Roof Truss Calculator
Calculate how many roof trusses you need for a building from its length and truss spacing on center.
How this calculator works
Roof trusses are placed at a set spacing along the length of a building, measured center to center. To find the total count, convert the building length to inches, divide by the spacing in inches, and add one truss for the starting end — since a truss sits at both ends of the run, not just in the gaps between them.
Formula: Truss count = floor(building length in inches ÷ spacing in inches) + 1 (one truss at each end plus the ones in between).
Worked example
A 40 ft building with trusses spaced at 24 in on center:
- 40 ft × 12 = 480 in
- 480 ÷ 24 = 20
- 20 + 1 = 21 trusses
Notes
24 in on center is the most common truss spacing used on residential roofs, which is why it’s a typical default when estimating.
This count covers standard common trusses only. A gable roof also needs gable-end trusses, which are usually specified and counted separately by the truss supplier rather than included in a straight length-and-spacing count.
Truss engineering — member sizes, connector plates, and load design — is handled by the truss manufacturer based on your specific roof, not by this calculator.
How to use
Enter your building’s length and the truss spacing you’re planning to use (commonly 24 in on center for residential roofs). The calculator returns the number of common trusses needed along that run, which you can use as a starting point when getting quotes from a truss supplier.
This counts standard trusses by spacing only. Truss engineering and design load are the truss manufacturer’s responsibility — confirm your building’s actual truss count and design with your supplier.
Frequently asked questions
Why does the count add 1 to the division result?
Because a truss sits at both ends of the building, not just spaced out in between. If you only divided the length by the spacing, you'd count the gaps between trusses instead of the trusses themselves. Adding 1 accounts for that starting truss at the first wall, matching how framers actually lay out a truss roof.
What is truss spacing 'on center' measuring?
On center means the distance from the centerline of one truss to the centerline of the next, not the gap between their edges. This is the standard way truss spacing is specified because it lines up with how sheathing and other materials are measured and fastened across the roof.
Why isn't this count exact for every roof style?
This calculator counts standard common trusses spaced evenly along a simple building length. Roofs with hips, valleys, dormers, or other breaks in the roofline need additional specialty trusses that aren't part of this straight-line count, so treat the result as a baseline for a simple gable or shed roof.
Do gable ends need extra trusses beyond this count?
Yes, typically. A gable roof needs gable-end trusses (or a conventionally framed gable end) in addition to the common trusses spaced along the building's length. These are usually specified and counted separately by your truss supplier when they engineer the full package.
Estimates only. Verify quantities with your supplier before purchasing.