Roof Area Calculator (UK)

How to use this calculator

Measure your home’s exterior footprint at the wall plate or just outside the gutter line:

1. Footprint length — the longest external dimension in metres
2. Footprint width — the shorter external dimension in metres
3. Pitch — enter degrees (UK convention) or use the roof pitch calculator if you have rise and run
4. Eaves overhang — typically 0.2 to 0.45 m on UK domestic roofs

The calculator returns:

• Roof on-slope area in square metres (the actual area you batten and cover)
• Tile and slate quantity based on the most common UK gauges
• Underlay rolls at standard NFRC roll sizes (45 m² breathable membrane)
• Battens in linear metres at the gauge for your chosen product

Why this calculation gets done wrong on UK estimates

Two errors cost real money on British roofing jobs:

Error 1: Quoting from plan area instead of on-slope area. A 10 m by 8 m bungalow has an 80 m² footprint. A roofer quoting plain clay tiles at a 40-degree pitch from the footprint alone underorders by more than 30 percent — the actual on-slope area is 104 m². Plain tiles at 60 per m² means 1,440 missing tiles. That is a full pallet shortfall and a delayed completion.

Error 2: Ignoring eaves and verge projection. A typical 350 mm eaves overhang on a 10 by 8 footprint adds roughly 5 m² to the area. Combined with the slope factor that is another 6 to 7 m² of cover. NFRC technical bulletins recommend always taking measurements at the eaves line, not at the wall.

The formula

Plan area (m²) = (length + 2×overhang) × (width + 2×overhang)
Slope factor = 1 / cos(pitch in degrees) = √(1 + (rise/run)²)
On-slope area = plan area × slope factor
Squares (UK trade) = on-slope area / 9.29 m² (1 square = 100 sq ft)

The calculator does all of this; you supply the four inputs.

How material counts work in the UK

Plain clay tiles (Marley Acme Single Camber, Sandtoft, Tudor): 60 per m² at 100 mm gauge, 65 per m² at 90 mm gauge for exposed locations under BS 5534 wind zones D and E.

Concrete interlocking tiles (Marley Modern, Redland Cambrian, Russell Lothian): 9.7 per m² for Modern, 12.5 per m² for Mini-Stonewold, 16.4 per m² for Plain. Check the manufacturer’s data sheet for your specific batten gauge.

Natural slate (Welsh, Spanish, Brazilian): 500x250 mm at 75 mm lap = 21.5 per m². 600x300 mm at 100 mm lap = 13.4 per m². Smaller sizes for steep pitches and severe exposure zones in Scotland.

Underlay (Klober Permo, Tyvek Supro, Cromar Vent 3): standard roll is 1.5 m × 50 m = 75 m² gross, 45 m² effective coverage with 100 mm horizontal lap and 150 mm side lap.

Battens (CLS treated 25x50 or 38x25 to BS 5534): linear metres = on-slope area divided by gauge in metres. At 100 mm gauge plain tile that is 10 linear metres of batten per m² of roof. Buy 4.8 m lengths.

Ridge tiles: linear metres of ridge — usually the building length on a simple gable. Mortar bedded or dry-fix systems both calculated the same way.

Verge undercloak or dry-verge: linear metres of gable ends — building width × 2 for a simple gable.

Adjusting for complex UK roofs

This calculator assumes a simple cut-and-pitched gable. For other configurations:

• Hipped roof: same footprint area as a gable for the same pitch and outline. Add 10 percent extra material to cover bonnet hips, hip irons and valley cuts.
• L-shape or T-shape: split into rectangles, calculate each, sum the results. Add 5 percent extra for the additional cut at the internal valley.
• Mansard or French roof: two pitches per side. Calculate upper and lower slopes separately using the gambrel roof calculator, which uses the same maths.
• Cottage with dormers: add the dormer roof areas separately, deduct nothing for the dormer void (the main roof still has tiles up to the dormer cheeks).

Sources used in this calculator

The slope factor maths is pure trigonometry — it does not change between countries. The tile and slate coverage rates come from current BS 5534:2014+A2:2018 manufacturer data sheets (Marley, Redland, Russell, SIGA Slate). NFRC technical bulletins inform the waste allowances, and Construction Skills CITB rate book figures cross-check the underlay and batten quantities for typical UK jobs.

For pricing per m² in your area, check Checkatrade or MyBuilder local quotes. For BBA Agrément certification on synthetic underlays, visit the BBA database.