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Roof Truss Calculator (UK)

Free UK roof truss calculator. Size trussed rafters, fink, attic and scissor trusses to BS 5268-3 and BS EN 14250. Member lengths, count and delivered cost in GBP.

Roof Truss Calculator

Estimate truss member lengths, the number of trusses needed for your building, total lumber, and a delivered prefab cost.

Top chord (each)
5.2
m per side
Bottom chord
9
m
Web members total
5.58
m per truss
Lumber per truss
24.97
m
Truss count
21
incl. end pair
Total lumber
524
m (linear)
Peak height
2.7
m above plate
Per truss
£252
prefab, delivered
Total truss cost
£5,292
excludes crane / install

Slope factor used: 1.155. Lumber LF is the linear sum of all chords and webs per truss, useful for comparing site-built vs prefab. Add 5–10% for cuts and connectors.

Roof truss calculator — UK trussed rafters

This tool sizes prefabricated trussed rafters for residential and light-commercial roofs in the UK to BS EN 14250 and BS 5268-3. Set your span, pitch in degrees and trussed rafter centres to get:

  • Top chord (rafter) length per side
  • Bottom chord (ceiling tie) length
  • Web member total by truss type — fink, mono-pitch, attic, scissor, queen
  • Truss count based on building length and centres
  • Total carcassing run in linear metres
  • Per-truss and total delivered cost in GBP

Pitch is in degrees throughout, in line with UK construction drawings and BS 5534 specification practice.

How the math works

Trussed rafters are triangulated stress-graded softwood assemblies pressed with metal connector plates (“punched metal plate fasteners” in BS 5268-3 language). The geometry is fully determined by span, pitch and depth.

Top chord. For a symmetric duo-pitch truss, each top chord runs from the wall plate to the apex. With the slope factor f = √(1 + tan²θ) = 1/cos θ, the top chord length is:

topChord = (span / 2) × (1 / cos θ)

For a 9 m span at 30°, that’s 4.5 ÷ cos 30° = 5.20 m per side. The mill orders the carcassing at the next standard length up — typically 6.0 m C24 stress-graded softwood — and trims on the docking saw before plating.

Bottom chord (ceiling tie). Equal to the span plus a small bearing allowance (50 mm typical). Manufacturers leave the chord as one piece up to 9 m, then go to a finger-joint or two-piece splice with a punched metal plate above the splice.

Webs. Internal diagonals depend on truss type. For the standard fink (W) truss, web length total ≈ 0.62 × span. Mono-pitch (half-trusses) ≈ 0.30 × span; scissor ≈ 0.70 × span; attic ≈ 1.10 × span thanks to the vertical side webs that frame the room-in-roof.

Truss count. Count = ceil(building length ÷ centres) + 1. A 12 m long extension at 600 mm centres needs 21 trussed rafters.

Truss types — when to specify which

TypeBest forSpan limitCost vs fink
Fink (W-truss)Standard semi-detached, terrace11 m1.00×
Mono-pitchLean-to extensions, garage roofs8 m0.80×
Queen-postPeriod-style cottages, simple barns9 m0.85×
Howe / PrattLong-span industrial buildings18 m1.10×
ScissorVaulted living rooms (cathedral effect)9 m1.30×
Attic / room-in-roofConditioned loft conversions9 m1.85×
Gable spandrelGable end with integral wall framing11 m1.20×

For long-span Howe and Pratt trusses on agricultural or industrial buildings, you’ll get a custom-engineered quote against BS EN 1990 / BS EN 1995-1-1 (Eurocode 5) rather than the typical-pricing estimate above.

UK pricing — 2026 reference data

Carcassing softwood prices through Q1 2026 have stabilised after the 2022–2024 swings, with Forest Trade Council (FTC) C24 KD averaging £318/m³ for the 47×100 sections that dominate trussed rafter manufacture. Translated to delivered prefab fink trusses on a 60 mile radius, TRA-member yards in the South East and Midlands are quoting £25–£32 per metre of span for residential 25°–45° pitches under 0.6 kN/m² snow.

Worked example (the calculator’s reference test): a 9 m span × 12 m long two-storey extension at 30°, 600 mm centres:

  • 21 fink trusses at £28/m × 9 m span = £252 each → £5,300 truss order
  • Delivery: included for orders above 15 trusses within 60 mi
  • Crane and 3-banksman set day: £450–£700
  • Total prefab roof structure: £5,800–£6,000

By comparison, a cut roof using site-cut C24 rafters and a doubled 47×195 ridge runs £2,400 in carcassing + £3,400 in joiners’ labour (3 carpenters × 4 days at £280/day) = £5,800. Material cost is similar; trussed rafters save you most of a week on programme.

