Model a common rafter on a symmetric gable
A common rafter follows the roof slope from an outside wall toward the ridge. In a symmetric gable, the ridge lies halfway across the building span. A ridge board has thickness, so the rafter meets its near face rather than the roof centreline. This calculator subtracts that thickness from the wall-to-wall span before halving the remainder. It models the reference line of the rafter, not the full joinery of a cut timber.
Pitch converts horizontal run into sloping distance
Run = (span − ridge thickness) ÷ 2; slope factor = √(1 + (pitch ÷ 12)²)The pitch field is rise per twelve units of horizontal run in either unit system. A six-in-twelve pitch therefore means a rise of one half the run, not an angle of six degrees. Multiply the run by the slope factor for the wall-to-ridge line. The eave entry is a horizontal projection beyond the wall. It is added to the run before multiplying, which gives a longer sloping tail than simply adding the projection to the timber length.
An eight metre building with a half metre eave
For an 8 metre span and a 50 millimetre ridge board, the horizontal wall-to-ridge run is 3.975 metres. A six-in-twelve roof raises the ridge line 1.9875 metres above the wall reference. Its slope factor is approximately 1.118034. Adding a half metre horizontal eave gives 4.475 metres of combined run and a sloping reference line of approximately 5.00331 metres. All lengths refer to the same ideal roof plane.
Prepare a cut list from the actual details
Birdsmouth geometry, fascia position, ridge connections and plumb cuts affect the stock needed and where marks belong. Include appropriate cutting room when ordering; the displayed line is not a shop-ready cut length. This equation does not apply unchanged to hips, valleys, unequal roof pitches or an off-centre ridge. Measure the correct span reference and use a separate geometry model for those arrangements. The page does not choose timber depth, spacing, allowable span or connections. A zero pitch remains a valid horizontal line mathematically, but it does not describe a pitched gable. Ridge thickness must leave a positive run.
Frequently asked questions
Do I enter full span or half span?
Enter the full outside wall-to-wall span. The calculator subtracts ridge thickness and divides by two to obtain one rafter's run.
Is overhang measured along the slope?
No. Enter the horizontal eave projection. Its sloping length is calculated using the same pitch factor as the main rafter.
Does six-in-twelve mean six degrees?
No. It is a ratio of vertical rise to horizontal run. An angle must first be converted to that ratio.
Can I use this for hip rafters?
No. A hip travels diagonally in plan and requires different geometry. This page models common rafters on symmetric gables.
Does the result include cut allowance?
No. It is a reference-line length. Add the stock and end-detail allowance required by your own cutting layout.
Last reviewed: 2026-10-05 · How we calculate