Roof Rafter Calculator (UK)

Roof rafter calculator — what this returns

This tool sizes cut-roof common rafters and gives you the order quantity for the timber merchant:

• Rafter length per piece, including the eave overhang tail
• Count per slope and total (both sides combined)
• Total linear metres of timber
• Recommended section (38×140 / 184 / 235 / 286 mm) auto-selected for your snow load
• Timber cost in pounds at 2026 C24 KD pricing, with an 8% waste allowance

The math follows BS 5268-7.5 simplified span tables with snow-load and spacing de-rating applied so the size recommendation tracks your actual ground snow load (sk from BS EN 1991-1-3 NA Fig NA.1) rather than a generic 0.6 kN/m² default.

How rafter sizing works under UK practice

Modern UK rafter design lives at the boundary of two documents: the prescriptive span tables in BS 5268-7.5 (still permitted under transitional provisions) and the engineered Eurocode 5 calculations to BS EN 1995-1-1 that Approved Document A nominates as the primary route. For domestic rafters at typical spans, both routes give the same answer to within 5%; for atypical loads or large spans, only the engineered route applies.

Loads. Use BS EN 1991-1-1 Table NA.A1 for permanent (dead) loads and BS EN 1991-1-3 NA for snow. NA Fig NA.1 maps the UK into five snow zones, with characteristic ground snow ranging from 0.4 kN/m² (south coast) to 1.0+ kN/m² (Highlands). Most of England south of Manchester is 0.5 kN/m²; most of Scotland is 0.7+. Add altitude correction above 200 m per NA.4.

Span direction. The span in BS 5268-7.5 is the horizontal projection, wall plate to ridge measured flat. The calculator divides your full building span by 2 and adds the eave overhang for the actual rafter length, then sizes the section against the horizontal half-span only.

Section choice. UK timber merchants stock C16 and C24 in metric sizes 38×89, 38×140, 38×184, 38×235 and 38×286 mm. C24 dominates new orders; specify C16 explicitly if you want it. Pre-2003 stock uses imperial 4×2 (≈100×50), 6×2 (≈150×50), 8×2 (≈200×50) which are no longer milled — use the metric equivalents.

Spacing. 400 or 600 mm centres in line with NHBC Standards 7.2. The base table is 600 mm; tighter spacing (400 mm) gives a ~10% bonus to allowable span; 800 mm is rare in cut-roof work and drops allowable span by 12%.

Worked example — 8.5 m × 10 m semi-detached, 30° pitch

Inputs: 8.5 m span, 10 m long, 30° pitch, 600 mm centres, 0.6 kN/m² snow, C24, 0.3 m overhang.

Computation:

slopeFactor      = √(1 + tan(30°)²) = 1.155
half-span run    = 4.25 m
rafter length    = (4.25 + 0.30) × 1.155 = 5.25 m each
count per side   = ceil(10 / 0.6) + 1 = 18
total rafters    = 36
total timber     = 36 × 5.25 = 189 m
38×235 C24 max   ≈ 4.6 m at 0.6 kN/m², 600 mm — outside prescriptive table
                  → engineer required, or specify 38×286 / TJI 240
peak height      = 4.25 × tan(30°) = 2.45 m above plate

This is the realistic scenario where prescriptive tables run out. For a semi at 8.5 m clear span, you’d use either a structural ridge beam (steel UB or LVL) reducing the rafter span to 4.25 m, or engineered I-joists at 600 mm with mineral wool fully filling the 240 mm depth for U-value compliance with Approved Document L.

Section table — BS 5268-7.5 / Trada at 0.6 kN/m² snow, 600 mm centres, C24

Source: Trada Eurocode 5 Span Tables (2018 edition, current). Values for L/300 instantaneous deflection limit, no ceiling load reduction. Above 5.45 m, engineered TJI 240/300 mm or LVL members are standard.

