units vertically for every 12 units of horizontal run. This is approximately 18.4 degrees. The X:12 ratio is a US/imperial convention — in the UK roof pitch is normally given in degrees, so we show the ratio mainly for cross-reference. For comparison, a 6:12 pitch is 26.6° and an 8:12 pitch is 33.7°, both moderate to standard UK pitches.
Roof pitch can affect planning permission, particularly for extensions and conversions. Many Local Planning Authorities require new roof slopes to match the existing roof pitch. Permitted Development rights for rear extensions often specify that the roof pitch must match the original house. Altering the pitch of an existing roof usually requires planning permission.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.