Roof Truss Calculator

Roof truss calculator

A roof truss calculator counts the trusses a roof needs along the length of a building. The count is ceiling(length in inches ÷ spacing in inches) + 1. A 40 ft building with trusses 24 inches on center has 20 gaps, so it needs 21 trusses.

It is a quick count for budgeting and planning. The truss manufacturer produces the real layout.

The formula in plain words

1. Building length in inches = feet × 12.
2. Gaps = length in inches ÷ spacing in inches, rounded up.
3. Trusses = gaps + 1.

The +1 is the truss at the starting end. Every gap has a truss at its far side, so n gaps need n + 1 trusses.

Worked examples

A 40 ft building at 24 in on center, the default:

• 40 × 12 = 480 in
• 480 ÷ 24 = 20 gaps
• 20 + 1 = 21 trusses

A 30 ft garage at 16 in on center:

• 30 × 12 = 360 in
• 360 ÷ 16 = 22.5, rounded up to 23 gaps
• 23 + 1 = 24 trusses
• The last bay is 8 in narrower than the rest, so the supplier may adjust the spacing to even it out

A 41 ft building at 24 in:

• 492 ÷ 24 = 20.5, rounded up to 21 gaps
• 21 + 1 = 22 trusses
• The 22nd truss closes the end wall, and the last bay is 1 ft wide instead of 2 ft

Truss counts by length and spacing

Building length	16 in	19.2 in	24 in
20 ft	16	14	11
30 ft	24	20	16
40 ft	31	26	21
50 ft	39	33	26
60 ft	46	39	31

Tighter spacing means more trusses, but each one may be lighter. The designer balances truss size, spacing, roof loads and sheathing thickness.

Which spacing to enter

Use the spacing on your plans. If the plans do not say, ask your truss supplier. Common choices in house building are 16 in, 19.2 in and 24 in. Wider spacing is common for roof trusses, and tighter spacing may be required in heavy snow areas.

Special trusses not in this count

This count covers evenly spaced common trusses. Real roofs add:

• Hip girders and jack trusses on hip roofs.
• Valley framing where roof planes cross.
• Dormer and porch trusses.
• Scissor or attic trusses if the plan calls for them. The attic truss calculator covers those.
• Extra trusses under a heavy load, such as a water heater or HVAC unit in the attic.

How to measure

• Measure the building length along the wall that runs parallel to the ridge, outside to outside.
• Include any overhang only if the trusses extend over it.
• For an L-shaped house, count each wing as a separate gable roof.

The span, meaning the width the truss crosses, is a separate dimension that the supplier needs. You do not enter it here.

Common mistakes

• Using the span instead of the length.
• Forgetting the +1.
• Forgetting that the last bay is narrower when the length does not divide evenly.
• Forgetting special trusses for hips, valleys and dormers.
• Using the count to design the roof. A truss package is engineered for your loads.

Safety

Trusses are engineered parts. Never cut or notch them, and brace them as the manufacturer instructs while you set them, because a truss that is not braced can fall over during installation. Lifting trusses overhead is a job for an experienced crew with the right equipment.

Related calculators

The truss cost calculator prices a package, the roof pitch calculator finds the slope, and the roof area calculator gives the sheathing and roofing area. If you frame with rafters instead of trusses, use the rafter calculator.