What this calculator does
Truss count is the building length divided by the spacing, plus one for the closing end. A 12 metre building at 24 inch centres needs 21 trusses.
The plus one is the same rule that governs fence posts and wall studs: a run of twenty bays needs twenty-one members. It is the most commonly forgotten term in any spacing calculation, and on a truss order it is an expensive omission because a single truss is not a stock item.
The formula
The building length is converted to feet, multiplied by twelve and divided by the spacing in inches, rounded up to give the bays, with one added for the final truss. Spacing can be entered in inches or millimetres. The count covers common trusses only, not girder trusses, gable end frames or any hip assembly.
| Term | Meaning |
|---|---|
| On-centre spacing | Truss centres. 24 inches or 600 mm is standard for domestic roofs. |
| Common truss | The standard repeating truss. Special trusses at ends and openings are extra. |
| Girder truss | A heavier truss carrying others, used where a roof changes direction. |
| Gable end frame | The vertical frame at each end, which is not a standard truss. |
The inputs explained
| Field | What to enter |
|---|---|
| Building length (m) | Building length in metres, along the direction the trusses repeat. |
| On-centre spacing | On-centre spacing. 24 inches and 600 mm are the common standards and are nearly identical. |
| Spacing unit | Whether the spacing is in inches or millimetres. |
When to use it
Ordering roof trusses
Trusses are made to order, so getting the count right before the order goes in matters.
Comparing spacings
Tighter spacing means more trusses but lighter members and thinner roof battens.
Estimating a roof budget
Trusses are a significant line item and the count drives it directly.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How many trusses for each building length?
A range of building lengths at standard spacing.
| Building length | Trusses needed | Building length | Spacing used |
|---|---|---|---|
| 8 m | 15 | 26.25 ft | 24.00 in on-centre |
| 12 m | 21 | 39.37 ft | 24.00 in on-centre |
| 20 m | 34 | 65.62 ft | 24.00 in on-centre |
Questions
What spacing do roof trusses use?
24 inches or 600 mm on centre is standard for domestic roofs, and the two are close enough to be interchangeable in practice. 16 inch or 400 mm spacing is used for heavier loads or thinner roof battens, at the cost of half again as many trusses.
Why one more truss than the bays?
Because a run needs a truss at both ends, and adjacent bays share the trusses between them. Twenty bays means twenty-one trusses. It is the same arithmetic as fence posts, and forgetting it is the classic estimating error.
Does this include gable end frames?
No. The gable ends are usually vertical frames rather than standard trusses, and hip roofs need a whole assembly of shortened trusses plus a girder. This counts common trusses only, which is the repeating bulk of a simple gable roof.
Can I change the spacing to save money?
Within limits set by the roof covering and the batten span. Wider spacing means fewer trusses but needs heavier battens and may exceed what the roof material supports. The truss manufacturer designs to the spacing, so it is settled before the order rather than on site.
For the roof geometry, see the roof pitch calculator. For shingle quantities, see the roof shingle calculator.