Roof Truss Calculator — Quantities & Spacing

Our roof truss calculator determines the number of trusses needed based on your roof length and the standard 600mm spacing used in UK domestic construction. It also accounts for gable trusses at each end.

The formula

Total trusses = (Roof length ÷ 0.6m) + 1
Gable trusses = 2 (the two end positions, one per gable end)
Standard trusses = Total − Gable trusses

The +1 means the count includes a truss at each end of the roof, so a 10m roof gives (10 ÷ 0.6) + 1 = 18 total. The gable trusses occupy those two end positions rather than being added on top — they carry the gable wall — so 18 total is made up of 16 standard + 2 gable (per MiTek’s UK Trussed Rafter Handbook and Trussed Rafter Association guidance). For mono-pitch roofs only one gable truss is typically needed.

Truss types

• Fink (W): Most common domestic truss, suitable for spans up to ~11m. Internal W-shaped web members.
• Attic / Room-in-Roof: Designed to create usable space in the loft. Heavier and more expensive.
• Mono-Pitch: Single slope, used for lean-to extensions, porches and garages.
• Raised Tie: Bottom chord raised to give more headroom in the loft space, useful for storage.

Important notes

All trusses must be designed by a qualified truss manufacturer or structural engineer. They are made to order based on your specific span, pitch, loading (tile type, snow load, wind load) and any special requirements. This calculator gives an estimate of quantities only — always confirm with your truss supplier.