Roof Truss Calculator

This calculator gives the two numbers you need before phoning a truss supplier: how many trusses the building takes, and what shape they are. Enter the building length and the on-centre spacing for the count, and the span, pitch and overhang for the run, rise, roof angle, top and bottom chord lengths and the lineal feet of chord material. It reports the short last bay rather than hiding it, since a building length rarely divides exactly by the spacing, and it can allow for gable end frames at the two ends. This is geometry and quantity only, not engineering: web layout, lumber grade, plates and load capacity come from the truss manufacturer's stamped drawing for your snow and wind loads.

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Measurements In
Building length, span and every chord length in feet; overhang and heel in inches. The pitch is a ratio and reads the same either way, and the spacing list carries both the inch and the millimetre modules whichever you pick here.

Building length and spacing give the count; span and pitch give the shape and the chord lumber that goes with it.

Along the ridge, the direction the trusses repeat in. Not the span.
Both modules are listed because they are different products, not one spacing written twice: the metric entries are 300 to 1200 mm as built, not converted inches.
The bottom chord length: outside of one bearing wall to outside of the other.
A 6/12 roof rises 6 for every 12 across. Enter 6 for 6/12. This is a ratio, not a length, so it is the same number whether you are working in inches or in millimetres: 6 in 12 is 150 mm in 300.
How far the top chord runs past the wall, measured horizontally in inches. Blank takes a 12 in eave, and 0 means no overhang.
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Measurements In
Building length, span and every chord length in feet; overhang and heel in inches. The pitch is a ratio and reads the same either way, and the spacing list carries both the inch and the millimetre modules whichever you pick here.

The shape of a single truss: run, rise, chord lengths and the overall height, with the heel taken into account.

The bottom chord length: outside of one bearing wall to outside of the other.
A 6/12 roof rises 6 for every 12 across. Enter 6 for 6/12. This is a ratio, not a length, so it is the same number whether you are working in inches or in millimetres: 6 in 12 is 150 mm in 300.
How far the top chord runs past the wall, measured horizontally in inches. Blank takes a 12 in eave, and 0 means no overhang.
The vertical depth of the truss where it sits on the wall, in inches. A standard heel is about 3.5 in; a raised or energy heel is often 9 to 16 in, so the insulation carries over the wall plate.
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