UK code and standards

  • BS 5268-3:2006 — Code of practice for trussed rafter roofs. The historic UK design code, still used alongside Eurocode 5 (BS EN 1995-1-1) for residential trussed rafters under TRA Technical Handbook 8.
  • BS EN 14250:2010+A1:2018 — Timber structures — Product requirements for prefabricated structural members assembled with punched metal plate fasteners. The product standard the truss is CE/UKCA-marked against.
  • BS 5534:2014+A2:2018 — Slating and tiling for pitched roofs and vertical cladding — Code of practice. Drives the dead-load design assumption (45–80 kg/m² for plain-tile, 28 kg/m² for concrete interlocking, 18 kg/m² for natural slate at 26°+).
  • Approved Document A (Structure), 2013 Edition (incorporating 2024 amendments) — Mandates engineered design for trussed rafters and prohibits field modification without a sealed engineer’s approval.
  • Approved Document C (Resistance to moisture) — Ventilation provisions for cold and warm pitched roofs; sets the eaves airway requirements that the heel height must accommodate.
  • NHBC Standards 7.2 (and LABC Warranty / Premier) — Guarantee provider technical requirements for trussed rafter roofs in new-build housing. Most are aligned with TRA Technical Handbook 8.

Buying tips

  1. Get three quotes — TRA membership doesn’t standardise pricing. A 15% spread on a £6,000 order is real money. Members’ directory at tra.org.uk.
  2. Specify heel height for energy — a 200 mm energy heel lets you maintain 270 mm rockwool over the wall plate with the 25 mm airway BS 5250 wants. Standard 90–125 mm heels force a compromise.
  3. Order the eaves detail — let the manufacturer cut the rafter tails to your soffit profile. Saves a day of fascia carpentry.
  4. Confirm CE/UKCA marking — every truss should ship with the design certificate referenced to BS EN 14250 and the project-specific structural pack.
  5. Crane the load — a one-day Hiab on a 9 m span is £350–£500 in metro areas, and avoids the manual handling assessment that BS 5268-3 expects above 6 m.

Pair with these calculators

Outputs update live as you change inputs. Print the result and pass it to a TRA-member manufacturer for a comparable quotation.

Frequently asked questions

How many trussed rafters do I need for a 12 m long extension at 600 mm centres?
Number of trusses = ceil(building length ÷ spacing) + 1. For 12 m at 600 mm centres that's ceil(12 ÷ 0.6) + 1 = 21 trusses, including one at each gable. UK Trussed Rafter Association practice puts the gable trusses tight to the masonry, with internal trusses at 600 mm. Half-trusses (mono pitches) are sometimes substituted at gables on cost-sensitive jobs.
What is a fink trussed rafter?
A fink trussed rafter is the W-shaped engineered truss used in around 90% of UK residential roofs. It has two sloping top chords (rafters), a horizontal bottom chord (ceiling tie) and four diagonal webs forming a W. It's manufactured to BS EN 14250 by members of the Trussed Rafter Association (TRA), pressed with metal connector plates and stamped with a CE/UKCA design certificate. Spans up to about 11 m are routine; above that you move to attic or fan trusses.
How much do trussed rafters cost in the UK in 2026?
Delivered fink trussed rafters from a TRA member are running £25–£32 per metre of span in 2026, so a 9 m span fink is roughly £225–£290 per truss. A typical 9 m × 9 m semi-detached needs 16 trusses at 600 mm centres, so a £4,000–£4,800 truss order plus £350–£550 crane hire on the day. Attic trusses cost 60%–90% more, scissor trusses around 30% more. Carcassing softwood prices set by the FTC index drive the variation; ring three TRA members for live quotes within 60 days of order.
Do I need building regulations approval for trussed rafters?
Yes — the truss design has to satisfy Approved Document A (Structure) and the manufacturer issues a structural calculation pack stamped to BS EN 14250 and BS 5268-3. Building Control will want sight of the calcs and the supplier's TRA membership before signing off the carcass inspection. The Trussed Rafter Association's Technical Handbook (Section 8) is the standard reference. Don't cut, notch or drill trusses on site without written engineer's approval — Approved Document A 1.5 forbids field modification.
What is the difference between a trussed rafter and a cut roof?
A cut roof uses individual rafters and ceiling joists with a ridge board, purlins and intermediate supports — site-built joinery. A trussed rafter is a triangulated factory-built unit that clear-spans the whole house with no internal load-bearing walls. Trussed rafters are 30–40% cheaper in labour, install in a day instead of a week, and free up the ground-floor plan. The trade-off: the loft is filled with webs and unusable for storage unless you specify attic trusses. Approved Document A treats both as equivalent provided the design is signed off.
What's the standard centres for UK trussed rafters?
600 mm centres is the residential standard, set so the standard 1200 × 2400 mm OSB sheet aligns at every other rafter. 400 mm centres are used in heavy snow zones (Scottish Highlands, Pennines) or where the architect wants 9 mm OSB instead of 11 mm. Commercial buildings sometimes use 1200 mm centres with 18 mm decking. Always size your sarking to the centres — BS 5534 Table 8 gives the matrix.
How does heel height affect attic ventilation?
A taller heel (the depth of the truss at the wall plate) keeps the insulation depth continuous over the wall plate without choking the cold-roof ventilation gap. Approved Document C and BS 5250:2021 require a 25 mm continuous airway at the eaves for cold-roof construction, sized at 10 mm continuous or 25 mm at high level for warm pitched roofs. A 200 mm energy heel lets you fit 270 mm of mineral wool over the wall plate with a 25 mm airspace — a code-compliant cold roof. Standard 90–125 mm heels run out of room above the wall plate and force a compromise on insulation depth.

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