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The length of the building run the trusses are spaced along.
The run the trusses march along, which is perpendicular to the direction they span — the gable-to-gable dimension on a simple roof. A hip end changes it: the trusses stop short of the hip and the corner is framed differently, so the run for spacing purposes ends where the standard trusses end and not at the building's corner.
The on-center spacing between trusses, per your design (commonly 24 in / 610 mm for roof trusses).
Centre to centre, from the truss design rather than from habit — spacing is a design output that follows from the loads, the span and the truss's own capacity, and 600 mm is a common answer rather than a rule. It also has to suit whatever spans between the trusses: battens, sheathing and any ceiling below all have their own limits, and the smallest of those governs.
How many of the counted positions are gable ends: 2 on a gable roof, 0 on a hip roof.
A gable end stands at the first and last position, so it is one of the counted trusses, not an extra; the breakdown splits it out because it is a different frame (studded under its top chord for the sheathing), ordered as its own line. Each gabled wing adds one more at its end.
How many runs of continuous lateral bracing the truss drawings call for; two is an example.
Each run is a line of timber along the roof, square to the trusses, fixed to the webs or chords the truss maker marks. The page gives their lineal total at the building's length; how many, where and the diagonal bracing that stiffens them are the truss design's.
From a wing's end wall to the main roof's wall line, for an L, T or U-shaped house.
A wing's trusses march along its own length, one at every spacing from its end wall inward; where the wing meets the main roof the valley set over the main trusses is the truss maker's layout and is not counted.
A second wing's length to the main wall line; 0 for none.
Measured like the first wing.
Trusses needed
21 trusses
- Whole truss bays along the run
- 19
- Short bay left at one end
- 12 in
- Common trusses
- 19 trusses
- Gable-end trusses
- 2 trusses
- Lateral bracing, lineal
- 78 ft
They open the calculator with your figures already in it
Roof Truss Spacing & Count Calculator: 21 trusses — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Truss count = ceil(building length ÷ spacing) + 1, the standard method for spacing repetitive framing members along a run
Inputs used
- Building Length
- 39 ft
- Truss Spacing (o.c.)
- 24 in
- Gable-End Trusses Among Them
- 2
- Lateral Bracing Lines
- 2
- First Wing's Length (0 for none)
- 0 ft
- Second Wing's Length (0 for none)
- 0 ft
Intermediate steps
- Whole truss bays along the run
- 19
- Short bay left at one end
- 12 in
- Common trusses
- 19 trusses
- Gable-end trusses
- 2 trusses
- Lateral bracing, lineal
- 78 ft
What this calculation does not cover
- The length asked for is the run the trusses are spaced along, not the distance each truss spans. Entering the building width instead returns a count that looks perfectly reasonable but belongs to a roof turned ninety degrees, and nothing in the result flags the swap.
- The short bay above is the whole remainder, pushed into one end rather than eased along the run: an 11.8 m run at 24 in centres is nineteen full bays and about 220 mm left. Which end it lands on is the part still missing, and it is a real decision — a gable end, a girder truss or a hip set can each require the odd bay at one particular end, and the page does not know which you have.
- Spacing is treated as an instruction and never tested. The load carried by each truss rises in proportion to its centres, so a truss certified at 610 mm under a light covering is not automatically valid at those centres under concrete tile, and the page will count members at any spacing from 305 mm to 610 mm without reference to the truss design, span or snow and wind loading it belongs to.
- The trusses are counted, and the lateral bracing as runs the length of the roof at the number of lines entered. The rest of the stability bracing the truss designer specifies, the diagonal rafter bracing and the chevron web bracing, and the truss clips, wall-plate straps and hangers, are a separate timber and metalwork take-off.
- A wing's trusses are counted from its end wall to the main roof's wall line; the valley set framed over the main trusses where the wing meets them, and any girder carrying them, are the truss maker's layout.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Part of bigger jobs
This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.
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Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-11 · v1.0.1
Regulatory standards & verification citations1
- Truss count = ceil(building length ÷ spacing) + 1, the standard method for spacing repetitive framing members along a run
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Tools and safety for this job
To frame the roof structure. Generic types, no brands, no prices.
Protection this work requires
- Hardwood dust is a recognised carcinogen: extract at the tool and wear a P2/N95 respirator when cutting or sanding indoors.
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
Frame the roof structure: Rafters, joists, purlins and trusses, and the ties and straps that hold them, are sized by an engineer, a truss maker or accepted span tables, and this site never sizes them. A count of rafters or trusses at a chosen spacing is a quantity to order against; what each member must be is not something a count can say.
Show the 6 tools this job needsHide tools
Essential
Circular saw
Combination or framing square
Tape measure
Recommended
Framing nailer
String line and pins
Tower scaffold or trestles
Also needed as materials: circular saw blades, framing nails, fuel cells.
what each carpentry tool is for, and the spec that decides which to buy where one does.