Question 1

How do I calculate rafter length?

Accepted Answer

The rafter follows the hypotenuse of a right triangle: run is half the building width less half the ridge thickness, rise is run times pitch over 12, and the rafter length is the square root of run squared plus rise squared. Then add the overhang measured along the slope.

Question 2

What is line length versus rafter length?

Accepted Answer

Line length is the theoretical measurement from the outside of the wall plate to the centre of the ridge. Rafter length is what you actually cut — line length shortened by half the ridge thickness and lengthened by the overhang.

Question 3

How do I account for the ridge board?

Accepted Answer

Subtract half the ridge board thickness from the run before calculating. With a 2x ridge that is 3/4 inch off each rafter. Forgetting it pushes the walls out by an inch and a half.

Question 4

How much overhang should a roof have?

Accepted Answer

Twelve to twenty-four inches is typical. Wider overhangs shade walls and keep water off siding, but increase uplift exposure in high-wind areas. Note that the overhang measured along the rafter is longer than the horizontal projection.

Question 5

How many rafters do I need?

Accepted Answer

Divide the building length in inches by the on-centre spacing and add one, then double it for both sides of a gable roof. A 40 ft building at 16 in on centre needs 31 rafters per side, 62 in total.

Question 6

What size rafters do I need for a 24-foot span?

Accepted Answer

It depends on species, grade, spacing, and snow load — consult the AWC span tables rather than a rule of thumb. As a rough guide, 2×8 at 16 in on centre often works for a 12 ft horizontal run in moderate snow country, but this must be checked against your local load.

Question 7

What is a birdsmouth cut?

Accepted Answer

The notch where the rafter sits on the top plate, made of a level seat cut and a vertical heel cut. Code limits the seat cut depth to no more than one-third of the rafter width, or you compromise the remaining section.

Question 8

Should I buy rafters longer than the calculated length?

Accepted Answer

Yes. Buy the next standard length up — you lose material to the plumb cut, the birdsmouth, and the tail cut, and a rafter cut short is scrap. The calculator shows the stock length to order.

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