Rafter length for a 24 ft wide building with a 6/12 roof

Twenty-four feet is the classic two-car garage width. At 6/12 the rafters come out at about 14 ft 5-9/16 in along the top edge — longer than a 14 ft board before you make a single cut — so this roof is framed from 16 ft stock. With 2×8s the plumb-cut tail and an inch of trim bring the board you need to about 178-3/16 in, comfortably inside 192 in.

Rafters this long need the right size and grade for your snow load, and a 24 ft span usually calls for ceiling ties or a structural ridge — take those from your code’s span tables or an engineer. The 2×8 here is only an example size.

What to know about rafters for a 24 ft wide roof

Each side of the roof covers 12 ft of level run, less ¾ in for half the ridge board, so the line length from ridge to wall is 160.16 in. The 12 in overhang, measured level, adds about 13.4 in along the slope. The rise on the measuring line is 72 in (144 in of run × 6/12), at an angle of about 26.6° — the angle you set on a speed square or saw for the plumb cuts. That 72 in is not the height of the ridge board, though: 2×8 rafters with a 3½ in seat leave a height above plate (HAP) of 6-3/8 in, so the top of the ridge board sits about 78 in above the top plates.

A 2×8 at 6/12 has a plumb cut of 8-1/8 in, and the IRC wants the ridge board at least as deep as that cut end (R802.3) — a 2×10 ridge. The birdsmouth is fine: a full 3½ in seat gives a 1-3/4 in heel and a notch 1-9/16 in deep measured square to the rafter, inside the ¼-depth limit of 1-13/16 in. The same seat on a 2×6 would not pass; its limit is 1-3/8 in.

Rafter size is where this calculator stops. Section R802 of the IRC covers wood roof framing: rafter span tables by species, grade, spacing and snow load, the ridge, and the ties or ceiling joists that stop the walls from spreading. The horizontal span here is about 11 ft 11 in per side; find that span in the table for your species, grade and snow load rather than picking a size by eye. On a garage with no interior walls, the ties matter as much as the rafters.

The count is 19 per side: 288 in of building length ÷ 16 in gives 18 spaces, plus one to close the last. That makes 38 rafters of 16 ft stock. At 24 in on center it drops to 26, a common spacing for trusses but one that needs a bigger or stiffer rafter under the same load.

The long walls under this roof are on How many studs for a 24 ft wall with a door and a window. The same math on a smaller building is Rafter length for a 12 ft shed at 4/12.