Roof Truss Calculator

A roof truss calculator returns how many trusses a roof needs at a given spacing. It converts the roof length to inches, divides by the 16 in. or 24 in. on-center spacing, rounds up, and adds one truss for the starting end. A 30 ft roof at 24 in. spacing needs 16 trusses.

Last reviewed September 2026Method & sources Report an error

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Trusses Needed

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Roof Trusses16 trusses

Breakdown
Trusses Needed	16

How to Calculate Roof Truss

Trusses are spaced along the roof's length (the direction running along the ridge) at a fixed on-center interval, most commonly 16 or 24 in. The same count formula used for framing studs and rafters applies. Divide the roof length (converted to inches) by the spacing, round up, and add one for the truss at the starting end.

Example: 30 ft long roof, 24 in. truss spacing

A 30 ft long roof, 24 in. truss spacing: Truss count: ceil(30 × 12 ÷ 24) + 1 = ceil(15) + 1 = 16 That roof needs 16 trusses.

Roof Truss: Compare Scenarios

Scenario	Trusses Needed
Small (20 ft roof length)	11
Medium (30 ft roof length)	16
Large (60 ft roof length)	31

What Is Roof Truss?

A roof truss calculator converts a roof's length and chosen truss spacing into the number of prefabricated trusses needed. It is pure count-by-spacing geometry, not a structural truss design (span, pitch, load rating). That design is an engineered product decision made with your truss manufacturer, not a formula this calculator applies.

Roof Truss: Pros and Cons

Pros

Uses the same reliable on-center spacing formula this site already uses for framing studs and rafters
Works for both common residential truss spacings (16 in. and 24 in.)
A quick material-list starting point before finalizing an order with a truss manufacturer

Cons

Doesn't include the gable end truss, which is typically a different, flat-profile truss than the standard repeating trusses. Check with your truss manufacturer on whether it's counted separately from this figure
Doesn't design the truss itself (span, pitch, chord size, webbing pattern, load rating). Trusses are an engineered, manufactured product specified with a truss company, not calculated from a simple formula
Doesn't account for extra trusses needed at a hip, valley, or other roof-shape complexity beyond a simple gable roof

Roof Truss: What a Pro Checks

Confirm with your truss supplier whether the gable end truss is counted separately from this figure. Many gable roofs use a different, flat-profile truss at each end in addition to the regularly-spaced trusses counted here.
This only counts trusses by spacing. The truss's actual span, pitch, chord size, and load rating is an engineered decision made with your truss manufacturer, not a formula.
24 in. on-center spacing is common for trusses (versus 16 in. for stick-framed rafters) since engineered trusses can typically span the wider spacing without extra intermediate support.

Frequently Asked Questions

How many roof trusses do I need?

Divide your roof's length (in inches) by the truss spacing, round up, and add one for the starting end. A 30 ft roof at 24 in. spacing needs 16 trusses.

Which is more common, 16 in. or 24 in. truss spacing?

24 in. on-center is very common for residential roof trusses, since trusses are engineered structural units (unlike stick-framed rafters) that can typically span the wider spacing without the extra intermediate support stick framing would need.

Does this calculator design my roof truss for me?

No. It only calculates how many trusses fit your roof's length at a given spacing. The truss's actual design (span, pitch, chord and web member sizing) is engineered by a truss manufacturer based on your roof's span, pitch, and local load requirements.

Do I need an extra truss for the gable end?

Often yes. Many gable roofs use a flat, ladder-style gable end truss at each end, in addition to (or sometimes counted within) the regularly-spaced trusses this calculator counts. Confirm your specific roof design with your truss supplier.

What's the difference between roof trusses and rafters?

Trusses are pre-engineered, factory-built triangulated structures that typically span the full roof width without interior load-bearing walls. Rafters are individual framing members cut and assembled on site, usually needing a ridge beam or interior bearing wall for support. See the Rafter Length calculator for stick-framed rafter sizing instead.

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