Rafter length calculator
Rafter length, roof angle and slope for a gable roof.
Fill in the fields and the result will appear here automatically.
A symmetric gable rafter runs from ridge to support, so its right triangle uses half the span. Using the whole span would overestimate the length, but not necessarily double it: the rise also matters. The overhang here is measured along the inclined rafter and added after the hypotenuse. Angle and percentage slope are shown together to distinguish the two ways of describing pitch.
How it works
Formula and logic
Symmetric gable model: run a = span/2, length l = √(a² + rise²) + e, angle α = atan2(rise, a), slope = 100·rise/a. e is the overhang length along the inclined rafter, not its horizontal projection. Span and rise are positive, e ≥ 0, and inputs are finite. Length does not determine section size, spacing, connections or load capacity.
Example
An 8 m span with a 2.4 m rise and 0.5 m overhang needs a 5.165 m rafter at 30.964 degrees.
Fields and units
- Building span — m
- Ridge rise — m
- Overhang along the rafter — m
How to use
- — Enter the span of the building between the outer walls.
- — Enter the rise from the wall plate to the ridge.
- — Enter the overhang length along the inclined rafter, not its horizontal projection.
- — Add a cutting allowance before ordering timber.
Method and limitations
- Calculation method
- Formula and logic
- Limitation
- This is a geometric quantity calculation within the described model. It does not establish load capacity, safety or project compliance; check design parameters separately.
FAQ
Is the span measured between walls or across the whole building?
Across the building, between the outer wall plates. The calculation halves it, because one rafter covers one slope.
Why add the overhang after the hypotenuse?
The field is the length along the inclined rafter. If you measured a horizontal projection eₓ, first convert it to along-slope length e = eₓ/cos α; entering eₓ directly would understate this model.
What roof slope is usable?
The minimum depends on the specific covering and installation system. Use its current manufacturer instructions and the design; this calculator converts the chosen geometry and does not select a suitable covering pitch.
Does this cover hip roofs?
No. This is a gable roof with two equal slopes. Hip rafters run diagonally and are longer than the figure here.