BuildCalcs Roof Tile & Slate Calculator UK
Estimate roof covering quantities from the true slope area, the manufacturer’s tiles-or-slates-per-square-metre figure and your chosen waste allowance.
Reviewed: 20 September 2026 · How we check calculations
How the roof tile and slate calculation works
The calculator multiplies eaves length by slope length and the number of similar roof slopes. It then multiplies that area by the selected units-per-square-metre rate and applies the waste allowance.
Coverings required = eaves length × slope length × number of slopes × units per m² × (1 + waste %). The final quantity is rounded up because whole tiles or slates are ordered.
Worked example: two 8 m × 4.5 m roof slopes
Two matching slopes each measuring 8 m along the eaves and 4.5 m from eaves to ridge have a combined area of 72 m². At 10 tiles per m², the base quantity is 720 tiles. A 10% allowance produces a planning quantity of 792 tiles.
Why units per square metre varies
Covering dimensions, headlap, side lap, batten gauge and roof pitch can all change the number of tiles or slates needed per square metre. Use technical data for the exact product and fixing arrangement rather than treating a generic rate as universal.
Using the calculator for natural or fibre-cement slates
The same area method works for slates when the correct coverage rate is entered. Slate coverage is especially sensitive to slate size, headlap and gauge. Check the manufacturer or supplier specification for the intended pitch and exposure, and keep ridge, hip, verge, valley and ventilation components separate from the field-slate count.
Frequently asked questions
Does this work for interlocking tiles and slates?
Yes for a basic area estimate if you enter the correct units-per-square-metre figure supplied for the product and installation gauge.
Should I add extra for breakages?
A waste allowance can cover cuts and breakages, but complex hips, valleys and detailing may require a larger allowance.
Related calculators
Using tiles per square metre
Tile coverage is controlled by the manufacturer's permitted gauge and lap, so use the tiles-per-square-metre figure for the exact tile and roof pitch rather than assuming all tiles of a similar size cover the same area. The calculator multiplies the slope area by that coverage rate and adds the selected waste allowance.
Calculate the roof slope area, not the plan area
The length entered from eaves to ridge is measured along the slope. A pitched roof therefore has more surface area than its horizontal footprint. For a simple dual-pitch roof with two matching faces, set the number of similar slopes to two; extensions, valleys and hips should be broken into appropriate roof areas.
Tiles are only part of the roof order
Cutting around verges, valleys, rooflights and abutments can increase waste. Ridge, hip and verge units are normally ordered separately because they are linear components rather than field tiles. Battens, membrane, fixings and ventilation products are also outside the calculation. Always check the tile manufacturer's minimum pitch and fixing specification for the location.