Roof Tile Calculator

How to Use the Roof Tile Calculator

1. Measure the roof on plan. Measure the length and width of the area the roof covers, including the overhang at the eaves and verges. You can measure from the ground or from drawings; you don't need to go on the roof.
2. Enter the roof pitch in degrees. If you don't know it, a typical UK house roof is 30–40°.
3. Choose the roof shape: a simple gable (duo-pitch) roof, a hipped roof, or a single lean-to (mono-pitch) roof.
4. Choose your tile or slate. The calculator fills in its coverage per m², batten gauge and minimum pitch. You can also enter your own figures from a data sheet.
5. Pick the headlap. 75mm or 100mm for interlocking tiles; plain tiles use about 65mm.
6. Set the waste allowance (5% for simple gable roofs, 10% or more for hips, valleys and dormers).

The result shows the true roof area, tiles to order, battens, membrane, ridge and hip tiles, and the estimated cost.

How to Work Out Your Roof Area

Roof area is larger than the floor area underneath, because the roof slopes. Multiply the plan area by a pitch factor:

This works for gable, hipped and lean-to roofs, as long as all the slopes have the same pitch. A 10 m × 8 m roof on plan at 35° has a true area of 80 × 1.221 = 97.7 m².

Remember to subtract areas you won't tile, such as large rooflights or a flat-roofed section, and add separate areas for dormers or porches.

How Many Roof Tiles Per m²?

Coverage depends on the tile size and the headlap (how much each course overlaps the one below). Typical figures from manufacturers' data:

Natural and fibre-cement slates vary with size and headlap. Slates are double-lapped, so the gauge is (slate length − headlap) ÷ 2. As a guide, 500 × 250mm slates need about 18.5–19.6 per m² at 75–100mm headlap, and 600 × 300mm slates about 12.5–13.1 per m², allowing a 5mm side gap. The headlap you need depends on your roof pitch and exposure. Always use the manufacturer's figure for your exact product and site.

Plain tiles use far more tiles per m² because each tile is small and the roof is double-lapped: at any point there are at least two layers of tile. That's why plain tile roofs are heavier and more expensive to lay.

Battens, Membrane and Fixings

Battens. The batten gauge (the spacing between battens) is set by the tile length minus the headlap. For a 420mm tile at 75mm headlap, the gauge is 345mm, so you need about 2.9 m of batten per m² of roof. Plain tiles at 100mm gauge need about 10 m per m². UK battens are usually 25 × 38mm or 25 × 50mm treated softwood, graded to BS 5534; check the size required for your rafter spacing.

Membrane. A breathable roofing underlay goes under the battens. Rolls are typically 1 m wide and 50 m long. Allow about 15% for laps, so a 100 m² roof needs about 3 rolls.

Fixings. Since the 2014 revision of BS 5534, all single-lap interlocking tiles must be mechanically fixed (nailed and/or clipped), and mortar alone can no longer be relied on to hold ridge, hip or verge tiles. In practice, most new roofs use dry-fix ridge, hip and verge systems. The number of nails and clips depends on a wind uplift calculation for your site, so ask your supplier or roofer for a fixing specification.

Ridge and Hip Tiles

Standard ridge tiles are about 450mm long, so you need roughly 2.25 ridge tiles per metre of ridge, plus a few spares.

On a hipped roof, the sloping hips need hip tiles too. For a simple hipped roof on a rectangular house, the calculator works out the ridge length (length minus width) and the four hip lengths from the pitch. A 10 × 8 m roof at 35° has a 2 m ridge and four hips of about 6.3 m each, so about 61 ridge and hip tiles in total. Hip irons or dry hip kits and a few extra tiles for cutting are needed as well.

How Much Does a New Roof Cost in 2026?

Typical installed prices per m² of roof, excluding VAT and access costs:

A 100 m² concrete tile re-roof typically costs about £8,500–£17,000 nationally, and more in London and the South East. The biggest items are usually the tiles, scaffolding and stripping and disposing of the old roof. Allow extra for:

• Scaffolding: often around £900–£3,500 for a semi
• Timber repairs found once the old tiles come off: often £750–£3,500
• Lead flashings, ventilation, dry ridge and verge systems
• Insulation upgrades if you're renovating the roof

Concrete, Clay or Slate?

• Concrete interlocking tiles are the cheapest and quickest to lay, with large tiles and low weight per m² of coverage. They suit most modern houses.
• Clay plain tiles are traditional in much of England, last a very long time, and weather attractively, but cost more to buy and lay.
• Natural slate is long-lasting and lightweight but expensive; Welsh slate costs more than imported slate.
• Fibre-cement and synthetic slates give a slate look for less money and lower weight.

If you're changing from a lightweight covering (such as slate) to a heavier one (such as concrete tiles), the roof structure may need checking, and Building Regulations approval is usually needed. In conservation areas or on listed buildings, a change of roof material often needs consent.

Planning and Building Regulations

Re-roofing with similar materials doesn't normally need planning permission. However:

• Re-covering more than 25% of the roof usually needs Building Regulations approval, because it counts as renovating a thermal element, and you may need to improve the roof's insulation.
• Changing the covering to a heavier material can need structural checks.
• Listed buildings and conservation areas can need consent for changes to the roof's appearance.

Many roofers are registered with a competent person scheme and can self-certify the work. Ask for the certificate when the job is done.

Tips for Ordering Roof Tiles

• Order 5–10% extra. Hips, valleys and verges create cuts, and some tiles break in delivery.
• Buy from one batch so the colour matches across the roof.
• Check the minimum pitch for your tile. Using a tile below its minimum pitch can void the warranty and let water in.
• Keep a few spares for future repairs. Discontinued colours can be hard to match later.

For the brickwork or render below the roof, see the Brick Calculator and Render Cost Calculator. Converting the loft at the same time? The Loft Conversion Cost Calculator covers that.

Frequently Asked Questions