Measure one slope of your roof and the calculator works out how many tiles or slates you need, plus the battens and breathable underlay to order. Double the result for both sides of a pitched roof.
Measure one slope of the roof — along the ridge, and up the slope (not across the house). We work out the tiles, underlay and battens for that slope; double everything for both sides of a pitched roof.
Waste
264 tiles
24.0m² slope (26.4m² with waste allowed)
Breathable underlay1 roll
Treated battens (50×25)≈77m — 16 × 4.8m lengths
Coverage figures are typical (plain tiles ~60/m², interlocking ~10/m², slate ~20/m² with 100mm headlap). Add ridge and verge tiles — roughly 1.5 per metre of ridge — plus nails and clips. Guide only — confirm the spec with your roofer before ordering.
How it works
1Measure the slope — the length from ridge to eaves (or verge), and the height up the slope, not across the house. On a pitched roof measure each side separately, or measure one and double it.
2Pick your covering. Clay plain tiles work out at about 60 per m²; concrete interlocking tiles about 10 per m²; natural or fibre cement slates about 20 per m² with a 100mm headlap.
3Add a waste allowance. Interlocking tiles waste little, so 5% is usually enough. Plain tiles and slates get cut at verges and hips — allow 10%.
4Check the underlay and battens. The calculator includes breathable membrane (45m² per roll once laps are allowed for) and treated 50×25mm battens cut to your tile gauge.
5Don't forget the extras — ridge and verge tiles (roughly 1.5 per metre of ridge), nails, clips and lead flashing where the roof meets a wall.
Common roofing quantities
Single garage roof, 6m × 3m one slope — about 18m², roughly 180 interlocking tiles, 1 roll of underlay and about 40 4.8m battens.
Pitched extension roof, 5m × 3.5m per side — about 17.5m² per slope, so roughly 350 plain tiles per side at 60/m² — 700 for the pair.
Slate re-roof, 8m × 4m per slope — about 32m² per slope, roughly 640 slates per side, 2 rolls of underlay and around 70 battens.
Steep or shallow roofs change the maths: the flatter the pitch, the smaller the slope height for the same house — so always measure up the slope itself rather than working it out from the floor plan.
Now find the cheapest place to buy
Take your quantities across to Price Planter and compare them across UK builders' merchants — including whether splitting the order across two merchants works out cheaper.
Tile and slate coverage figures are typical UK allowances and vary by manufacturer — always check the coverage stated on the product, and confirm the spec with your roofer before ordering.