Timber Truss Design Calculator

Horizontal Thrust is w x L^2 / (4 x H), so it is inversely proportional to Truss Rise -- doubling the rise for the same span and load halves the outward thrust at the supports. This is a real structural tradeoff traditional timber framers exploit: a steeper truss needs taller rafters and more lumber, but pushes far less outward force into the walls below, which is why very shallow trusses often need a stronger tie beam or steel tension rod instead of relying on the walls alone.

No. Rafter Axial Force, Bottom Chord Tension, Horizontal Thrust, and King Post Tension are all determined by the truss's geometry and applied load (span, rise, roof load, and truss spacing) through simple statics -- Rafter Width and Rafter Depth never appear in those force calculations. A bigger rafter only raises Rafter Compression Capacity, which lowers Rafter Utilization Ratio by giving the same force more cross-section to resist it with.

Yes. The rafter carries a single compression force along its length. At the heel joint, that force resolves into a vertical part (balanced by the support reaction) and a horizontal part (the rafter's outward push). The bottom chord's whole structural job is to pull back with exactly enough tension to cancel that horizontal part -- so Bottom Chord Tension is not an independent calculation, it is the rafter's horizontal component by definition of horizontal equilibrium at that joint.

Wood Species affects only the capacity side of the calculation -- Rafter Compression Capacity (via the species' Fc, compression parallel to grain), Min Bottom Chord Size, and Min King Post Size (via the species' Ft, tension parallel to grain), both taken from the American Wood Council's NDS-2018 reference design value tables for Douglas Fir-Larch and Southern Pine. It has zero effect on the actual internal forces (Rafter Axial Force, Bottom Chord Tension, Horizontal Thrust, King Post Tension), which are purely a function of the truss's geometry and load.

King Post Tension equals half the total load carried by the truss, and total load is the per-foot roof load times the span -- so King Post Tension rises in direct proportion to Truss Span across its full range, with no plateau or reversal. A longer truss puts proportionally more tension pull on the vertical king post that carries the apex load down to the bottom chord.