For higher snow loads:

• 0.8 kN/m² (north of M62, most of Wales): multiply allowable span by 0.87
• 1.0 kN/m² (Pennines, Lake District, Cairngorms foothills): multiply by 0.78
• 1.5 kN/m² (>500 m altitude, Snowdonia): multiply by 0.63 — engineer mandatory

Pricing — Q1 2026 reference data

Builders’ merchant pricing for C24 KD in metric sections, mid-range yards:

• 38×140 mm C24: £6.00–£6.40/m at Travis Perkins, Jewson, MKM
• 38×184 mm C24: £7.80–£8.20/m
• 38×235 mm C24: £11.20–£11.80/m
• 38×286 mm C24: £15.20–£15.80/m

Specials and engineered:

• TJI 240 mm I-joist: £18–£22/m (Steico Joist or James Jones LVL flange)
• Glulam 90×240 mm: £35–£42/m
• LVL 240×45 mm: £24–£28/m

For the 8.5 × 10 m worked example with 38×286 mm C24, lumber alone is about 189 m × £15.50 = £2,930, plus £180–£250 for ridge board (50×235 mm), straps, framing anchors and bracing. Crew time for a cut roof on this scale: 2 carpenters × 18–22 hours.

Trussed-rafter alternative for the same building: roughly £2,400–£2,800 for the truss order delivered, plus £400–£600 for crane and crew on set day. Cut roof is competitive when the merchant timber price is at the low end of the range or when listed-building consent rules out trussed rafters.

Code requirements (UK)

• Approved Document A — Structural Safety. Domestic rafters over 4.5 m span require structural calculation to Eurocode 5; under 4.5 m can use prescriptive span tables from BS 5268-7.5 or Trada.
• Approved Document L Volume 1 (2021) — Roof U-value 0.16 W/m²K maximum. For cut-roof construction with insulation between rafters, depth requirement is typically 200 mm of mineral wool (0.034 W/m·K) — pushes you to 38×235 mm rafters minimum to maintain a 25 mm ventilation gap.
• Approved Document C — Resistance to moisture. Class L breather membrane on top of rafters, vapour control layer below ceiling joists. NHBC Standards 7.2 elaborates the detail.
• BS 5534:2014+A2:2018 — Slating and tiling. Specifies rafter-to-batten loading paths, mechanical fixing schedules, and ventilation cross-section by tile type.
• BS EN 1995-1-1 (Eurocode 5) — Engineered design route. Required for spans, loads or geometries outside the prescriptive tables.
• CDM 2015 Regulations — Where the build value exceeds £1m or 30 days of work, a Principal Designer must coordinate Health & Safety. Rafter erection at height triggers Working at Height Regulations 2005 — scaffolding, edge protection and competent supervision.

Cut roof vs trussed rafter decision

Build rafters on site when:

• Your span is under 4.5 m and you’re inside the prescriptive tables — no engineer needed
• You’re framing a vaulted (cathedral) ceiling with exposed rafter feet
• Listed-building consent prohibits trussed rafters (most pre-1900 conservation areas)
• The roof has multiple hips, valleys and dormers that don’t fit standard truss profiles

Order trussed rafters when:

• Span is over 5 m — you’re outside the prescriptive tables either way
• Building length is over 8 m — labour savings dominate
• You want a clear-span loft conversion with no internal bearing wall
• The build is on a tight programme — trusses go up in a day, cut roofs take 3–4

For typical UK new-build semis (8 m span, 8 m long, 30°–35° pitch), the timber package costs about the same and trusses install in a third of the labour — the roof truss calculator gives you the side-by-side cost comparison.

Pair with these calculators

• Roof pitch calculator — convert between degrees, X/12 and slope factor before specifying.
• Roof truss calculator — get a trussed-rafter quote to compare against your cut-roof estimate.
• Roof area calculator — total roof surface for slating, battens and breather membrane on the same job.

When you change any input above, the output updates immediately. Print the page with your inputs and take it to the timber merchant for a take-off quote — Travis Perkins and MKM both honour pre-priced take-off lists for 14 days.