How many roof trusses do I need?
Enter your building length and on-centre truss spacing. The calculator gives you the number of roof trusses to order — instantly, no sign-up.
Last bay: 1.99 ft · 24 ft building
Formula: trusses = ⌈ length ÷ spacing ⌉ + 1. Both end walls counted. Confirm spacing with your structural engineer — spans over 10 m (33 ft) typically need engineered trusses with a structural review.
The roof truss formula
Trusses = ⌈ building length ÷ spacing ⌉ + 1. Round up before adding the end truss — you cannot order a fraction of a truss. The "+1" accounts for both end walls: a roof with 12 bays needs 13 trusses.
Want the full working? Read the roof truss guide →
Common truss spacing reference
| Spacing | Typical use | Trusses per 10 m (33 ft) |
|---|---|---|
| 24" OC (610 mm) | Standard residential NA | 18 |
| 16" OC (406 mm) | Heavy loads, long spans | 26 |
| 600 mm OC | Standard residential UK/AU/NZ | 18 |
| 1200 mm OC | Engineered / commercial | 10 |
Common questions
- How many roof trusses do I need?
- Divide the building length by the on-centre truss spacing, round up, then add 1 for the second end wall. For example: a 24 ft building with 24" OC spacing needs ⌈24 ÷ 2⌉ + 1 = 13 trusses. A 7 m building at 600 mm OC needs ⌈7 ÷ 0.6⌉ + 1 = 13 trusses. The calculator above does this automatically.
- What is standard roof truss spacing?
- 24" on-centre (610 mm) is the standard spacing for most residential roofs in North America. 16" OC is used for heavier loads or longer spans. In the UK, Australia and New Zealand, 600 mm OC is the common standard. Your engineer or building designer will specify the required spacing for your design loads.
- How far apart should roof trusses be?
- For most residential roofs, trusses are spaced 24" (610 mm) on-centre. Heavier roofing materials such as concrete or clay tiles may require 16" (406 mm) OC to keep deflection within code limits. Always follow the spacing specified on your approved truss drawings.
- Do I include the end trusses in my count?
- Yes — the formula already includes both end walls. A roof with N bays (spaces between trusses) always needs N + 1 trusses. The calculator counts both end trusses automatically.
- When do I need an engineer for roof trusses?
- Engineered truss drawings are required for all manufactured trusses in most jurisdictions — the truss manufacturer typically supplies them. For spans over roughly 10 m (33 ft), or any complex roof geometry (hips, valleys, attic trusses), a structural engineer should review the truss layout and connections.
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.