The rafter is the hypotenuse of a right-angled triangle — here is how to find the length and the pitch from the span and the rise, and why the sloping eaves overhang is always a touch longer than the horizontal dimension.
Introduction
A symmetrical gable roof can be split lengthwise down through the ridge into two identical right-angled triangles. Each triangle has half the span as the horizontal leg, the rise (the height from wall plate to ridge) as the vertical leg — and the rafter itself as the hypotenuse.
The rafter length
Example: span 8.0 m and rise 2.0 m. Half span = 4.0 m. Rafter length = √(4.0² + 2.0²) = √(16 + 4) = √20 ≈ 4.47 m from wall plate to ridge.
The slope
With the same figures: slope = atan(2.0 / 4.0) = atan(0.5) ≈ 26.6°, corresponding to about a 50 % fall.
The overhang
A horizontal overhang follows the same pitch as the rafter. The sloped length that the overhang adds to the rafter is overhang / cos(pitch) — which is why the sloped overhang is always slightly longer than the horizontal measurement.
Lock types
- 01Collar-tie rafter: two rafters connected by a horizontal collar tie that absorbs the outward thrust — the classic construction for habitable attics.
- 02Lattice rafter (factory rafter): prefabricated rafters with nail plate connections; supplied with static documentation (ETA/manufacturer certificate) and span far without internal load-bearing walls.
- 03Scissors rafter: the underside follows a flatter slope than the top and creates a vaulted sloped ceiling.
- 04Renovation/collar-tie rafter: used when utilizing existing roof space where ceiling height must be preserved.
Eaves, roof tie and anchoring
The rafters rest on the wall plate along the outer wall's top edge. The plate distributes the load and is the link between roof and wall. Because a roof is also lifted by wind suction, rafters and plate must be anchored down into the load-bearing wall — typically with straps, rafter ties or ties. The bracing against wind load is dimensioned according to Eurocode 5 (DS/EN 1995) with wind loads from Eurocode 1 (DS/EN 1991-1-4) — it is not a nail strike on feel.
Stiffening of the roof surface
Individual rafters do not stand alone against wind from the gable. The roof plane is therefore braced in its plane — with wind bracing, with roof lining and battens that tie the rafters together, and with collar ties. Together this creates a rigid diaphragm that leads the horizontal forces down into the gables and walls. Bracing is part of the structural design.
Exchange at chimney and skylight
Where a chimney, a roof light or a roof tie breaks the roof, a rafter cannot run through. Instead, a header is made: the rafter is interrupted at the hole, and the load is carried around with a header (transverse beam) carried by the two adjacent rafters, which are often doubled. The dimension of the header itself belongs to the structural calculation — the geometry (where the hole is and how long the remaining rafter pieces are) is the carpenter's measurement.
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