How many roof trusses do I need?
Divide the building length by the on-centre spacing, round up, then add 1 for both end walls. A standard 24 ft building with trusses at 24" OC needs 13 roof trusses — 12 bays plus 1 end truss.
Skip the arithmetic: use the roof truss calculator → Enter the building length and spacing — get the count instantly.
The formula
The number of roof trusses for a building of length L with on-centre spacing S is:
Trusses = ⌈ L ÷ S ⌉ + 1
The ceiling function (⌈ ⌉) rounds up to the next whole number — you cannot order a fraction of a truss. The "+1" accounts for both end walls: a roof with 12 bays (12 spaces between trusses) needs 13 trusses.
Worked example — 24 ft building at 24" OC
- Building length: 24 ft
- Spacing: 24" = 2 ft
- Bays: ⌈ 24 ÷ 2 ⌉ = ⌈ 12 ⌉ = 12
- Trusses: 12 + 1 = 13 trusses
- Last bay: 24 − (11 × 2) = 24 − 22 = 2 ft (exact multiple, no short bay)
Common truss counts — imperial
Quick reference for common building lengths and spacings.
| Building length | Spacing | Trusses |
|---|---|---|
| 20 ft | 24" OC | 11 |
| 24 ft | 24" OC | 13 |
| 30 ft | 24" OC | 16 |
| 40 ft | 24" OC | 21 |
| 24 ft | 16" OC | 19 |
| 30 ft | 16" OC | 24 |
What affects how many trusses you need?
- Truss spacing
- The most common spacing in North America is 24" on-centre (610 mm). Heavier roofing materials — concrete or clay tiles — often require 16" OC (406 mm) to limit deflection. In the UK, Australia and New Zealand, 600 mm OC is standard.
- Building length
- The length measured along the ridge (ridge length), not the building's overall footprint depth. For a simple gable roof, this is the length of the building parallel to the ridge.
- Hip roofs and valleys
- Hip roofs and intersecting roof planes require additional specialised trusses — hip sets, valley sets, and jack trusses — beyond the standard count. A structural engineer or specialist truss supplier should design these layouts.
- Span and load
- Longer spans or heavier loads (snow, rooftop equipment, attic storage) may require closer spacing or deeper truss profiles. Spans over roughly 33 ft typically need engineered truss design with a structural review.
Common questions
- How many roof trusses do I need?
- Divide the building length by the on-centre truss spacing, round up to the next whole number, then add 1 for the second end wall. For a 24 ft building at 24" OC: ⌈24 ÷ 2⌉ + 1 = 13 trusses. For a 7 m building at 600 mm OC: ⌈7 ÷ 0.6⌉ + 1 = 13 trusses.
- What is the standard roof truss spacing?
- In North America, 24" on-centre (610 mm) is standard for most residential roofs. 16" OC is used for heavier loads or longer spans. In the UK, Australia and New Zealand, 600 mm OC is the common standard. Always follow the spacing on your approved structural drawings.
- Why do I add 1 to the number of bays?
- A roof with N bays (spaces between trusses) always needs N + 1 trusses — one at each end wall plus one at every intermediate point. Think of fence posts and fence panels: 3 panels need 4 posts.
- Do end trusses count the same as interior trusses?
- Yes, for ordering purposes you count all trusses including the two end trusses. In some designs the end trusses are gable-end frames rather than standard pitched trusses, but you still need to order the same number of structural members.
- When do I need an engineer for roof trusses?
- Manufactured trusses always require engineered drawings — the truss manufacturer produces these. For spans over roughly 10 m (33 ft), complex roof geometry (hip roofs, attic trusses, valley sets), or unusual loading conditions, a structural engineer should review the design. Your local building authority will confirm what sign-off is needed.
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-08.