Truss and Rafter Count Calculator
Last reviewed How we calculate
Trusses and rafters are set at a fixed spacing along the length of the roof, usually 16 in or 24 in apart. This counts how many a roof needs, which is useful for checking a framing quote or a truss order.
Trusses or rafters for a roof
How many trusses or rafters a roof needs at 16, 19.2 or 24 inch spacing.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Trusses or rafters needed
- Positions along the roof Including one at each end.
- Spaces between them
A count, not a design. Trusses are engineered by the truss maker for your spans and loads, gable-end trusses included, and rafter sizes come from your span tables. Doubled members around skylights, chimneys and other openings are extra.
How the calculation works
Divide the length of the roof by the spacing and round up to get the number of spaces, then add one, because there is a truss at each end as well as between them. Rafters come in pairs, one each side of the ridge, so a rafter roof needs twice as many members as there are positions.
A worked example
A 40 ft roof at 24 in spacing has 20 spaces and 21 positions, so it takes 21 trusses, or 42 rafters. At 16 in spacing the same roof takes 31 trusses.
A count, not a design
Trusses are engineered by the truss manufacturer for your span, pitch and loads, and they arrive with drawings that show where each one goes, including any special gable-end or girder trusses. Rafter sizes come from the span tables in your building code. Openings such as skylights and chimneys need doubled members around them, which are extra to this count.
Frequently asked questions
Why is there one more truss than the number of spaces?
Because there is a truss at each end as well as between them. A 40 ft roof at 2 ft spacing has 20 spaces and 21 trusses, like the posts of a fence.
Should trusses be 16 or 24 inches apart?
Trusses are commonly set at 24 inches and rafters at 16 or 24, but the truss or framing design decides. The truss manufacturer drawings show the spacing each truss was designed for.
Does the count include the gable-end trusses?
Yes, the positions at each end are counted. Gable-end trusses are often a different design from the rest, so they are ordered as a separate item.