What Is Rafter Length?
Rafter length is the length of the load-bearing timber beam (rafter) that extends from the eave (lower roof edge) to the ridge (upper roof edge). The rafter is the most important structural element of the roof frame and determines the sloped surface of the roof.
Precisely speaking, two rafter lengths are distinguished: the pure rafter length (from exterior wall to ridge) and the rafter length with overhang (including the projection beyond the exterior wall).
The Basic Formula: Pythagorean Theorem on the Roof
Rafter length derives from a right-angled triangle. The adjacent side is half the building width (for gable roofs), the opposite side is the ridge height above eave, and the hypotenuse is the rafter.
Therefore: Rafter length = square root of (half width^2 + ridge height^2).
Or, if the pitch angle is known: Rafter length = half width / cos(angle).
Both formulas yield the same result. The second is more practical when the angle is directly specified (e.g., by the zoning plan).
Worked Example Without Overhang
Building width: 10 m. Roof pitch: 35 degrees. Half width: 5 m.
Rafter length = 5 / cos(35) = 5 / 0.8192 = 6.10 m.
Alternatively via Pythagorean theorem: Ridge height = 5 x tan(35) = 3.50 m. Rafter length = sqrt(5^2 + 3.50^2) = sqrt(25 + 12.25) = sqrt(37.25) = 6.10 m.
The Overhang: Why It Affects Length
In practice, the rafter extends beyond the exterior wall forming the roof overhang. This overhang protects the facade from rain and sun. Typical values range from 30 to 80 cm (measured horizontally).
The overhang extends the horizontal run (adjacent side), not the rafter directly. The corrected formula is: Rafter length = (half width + overhang) / cos(angle).
Example with 50 cm overhang: (5 + 0.5) / cos(35) = 5.5 / 0.8192 = 6.71 m.
The 50 cm overhang (horizontal) results in a rafter extension of 6.71 - 6.10 = 0.61 m. The rafter becomes longer than the overhang itself because it runs at an angle.
The extension factor is: 1 / cos(angle). At 35 degrees: 1 / 0.8192 = 1.221. Each centimeter of horizontal overhang yields 1.22 cm of additional rafter length.
Lean-To Roof: Full Width Instead of Half Width
For a lean-to roof, the rafter spans the entire building width, not just half. The formula changes to: Rafter length = (building width + overhang) / cos(angle).
For a 10 m wide lean-to roof at 15 degrees with 50 cm overhang: (10 + 0.5) / cos(15) = 10.5 / 0.966 = 10.87 m. That is nearly 11 m -- significantly longer than a gable roof. Rafters of this length require special timber cross-sections or intermediate supports (purlins).
Practical Tips for Ordering
Tip 1: Order rafters 20-30 cm longer than calculated. Connections at both ridge and eave require additional material.
Tip 2: Standard timber lengths are 4, 5, 6, 7, 8, 10, 12 and 13 m. Choose the next larger standard length to minimize waste.
Tip 3: For rafter lengths exceeding 7-8 m, have a structural engineer calculate the cross-sections. Without intermediate support, deflection and buckling loads can become problematic.
Tip 4: Rafter length directly determines roof area and therefore the material quantities for battens, underlay and covering. Accurate calculation saves materials and costs.