Roof Truss Calculator
Length, span, spacing and pitch in. Truss count and top chord length out, so you can sanity check a supplier's quote before you sign it.
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Common trusses only. Gable ends, girders and any hip set are extra, and a truss layout has to be engineered.
Talk to us about the buildHow the count works
Trusses sit at a fixed spacing along the building, and you need one at each end as well as every one in between. So the count is the length in inches divided by the spacing, rounded down, plus one. A 40 foot building at 24 inches on center takes 21 trusses, not 20.
Top chord length comes from the span and the pitch. Half the span is the run, the pitch gives the rise, and the chord is the hypotenuse of the two, plus the overhang measured along the slope.
What a calculator cannot do
Truss layouts are engineered. The fabricator stamps a drawing for your specific loads, span and connection points, and that drawing is what the inspector looks at. Use these numbers to check a quote or budget a frame, not to order timber.
Where it gets more complicated
Gable end trusses are built differently from common trusses even though they span the same distance. Girder trusses carry other trusses and are usually doubled or tripled. Any hip, valley or dormer brings a whole set of stepped-down trusses that no single figure covers.
Working out the roof surface above the frame? The roofing square calculator converts footprint and pitch into squares and bundles, and the pitch calculator handles the slope factor on its own.
Common questions
How many trusses do I need?
Divide the building length in inches by the spacing, round down, then add one for the truss at the far end. At 24 inches on center a 40 foot building takes 21.
What spacing do roof trusses use?
Twenty-four inches on center is standard for residential trusses. Sixteen and 19.2 show up where the loads are heavier or the sheathing is thinner, and the engineered drawing is what settles it.
How do I work out the top chord length?
Half the span is the run and the pitch gives the rise. The chord is the square root of run squared plus rise squared, then add the overhang measured along the slope rather than horizontally.
Are trusses cheaper than rafters?
On a straightforward roof usually yes, because they go up fast and use less timber for the same span. Rafters win where you want a vaulted ceiling or usable attic, since a standard truss fills that space with webs.
Can I cut or modify a truss?
No. Cutting a web or chord takes the engineering out of it, and the fix is a repair detail from the fabricator's engineer. This is the single most common thing that fails an inspection on a truss roof.
Do I need an engineer for a truss roof?
The truss manufacturer supplies sealed drawings for the layout, which covers the trusses themselves. Anything bearing on them, like a girder or a load path down to a footing, is a design question for the project engineer.
Talk to us about your roof frame
We get on the roof, photograph what is actually there, and hand you a written report with a price. No charge, and no obligation to book the work with us. If the roof has years left in it, that is what the report will say.
Give us a call or fill out our form today to schedule your free inspection!
Whether you need a roof replacement, roof repair, or annual inspection, we’ve got you covered.