Rafter calculator

How it is worked out

• Rafters run from each wall up to a ridge board along the peak, no further apart than the spacing you choose. The first stands just inside each end wall.
• Each rafter is notched to sit on the 2×4 plate on top of its wall, and runs on past the wall to the overhang.
• A sub-fascia closes the rafters’ ends along each eave. A barge rafter runs up each rake, carried on lookouts from the first rafter inside the wall.
• Each outside corner of the eaves has a short rafter at 45° from the corner in to the first rafter it meets.
• The gable ends are framed up to the roof in studs under a plate that follows the slope.
• Boards come up to 20 ft long. A rafter that would need to be longer, with no wall or beam under it to joint it over, is turned down, and we say why.

We lay out and count the sizes you pick, close enough to plan a weekend and price the lumber. Span tables, your local code and, on a bigger job, an engineer settle what size each board has to be.

Questions

How long is a rafter?

It depends on the half-width of the building, the pitch and the overhang. The list gives each rafter’s length as it is cut, from the ridge board to the end of the overhang.

Does it cut the birdsmouth?

Yes. Each rafter is drawn notched over the plate it sits on, and its length is measured with the notch in it.

Can it frame a hip roof?

It frames roofs whose shape is the same all the way along, such as gables, sheds, gambrels and saltboxes. A hip roof falls outside that.