Extension Roof Calculator — Materials & Quantities

Most extension pitched roofs are 15–40 degrees. The minimum pitch depends on the specific product, not just the tile type: concrete interlocking tiles typically need 15° to 22.5° minimum, though some granular finishes need 25–30°, 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. Always check your chosen tile manufacturer's fixing specification for the exact minimum pitch.

This varies significantly by tile type. Concrete interlocking tiles need about 10 per m², plain clay plain tiles about 60 per m² (they are much smaller). Natural slate depends heavily on size and headlap: a 400×250mm slate at 90mm lap is about 26 per m², a 500×250mm about 20, and a 600×300mm about 13 (Cupa Pizarras coverage chart); this calculator uses 22 per m² as a mid-range default, so check your slate datasheet. Always add 10% for waste, cuts at hips/valleys, and breakages.

Approved Document L Volume 1 (2021 edition incorporating 2023 amendments — the version current in 2026), Table 4.2 sets the limiting U-value for a new roof element in an existing dwelling or extension at 0.15 W/m²K. In practice that needs roughly 130–150mm of PIR (λ≈0.022 W/mK) in a warm pitched roof between and over rafters, or 150mm+ of PIR above the deck on a flat roof; many builders specify 150–200mm to leave a margin. The exact thickness depends on the insulation product's conductivity and the construction build-up — confirm with a U-value calculation.

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 typical 25 to 40 year service life (some manufacturers quote a 50-year design life) 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.

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.

Both are common on UK single-storey extensions. A flat roof (EPDM or GRP membrane) is typically 20–40% cheaper to build, sits at a lower overall height, and suits a contemporary look, but needs replacing sooner (25–40 years of typical service life for modern EPDM and GRP membranes, though some EPDM manufacturers quote a 50-year design life) and needs a minimum 1:40 fall for drainage. A pitched roof costs more and adds 1–3m to the ridge height, but lasts 50–100+ years and gives usable loft space above.

Permitted development rules often decide the choice on a single-storey rear extension: the roof must not rise above the highest part of the existing house roof, which can rule out a pitched roof where there is limited headroom. Check your dimensions with the Extension Permitted Development Calculator, and see the full Pitched vs Flat Roof comparison for costs, lifespan and drainage side by side.

For a 6m × 4m (span) extension roof at a 30° pitch, using concrete interlocking tiles (10 tiles/m²) and this calculator's default 10% waste allowance: rafter length = (4 ÷ 2) ÷ cos(30°) = 2 ÷ 0.866 = 2.31m. Slope area = 2 × rafter × roof length = 2 × 2.31 × 6 = 27.71 m². Tiles needed = 27.71 × 10 × 1.10 = 304.81, rounded up to 305. Felt/underlay area = 27.71 × 1.1 (10% overlap allowance) = 30.48 m². Batten rows per side = rafter length in mm (2309mm) ÷ 345mm gauge, rounded up, plus one for the eaves batten: (2309 ÷ 345 = 6.69, rounded up to 7) + 1 = 8; doubled for both roof sides = 16 rows, giving 16 × 6m = 96m of batten. Ridge tiles = roof length ÷ this calculator's 0.45m cover default (standard 450mm half-round or angular ridge) = 6 ÷ 0.45 = 13.33, rounded up to 14; confirm your exact ridge profile with the Ridge Tile Calculator before ordering. These are the exact formulas and defaults this calculator applies: enter your own dimensions above for your own figures. For the rafter-length maths on its own, see the Rafter Length Calculator.

Yes. Permitted development (GPDO Class A) caps a single-storey rear extension at 4m overall height, requires the eaves not to exceed the eaves of the existing house, and the extension must not be higher than the highest part of the existing roof. Rafter length and rise both increase with pitch for the same span, so a steeper pitch, for example 35–40° to match a Victorian roof, needs more headroom and can breach these limits, while a lower pitch of 15–22.5° or a flat roof is more likely to fit. Always check your specific dimensions with the Extension Permitted Development Calculator before fixing the pitch.

This calculator defaults to 10% waste, which suits a simple rectangular roof slope. For a plain rectangular extension roof with no hips or valleys, 5% is often enough. Roofs with hips typically need about 7.5%, and complex roofs with valleys, dormers or multiple hips need 10–12.5%, because more tiles are cut to fit the junctions. Adjust the Waste Allowance field in Advanced Options to match your roof's complexity, and round the tile order up to a full pack. See the Roof Tile Coverage reference for coverage, gauge and minimum-pitch figures by specific tile brand and size.