Understanding your result
The headline is the number of trusses, split into common and gable-end at the spacing you chose. The stats repeat the split and add the bracing length and the ties. The breakdown shows the division into bays, the extra truss for a short last bay, the lookouts if the rake overhang needs a dropped gable, and the bracing in feet and boards.
The number to remember is the common trusses: that is what the truss company quotes by the piece, with the gable-end trusses priced separately. The last bay row is the layout warning; a short bay belongs at the far end, not in the middle of the roof.
For a stick-framed roof the rafter length calculator is the equivalent page, and the count is the same arithmetic for rafter pairs. The gable ends themselves, and the siding they take, are sized by the gable area calculator.
How we calculate this
The “plus one” is the fence-post rule: twenty bays of 2 feet need twenty-one fence posts. The part-bay rule adds one more when the length divides unevenly, because the last truss must still sit at the end of the building. Gable-end trusses are not extra: the two at the ends are gable trusses instead of commons, so the common count drops by two.
The bracing estimate follows the industry’s building component safety guide (BCSI-B3), which calls for continuous lateral restraint on the bottom chord in rows at about 10-foot intervals: a 30-foot span gets two rows, a 40-foot span three. Web bracing and diagonals are specific to each truss design and come from the drawings, so they are noted rather than counted. The rafter maths guide covers the difference between a trussed and a stick-framed roof and when each is chosen.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Spacing | 24 in on centre (600 mm) | Standard residential truss spacing matched to 4 × 8 sheathing |
| Gable ends | 2 | A gable roof; 0 for a hip roof |
| Dropped gable threshold | rake overhang over 12 in | Framing practice: short overhangs use a ladder; longer ones use lookouts over a dropped gable truss |
| Lateral bracing rows | about 10 ft apart on the bottom chord | BCSI-B3 permanent bracing guidance |
| Bracing boards | 2×4 × 16 ft | Common stock length |
| Lookouts | 2×4 at 24 in on both slopes | Framing practice for rake overhangs |
| Hurricane ties | 2 per truss | Typical connector schedule; the plans govern |
Assumptions last reviewed October 7, 2026.
The calculator does not design the trusses, does not count the step-down and jack trusses of a hip roof’s ends or the girder trusses that carry them, and does not know your code’s connector requirements. The truss manufacturer’s drawings are the authority for all of that; this page gives the count to ask them for. The ridge height calculator gives the height of the trussed roof for the same span and pitch.
Two worked examples
A 40-foot gable building at 24 inches
Length 40 feet, span 30 feet, 24-inch centres, two gable ends, 12-inch rake overhang, bracing rows at 10 feet, 16-foot boards.
- Bays: 40 ÷ 2 = 20, no part bay
- Trusses: 20 + 1 = 21, of which 19 common and 2 gable-end
- Bracing: round up (30 ÷ 10) − 1 = 2 rows × 40 ft = 80 ft, 5 boards of 16 ft
- Ties: 42
With a 12-inch rake overhang the gable trusses stay full height and a ladder frame carries the overhang. At 24 inches the calculator would drop them and add 60 lookouts.
A 37-foot hip-roofed building at 16 inches
Length 37 feet, span 26 feet, 16-inch centres, no gable ends.
- Bays: 37 ÷ 1.333 = 27.75
- Trusses: 27 + 1 + 1 for the 12-inch part bay = 29, all common
- Bracing: round up (26 ÷ 10) − 1 = 2 rows × 37 ft = 74 ft, 5 boards
- Ties: 58
Twenty-nine common trusses along the ridge; the hip ends add step-down and jack trusses that the truss company lays out from the plan. Lay out from one end so the 12-inch bay lands at the far end.
Where to find your inputs
Building length. Along the ridge, outside of wall to outside of wall, from the plans or a tape at the top plate.
Span. Across the building, outside to outside; it sets the bracing rows and is what the truss company needs for the quote.
Spacing. From the plans. If none is given, 24 inches is the default and the sheathing thickness must suit it (7/16-inch OSB is rated for 24-inch supports).
Rake overhang. How far the roof extends past each gable wall, from the plans or the existing roof.
Common mistakes
- Forgetting the plus one. Twenty bays need twenty-one trusses. The division gives bays, not trusses.
- Adding the gable trusses. They replace the end commons. Nineteen plus two is twenty-one, not twenty-three.
- Ignoring the part bay. A length that divides unevenly needs one more truss and a layout that puts the short bay at the end.
- Putting the short bay in the middle. Sheathing sheets then land between trusses. Start the layout from one end.
- No bracing in the order. The trusses arrive with drawings that call for lateral and diagonal bracing; the 2×4s are not included.
- Counting hip ends as common trusses. The hip ends use step-down, jack and girder trusses; this count is the commons only.
Questions people ask
- How many trusses do I need for a 40-foot building?
- At 24-inch centres, 40 ÷ 2 = 20 bays, so 21 trusses: one at each end and one every 2 feet between, of which 19 are common trusses and 2 are gable-end trusses. At 16-inch centres it is 31, at 19.2 inches 26, at 12 inches 41. The calculator adds one more when the length is not a multiple of the spacing.
- What spacing are roof trusses?
- Residential trusses are nearly always at 24 inches on centre, which suits 4 × 8 sheathing and most ceiling materials; 16 inches is used where the ceiling finish or a heavy roof needs it, and 19.2 inches (five bays per 8 feet) is an in-between option. In metric practice 600 mm is the equivalent of 24 inches and 400 mm of 16. The truss manufacturer designs for the spacing you specify.
- What is a gable-end truss?
- A truss with vertical studs instead of diagonal webs, set at the end of the building so the gable can be sheathed and sided like a wall. It replaces the common truss at that position rather than adding to the count. When the rake overhangs more than about 12 inches, the gable-end truss is made lower (dropped) so 2×4 lookouts can run over it to carry the barge rafter.
- What bracing do trusses need?
- Permanent bracing, as set out in the truss drawings and the industry guide BCSI-B3: continuous lateral restraint along the bottom chord in rows roughly 10 feet apart (so two rows on a 30-foot span), diagonal bracing in the web plane at intervals, and restraint on any web the drawings mark. A 40-foot building with two rows takes 80 feet of 2×4, five 16-foot boards, plus the diagonals.
- Do trusses need hurricane ties?
- Most codes require a positive connection between each truss and the wall plate beyond toe-nails, and in wind regions a metal tie on each side of every truss; two per truss is the usual count, 42 on a 21-truss roof. The truss drawings or the plans specify the connector and its nails.
- What if the building length is not a multiple of the spacing?
- There is a short bay at one end. A 37-foot building at 16 inches has 27 full bays and a 12-inch bay, so 29 trusses rather than 28. Lay the trusses out from one end so the full bays fall under the sheathing sheets and the short bay lands at the far gable, where it is covered by the gable-end truss and the sheathing is cut anyway.