Roofing Materials Calculator

Work out how many tiles, battens, and underlay rolls your extension roof needs, and roughly what the materials will cost. Enter the width and length of the extension, the roof style and pitch, and your tile or membrane choice. The tool returns the slope area, a tile count with wastage, and a supply-cost range for the covering.

What the calculator works out

The starting point is the roof slope area, which is larger than the floor area because a pitched slope runs at an angle. From that area the tool counts tiles for your chosen roof covering: concrete interlocking tiles at £7 – £25, plain clay tiles at £53 – £90, or slate. It also sizes the battens the tiles hook over and the number of breathable underlay rolls, then applies your wastage allowance across the lot.

Concrete tiles are the cheapest and most common covering, so they need the fewest tiles per m². Clay costs more and needs many more tiles for the same area, which is why the supply figure jumps when you switch to it. Slate prices vary hugely by source, so treat the slate estimate as a mid-range guide.

Pitched or flat

Your roof pitch decides which covering works. Below roughly 10 degrees, water sits rather than runs off, so a flat-roof membrane replaces tiles. For a flat roof the tool switches to a supply rate per m²: EPDM rubber at £8 – £12, or GRP fibreglass at £30 – £50 per m². EPDM goes down as a single sheet and is the usual pick for a project-managed extension; GRP cures to one continuous rigid skin but wants a specialist to lay it.

What it leaves out

The estimate covers materials for the field of the roof only. It does not price ridge, hip, or valley tiles, verge trims, dry-fix systems, guttering, or the lead flashing where the new roof meets the existing wall. Flashing materials for a typical abutment add £100 – £180, and none of the figures include labour. A finished, installed roof for a single-storey extension is nearer £2,500 – £4,500 once every accessory and the roofer's time are counted.

A worked example

Take a 4 metre by 5 metre single-storey rear extension with a lean-to roof at a shallow pitch, finished in concrete tiles. The sloped area comes out a little above the 20 m² floor area, and the tool counts the tiles, batten runs, and underlay rolls for that slope with wastage added. At the £7 – £25 supply rate, the tile materials land in the range the calculator shows, with battens and underlay on top.

Swap to clay tiles at £53 – £90 and the covering cost climbs sharply, because clay needs many more tiles per m² than concrete. Add lead flashing at the house wall and guttering, and remember that the finished, installed cost sits closer to the £60 – £100 per m² full-roof rate than to the materials figure on its own.