Work out the number of roof trusses and the truss height for a roof from its length, span, pitch and truss spacing.
Optional – add span and pitch for truss height and rafter length:
The calculator works out how many trusses you need along the roof, then – if you provide the span and pitch – the rise at the ridge and the length of each rafter. Truss design must always be confirmed by the truss manufacturer or a structural engineer.
Trusses are typically spaced at 600 mm centres in the UK or 24" on centre in the US, though 400 mm / 16" is used for heavier loads. Always follow the truss manufacturer’s design.
Divide the roof length by the spacing and add one for the truss at the start. The calculator does this for you and rounds up so the run is fully covered.
The rise at the ridge equals half the span multiplied by the tangent of the pitch angle. A steeper pitch gives a taller truss for the same span.
UK pitched roofs are often 30–45°, while low-slope US roofs may be expressed as rise-over-run (for example 6:12 ≈ 26.6°). Enter the angle in degrees here.
Use it for planning quantities and rough sizes. Trusses are engineered components – the supplier will produce a design and cutting list based on your exact loads, span and pitch.
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