What pitch should an extension roof be?

Most extension pitched roofs are 15–40 degrees. The minimum pitch depends on the tile type: concrete interlocking tiles need 17.5° minimum, plain clay tiles 35°, and natural slate 20–25°. Flat roofs should have a minimum fall of 1:40 (about 1.4°) to ensure water drainage. Your chosen tile manufacturer will specify the minimum pitch.

How many tiles do I need per m² of roof?

This varies significantly by tile type. Concrete interlocking tiles need about 10 per m², plain clay tiles about 60 per m² (they are much smaller), and slate/synthetic slate about 22 per m². Always add 10% for waste, cuts at hips/valleys, and breakages. The tile manufacturer datasheet provides the exact coverage rate.

What insulation thickness do I need for a Part L compliant extension roof?

For 2026 Building Regulations Part L, extension roofs must achieve a U-value of 0.15 W/m²K or better. This typically requires 150–200mm of rigid insulation (PIR) between and over rafters for pitched roofs, or 150mm+ of PIR above the deck for flat roofs. The exact thickness depends on the insulation product and construction method.

What is the best flat roof membrane for an extension?

EPDM rubber and GRP fibreglass are the most popular for domestic extensions. EPDM is a single sheet with no joints (ideal for simple shapes), has a 50-year life expectancy and is DIY-friendly. GRP is seamless, extremely durable and suits complex shapes with upstands. Single ply (TPO/PVC) is common on commercial but increasingly used domestically.

What are firring strips and why do flat roofs need them?

Firring strips are tapered timber battens that create a fall (slope) on a flat roof deck so rainwater drains to outlets. Building Regulations require a minimum fall of 1:40. Firring strips are typically placed at 400mm centres across the roof width, with the taper calculated to achieve the required fall over the drainage distance.

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Extension Roof Calculator — Materials & Quantities

Calculate roofing materials for extension pitched or flat roofs including tiles, felt, battens, membrane, insulation and firring strips.

Roof type

Roof plan length

Roof plan width (span)

Pitch angle

Tile type

How We Calculate This

This calculator determines material quantities for extension roofs based on plan dimensions and roof type.

Pitched roof

Slope area = 2 x rafter length x roof length, where rafter = (span/2) / cos(pitch). Tile count = slope area x tiles per m² x waste factor. Batten length calculated from gauge (spacing) and rafter length.

Flat roof

Membrane area = plan area x 1.15 (for upstands and overlaps) x waste factor. Firring strips at 400mm centres across the width.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.