Roof Area Calculator (UK)
Free UK roof area calculator. Compute the actual roof surface in square metres for gable, hip, mono-pitch and L-shape roofs from footprint, pitch in degrees, and eaves overhang.
Roof Area Calculator
How this UK roof area calculator works
Input the building length, width, eaves overhang and pitch in degrees. The calculator returns the roof surface area in square metres, the rafter length, the slope factor, and an equivalent in roofing squares (for cross-reference with imported specifications). Pick the shape — gable, hip, mono-pitch or L-shape — and the geometry adapts.
It works for any pitch from 5° (lean-to / extension) up to 75° (mansard upper plane). For pitches below the BS 5534 minimum for your chosen covering, switch to a single-ply membrane and consult the flat roof replacement cost calculator.
The geometry, derived from first principles
The slope factor relates the inclined surface to its horizontal projection:
slope factor = 1 / cos(pitch in degrees)A 30° pitch gives 1.155; a 45° pitch gives 1.414. The roof area on a rectangular gable or hip is:
roof area (m²) = (length + 2·overhang) × (width + 2·overhang) × slope factorFor a mono-pitch (lean-to / shed) the same rectangle is a single plane and the formula is unchanged. For an L-shape the calculator computes each rectangle separately and sums.
UK-specific input guidance
- Length × width — measure to the gutter line in metres. Standard UK semi-detached is around 8 × 8 m, terrace 5–6 × 8 m, four-bed detached 10 × 9 m.
- Pitch — record from the gable bargeboard with a digital level. Typical UK pitches: 17.5° (low-pitch concrete tile), 22.5° (modern volume housebuilder), 30° (most established stock), 35°+ (Victorian and Edwardian). The NFRC technical bulletin 06 documents standard pitches per covering type.
- Overhang — UK eaves typically project 250–500 mm. Verge overhang at the gable is similar but can be reduced to a flush detail.
UK pricing context (2026)
Roof area is the input to material pricing. Recent NFRC member quotes and Checkatrade / MyBuilder data for 2026 give the following ranges for a re-roof on the calculated surface:
| Covering | £/m² supply + fit | Typical UK pitch |
|---|---|---|
| Concrete interlocking tile (Marley Modern, Redland Mini-Stonewold) | £55–£85 | 17.5°+ |
| Clay plain tile (Marley Acme, Sandtoft) | £85–£130 | 35°+ |
| Natural Welsh / Spanish slate | £130–£220 | 25°+ |
| Re-cycled cement-fibre slate (Cembrit, Marley Eternit) | £75–£105 | 20°+ |
| Standing-seam zinc (VMZINC, elZinc) | £180–£260 | 7°+ |
| Single-ply EPDM (Firestone RubberCover) | £75–£110 | 1°+ (low slope) |
So a 95 m² hipped roof in concrete interlocking tile at £70/m² is a £6,650 contract before scaffolding and access (typically £900–£1,400 for a two-storey detached) and skip hire (£250–£400). Add 5–10% for valleys and dormers — the calculator gives you the geometric area, and your roofer adds waste on top.
Common UK pitfalls
Confusing footprint with roof area. A 9 × 7 m footprint at 30° is 63 m² in plan but 73 m² of slate. Suppliers will not refund the missing 10 m² when the roof leaves you short.
Forgetting verge and eaves overhang. A 0.4 m overhang on a 9 × 7 footprint adds about 13% to the area before the slope factor. That is real material.
Mixing concrete and clay headlap. BS 5534 requires different headlaps for different tile types. The roof area is constant; the tile count per m² changes. Use the manufacturer’s coverage rate, not a generic 10/m² assumption.
Assuming flat-roof rules apply at the boundary. Below 10° most warranties switch to single-ply or built-up bitumen — see BS 6229 and Approved Document C for cold-deck / warm-deck rules.
Code and standards references
- BS 5534:2014+A2:2018 — Slating and tiling for pitched roofs and vertical cladding (Code of Practice). Headlap, batten gauge, fixings and minimum pitches.
- BS 8612:2018 — Dry-fixed ridge, hip and verge systems.
- Approved Document C — Site preparation and resistance to contaminants and moisture (England & Wales building regulations).
- Approved Document L1B (2024 amendment) — refurbishment U-values: pitched roof at rafter-line target 0.16 W/m²K.
- Work at Height Regulations 2005 — scaffolding, edge protection, harness rules. Always factor scaffold cost separately from the roof area takeoff.
- NFRC technical bulletins — current best practice for headlap, underlay and ventilation.
Pairing the area with the structure
Once you have the surface area, size the timber with the roof rafter calculator and the cost with the roof cost calculator. For listed-building work consult the conservation officer before specifying tile or slate type — area is unaffected by appearance constraints, but unit cost and supplier lead time can swing dramatically.
The roof area is the single calculated input every other stage of a UK roofing project depends on. Get it right here and the supplier order, scaffold quote, NFRC contractor estimate and Approved Document L compliance check all line up afterwards.