Roof Tile Calculator UK

Start with your roof size. If you know the area, switch to area mode and enter it. If you don't, enter the width along the eaves and the rafter length from eaves to ridge, then pick how many slopes you have. A standard gable roof has two. A lean-to has one.

Measure the rafter length up the slope, not the height of the roof. If you only have the span and the pitch, work the rafter length out first with the rafter length calculator, then come back.

Next, pick your tile. The presets cover the common UK types and fill in typical dimensions for each, but every one of those fields stays editable. If your tile isn't listed, or your merchant's datasheet gives different numbers, type them straight in. The three that matter are tile length, cover width and headlap.

Enter your pitch. The calculator uses it to flag whether you're below the usual minimum for that tile type. It doesn't change the tile count on its own, because the headlap does that, and the headlap is yours to set.

Leave wastage at 5% unless you've got a reason to change it. Cut-heavy roofs with valleys, dormers or hips need more, usually 7% to 10%.

If you want a ridge count too, enter the ridge length. The default 375mm cover suits a standard 450mm ridge tile at a typical lap.

Here's a real shape of roof, run end to end.

The house is a 1930s semi with a plain gable roof. Measured along the eaves it's 8.4m. The rafter length, eaves to ridge, is 4.2m. Two slopes, pitched at 22.5°, being recovered in interlocking concrete tiles at 420 × 330mm with a 300mm cover width.

Roof area is 8.4 × 4.2 × 2, which gives 70.56m².

The tile has a 75mm headlap. It's a single-lap tile, so the gauge is simply the length minus the lap: 420 − 75 = 345mm. That's also the batten spacing.

Tiles per square metre comes from the gauge and the cover width together: (1000 ÷ 345) × (1000 ÷ 300) = 9.7 tiles per m².

So the roof needs 70.56 × 9.7 = 682 tiles before wastage. Add 5% and round up: 716 tiles.

The ridge runs the full 8.4m. At 375mm cover per ridge tile, that's 8.4 ÷ 0.375 = 22.4, rounded up to 23 ridge tiles.

At a typical 2026 merchant price of around £1.70 per tile, the covering alone is roughly £1,217, before ridge tiles, battens, felt and fixings.

The takeaway: the gauge did the heavy lifting here. Change the headlap by 10mm and the tile count moves by around 20 tiles on this roof. Get the lap right first, and the order figure follows.

Gauge is the distance between batten centres, and it's what sets how much of each tile you actually see. Tighter gauge, more courses, more tiles.

How you get there depends on the tile. Single-lap tiles, meaning interlocking concrete tiles and pantiles, overlap only the course below, so the gauge is the tile length minus the headlap. Double-lap tiles, meaning plain tiles and slates, overlap the two courses below, so the gauge is the tile length minus the headlap, divided by two. That halving is why a plain tile roof needs roughly six times the tiles of an interlocking one.

Headlap isn't a free choice. BS 5534:2014+A2:2018, the code of practice for slating and tiling, ties minimum headlap to pitch and exposure. Shallower pitch means water moves more slowly and wind-driven rain travels further up the roof, so the lap goes up. The lap going up drives the gauge down, and the tile count up.

BS 5534's 2014 revision also changed fixing. Every tile in vulnerable zones, and in many cases across the whole roof, now needs mechanical fixing rather than relying on nibs and gravity. That doesn't change your tile count, but it changes your clip and nail order, and it's worth knowing before you price the job.

The takeaway: settle the headlap against the datasheet and your pitch before you calculate anything. Every other number on the page depends on it.