What rafters do

Rafters are the sloped framing members of a stick-framed roof. Each pair runs from the top of opposite walls up to the ridge, and the sheathing nails across them. The roof’s weight pushes rafters outward at the walls, so the code calls for a tie across the building, through ceiling joists or rafter ties, to hold the walls from spreading (R802.2).

The parts of a rafter

Part What it is
Ridge board The board at the peak the rafters frame against; at least 1 inch nominal thick and as deep as the rafter’s cut end (R802.3)
Plumb cut The vertical cut at the top, where the rafter meets the ridge
Birdsmouth The notch that seats the rafter on the wall’s top plate: a seat cut and a heel cut
Tail The part past the wall that makes the overhang
Common rafter A full rafter from wall to ridge
Hip and valley rafters The rafters along a hip or a valley; at least 2 inches nominal thick and as deep as the cut end of the rafters that meet them (R802.4.3)
Jack rafters Shorter rafters between a hip or valley rafter and the wall or ridge
Collar tie or ridge strap The tie that connects opposite rafters near the top (R802.4.2)

The guide to the parts of a roof shows where rafters sit under the deck, the ridge and the eaves.

How far rafters span

The code sizes rafters by their span measured level, not along the slope (R802.4.1). These are the code’s figures for two common lumbers, No. 2 grade, for a roof live load of 20 psf, a dead load of 10 psf and a ceiling not attached to the rafters (Table R802.4.1(1)):

Rafter 16 in apart, Douglas fir-larch 16 in apart, spruce-pine-fir 24 in apart, Douglas fir-larch 24 in apart, spruce-pine-fir
2x6 14 ft 7 in 14 ft 4 in 11 ft 11 in 11 ft 9 in
2x8 18 ft 5 in 18 ft 2 in 15 ft 1 in 14 ft 10 in
2x10 22 ft 6 in 22 ft 3 in 18 ft 5 in 18 ft 2 in
2x12 26 ft 0 in 25 ft 9 in 21 ft 4 in 21 ft 0 in

The table assumes the ceiling joists or rafter ties are at the bottom of the attic; set higher, the spans are cut by the factors in Table R802.4.1(9). Snow country, heavier roofing and other grades use the code’s other rafter tables.

Spacing

The code’s tables cover rafters 12, 16, 19.2 and 24 inches on center. Closer spacing lets the same board span farther, as the table shows. The rafter calculator counts the rafters for both slopes at the spacing you pick.

Rafter length and cuts

A rafter’s length along its top edge is its run times the pitch’s slope factor. The run is half the building’s width less half the ridge board’s thickness. A 6:12 roof’s factor is 1.118, so a 12 foot run takes a rafter about 13 feet 5 inches long before the tail. The plumb cut is the pitch’s angle from level: 26.6 degrees for 6:12. The rafter calculator works out the length, the tail, both cuts and the board to buy; the roof pitch guide covers the pitch.

Rafters vs trusses

The roof trusses guide covers truss parts, shapes and how many a roof takes.

Rafters Trusses
Built Cut and framed on site Made in a plant to an engineered design
Attic Open, room for storage or a room Webs fill much of the attic
Sizing The code’s span tables Truss design drawings, approved before they go up (R802.10.1)
Changes later Reframed to the code’s tables Not cut, notched or drilled without a registered design professional’s approval (R802.10.4)

Common questions

How far can a 2x6 rafter span?

At 16 inches apart, a No. 2 Douglas fir-larch 2x6 spans up to 14 feet 7 inches; at 24 inches, 11 feet 11 inches. That’s for a 20 psf live load, 10 psf dead load and no ceiling on the rafters.

Is span measured along the rafter?

No. The code measures a rafter’s span level, along its horizontal projection (R802.4.1).

How much does a rafter need to sit on the wall?

At least 1½ inches on wood or metal and 3 inches on masonry or concrete (R802.6).

Do rafters go on a roof estimate?

On a reroof, only the rafters that are damaged or rotted, as a repair. On new framing or a storm loss, the framing is priced by its lumber and labor along with the roof above it.