Roof Trusses from Poles: Geometry and Member Sizes for Spans from Four to Eight Metres

Where sawn timber is scarce or expensive, roof trusses are often built from round poles — and usually sized by copying whatever stood up last time. This paper originates a complete design method for one stated form, a king-post truss with two inclined struts at a 35 degree pitch and 3.0 m truss spacing, and computes its member sizes for spans from 4 to 8 m under two roof coverings. Member forces are obtained exactly, for the stated form, by solving joint equilibrium at all six joints simultaneously; members are then sized by stepping round-pole diameters upward until each passes an Euler buckling check with a safety factor of 3.0, an axial-plus-bending interaction check at stated allowable stresses for a stated pole class, and a joint check against stated pegged and lashed connection capacities. Under thatch (total stated load 0.90 kN/m² of slope) the lower rafter needs a diameter of 90 mm at 4 m span and 140 mm at 8 m; under tile (1.45 kN/m²) it needs 105 mm at 4 m and 165 mm at 8 m. The tie is never the problem: it requires only 60 to 90 mm across the whole range, and the king post passes at the 60 mm minimum diameter everywhere. Lashed joints, credited with a stated slip-limited capacity of 9.0 kN, suffice at every joint only for the 4 m thatch truss; everywhere else the primary joints need hardwood pegs, from 2 pegs at small spans to 4 pegs at the support of the 8 m tile truss. Extending the same method beyond the design range, the form stops working under tile between 9.0 m, which just passes with a 180 mm rafter, and 9.5 m, which no diameter up to the stated 180 mm maximum pole can carry; under thatch it stops between 11.5 and 12.0 m. In every failing case the lower rafter segment, in combined axial compression and bending, is the member that gives out — not the tie, not the king post, and not the joints. This study is simulated, not measured, and no truss described here has been load-tested.