Rafter Calculator

Rafters are cut from dimensional lumber, which comes in standard lengths, so the result is the starting point for your cut list and your lumber order. A rafter is measured along the slope, so it comes out longer than the flat distance it covers.

How to use this calculator

1. Measure the building width from the outside face of a wall to the outside face of the opposite wall, and enter it as the span.
2. Enter the pitch as the inches of rise for each foot of run. If you know only the rise and run, work out the pitch with the roof pitch calculator.
3. Enter the overhang as the flat distance from the outside of the wall to the edge of the eave, and check the unit selector beside it.
4. Read Rafter length as the cut length along the top edge, in feet. Use Without overhang when the rafter stops at the wall, and use Run and Ridge height to check the drawing.
5. Buy each rafter in the next standard length above the result you cut from. The plumb cut, the birdsmouth seat and any ridge board deduction all use up lumber.
6. Check the size, spacing and maximum span against the span tables in your building code, and confirm them with your local building department before you order.

Worked examples

Rafter length with overhang by building width and pitch

Each cell: Rafter length (ft). Inputs not in the table use the calculator's starting values.

How the length is worked out

For a common gable rafter, the run is the building width split down the middle, from the outside of the wall to the centre of the ridge. The calculator multiplies the run by a slope factor, which is the ratio of rafter length to run. The factor grows as the pitch gets steeper.

The overhang is measured flat, so the calculator adds it to the run before it applies the slope factor. Without overhang stops at the outside of the wall. The ridge height is the run in feet multiplied by the pitch, which gives inches, so you can check it by hand. The table uses the starting overhang, so enter your own eave width.

Cuts and lumber to buy

The result is the length before any cuts. The top end gets a plumb cut where it meets the ridge, and a ridge board shortens each rafter by part of the board's thickness. The bottom end gets a birdsmouth seat cut, a notch that sits on the top plate of the wall.

Every cut uses up lumber. The board feet calculator can total the lumber when you have the full list of lengths, and the roofing calculator covers the roof covering, which is a separate purchase.

Size and spacing come from code

This calculator covers geometry only. Rafter size, spacing and the longest allowed span depend on the lumber grade, the roof loads and the spacing, and the span tables in your building code set them out. Check local codes before you order, since your local building department confirms which tables apply.

Mistakes to avoid

• Cutting to Without overhang when the rafter tail runs past the wall. Use Rafter length there.
• Ordering exactly the result. The cuts take off length, so a board cut to the exact result comes up short.
• Reading decimal feet as feet and inches. Multiply the part after the decimal point by the inches in a foot before you mark the board.

How do I mark a birdsmouth and a tail cut?

Lay the rafter on the top plate and mark where the underside meets the outside corner of the plate. The birdsmouth has two cuts: a level seat that rests on the plate and a plumb heel cut up from the seat. At the tail, make a plumb cut, or a level cut if your fascia calls for one, at the end of the overhang. Keep the notch shallow, because a deep one weakens the board. The IRC caps a notch at the end of a rafter at 25 percent of its depth, so confirm the limit with your local building department.

Common questions