How this roof truss calculator works
Trusses repeat down the building like joists or studs, so the count is the same rule — the ridge length divided by the spacing, plus one — with the two ends being gable trusses:
common = trusses − 2 · gable-end = 2
At the defaults — a 40 ft building at 24 in on center — that is 21 trusses, split 19 common plus 2 gable-end, about $2,520; the full chain is in the worked example. Trusses are engineered to span the width — this sizes the count, not the truss.
Common, gable, and the rest of the roof
A trussed roof is a kit of repeating frames plus the pieces that tie them together:
- Common trusses: the interior frames that carry the roof and repeat on center — most of the count.
- Gable-end trusses: the two end frames, studded to bear on the end wall and carry the siding.
- Bracing: permanent lateral and diagonal bracing that keeps the trusses plumb and stable.
- Sheathing: the deck over the trusses — sized on the roof sheathing calculator.
- Roofing: shingles or panels on top — the roofing calculator prices the whole cover.
Truss spacing and what sets it
Spacing is a design decision, but the common cases are few:
- 24 in on center: the residential standard — suits ½ in sheathing and keeps the count down.
- 19.2 in: five per eight feet, in some engineered layouts.
- 16 in: heavy loads, tile roofs, or thinner decking.
- Load and snow: higher design loads push toward tighter spacing or beefier trusses.
- The truss designer governs: the sealed truss layout sets spacing and bracing — this is a planning count.
There is one piece the straight count doesn’t show: the two ends are a different animal. A gable-end truss is studded vertically to bear on the wall below and carry the siding, not to span, so it is quoted and priced on its own line — and a plan with extra gables, like an L-shaped house or a porch return, needs one pair per gable. Girder trusses that carry other trusses at a hip or valley are separate again, engineered to a point load. Read the number here as the field of common trusses plus the two standard gable ends, and add any specials straight off the roof plan.
Truss count by building length, and spacing options
Table 1 runs 24 in on center out to common building lengths; Table 2 is what each spacing is for.
| Building length | Trusses |
|---|---|
| 20 ft | 11 |
| 30 ft | 16 |
| 40 ft | 21 |
| 50 ft | 26 |
| 60 ft | 31 |
| On-center | Used for |
|---|---|
| 16″ | Heavy load, tile, thin deck |
| 19.2″ | Some engineered layouts |
| 24″ | The residential standard |
How to count trusses
- Measure the building length along the ridge — the direction the trusses march down.
- Pick the spacing — 24 in for most roofs — from the truss design or the spacing guide.
- Read the truss count, and note the two gable-end trusses are ordered separately.
- Confirm the span and pitch with the truss supplier — they engineer the truss itself.
- Add bracing, sheathing and hangers from the exclusions below.
Worked example: a 40 ft building
The default, worked line by line — a 40 ft-long building at 24 in on center, $120 a truss:
trusses = 20 + 1 = 21
split = 19 common + 2 gable-end
cost = 21 × $120 = $2,520
The plus-one catches people out: 20 spaces means 21 trusses, the same way a 40 ft fence has one more post than sections. Go to 16 in spacing for a snow or tile load and the count jumps to 31 — half again as many frames, and a bigger crane bill to set them.
What this truss estimate doesn’t include
The count is the trusses; a roof needs the pieces around them:
- Bracing. Permanent lateral and diagonal bracing, plus temporary bracing during the set.
- Hangers and connectors. Hurricane ties and truss clips at every bearing.
- Sheathing and roofing. The deck and the cover over the trusses.
- Delivery and crane. Truss delivery and the crane or boom truck to set them.
- Overhangs and specials. Extended tails, valley sets, and any girder trusses.
- Engineering. The sealed truss design — this sizes the count, not the truss.
Roof truss FAQ
How many roof trusses do I need?
Trusses = the building length ÷ spacing, rounded up, plus one to close the run. A 40 ft building at 24 in on center takes 21 trusses — 19 common trusses plus 2 gable-end frames. Tighten the spacing or lengthen the building and the count climbs.
How far apart are roof trusses?
24 in on center is the standard residential spacing — it suits ½ in roof sheathing and keeps the truss count down. 16 in is used for heavy loads, tile roofs, or thinner decking; 19.2 in (five per eight feet) shows up in some engineered layouts. The truss designer sets it for the load.
How many trusses for a 40 foot building?
At 24 in on center, a 40 ft-long building is 21 trusses — 480 in ÷ 24 is 20 spaces, plus the closing truss — of which the two ends are gable trusses. At 16 in on center the same building would take 31. The trusses span the building width; the length sets how many.
How many trusses for a 30x40 house?
It depends which way the ridge runs. Trusses span the width and are spaced along the length, so with the ridge along the 40 ft side you need 21 trusses spanning 30 ft; with the ridge along the 30 ft side, 16 trusses spanning 40 ft. Set the building length in the calculator to the ridge direction.
What is the difference between a common and a gable-end truss?
A common truss is the interior triangulated frame that carries the roof load and repeats across the building. A gable-end truss sits at each end and looks like a common truss with vertical studs filling it, since it bears on the end wall and carries the siding, not the roof span. The two ends are always gable trusses.
How much do roof trusses cost?
Common residential trusses run roughly $100 to $400 each in 2026 depending on span, pitch and load — the default $120 is a mid-size gable truss. Long-span, high-pitch, or attic and scissor trusses cost more. Delivery and a crane or boom truck to set them are separate.
Can roof trusses be spaced 24 inches apart?
Yes — 24 in on center is the standard, and roof sheathing and shingles are rated for it. It uses about a third fewer trusses than 16 in spacing, which is why it is the default for stick-framed and trussed roofs alike. Heavier loads or thinner decking are what push a design back to 16 in.
Sources and standards
The count is arithmetic; the truss itself is engineered:
- TPI / SBCA: the truss-plate and component standards behind the engineered design.
- Standard spacing: 24 in on center for residential roofs, 16 and 19.2 for heavier loads.
- IRC bracing: the permanent bracing requirements that keep a trussed roof stable.
- The sealed layout governs: the truss engineer’s drawing sets spacing, span and bracing.
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