How Far Apart Are Rafters?

The short answer

Stick-framed rafters in North American homes are usually spaced 16 or 24 in on center. Manufactured roof trusses are most often set 24 in on center. Some heavy-snow regions and older houses use 12 in or other spacings. In metric framing, 600 mm and 450 mm centers are common. The right spacing depends on the rafter size, span and loads, which come from your local code's span tables or the truss design.

Why 16 and 24 inches?

Both divide evenly into 48 in, the width of a sheet of roof sheathing and ceiling drywall. That way the edges of every 4 × 8 sheet land on a rafter. The metric 400 mm and 600 mm centers do the same for 1200 mm sheets.

16 in versus 24 in on center

Neither is better in every case. Wider spacing saves lumber and labor but puts more load on each rafter and on the sheathing between them.

How many rafters you need

Divide the building length by the spacing, round up and add one for the end rafter. That gives one side of a gable roof; double it for both sides.

rafters per side = ⌈building length ÷ spacing⌉ + 1

Gable-end overhangs, framed openings for skylights or chimneys, and doubled rafters add to this count.

Worked example

A 30 ft long workshop with a 20 ft span and a steeper 8/12 roof. At 16 in on center you need 24 rafters per side, 48 in all. At 24 in on center it drops to 16 per side, 32 in all: 16 fewer rafters, if the span tables allow the wider spacing for your rafter size and snow load. Each common rafter, with a 1½ in ridge board and a 16 in overhang, is about 13 ft 6 9/16 in long, so either way you buy 16 ft boards. In metric, a 9 m building at 600 mm centers needs 16 rafters per side. Run your own numbers in the rafter length calculator.

Checking the spacing in an existing roof

From the attic, measure from the center of one rafter to the center of one several bays away, then divide by the number of bays. Measuring across five bays and getting 80 in means 16 in on center. One bay alone can mislead you, because rafters next to chimneys, hatches and skylights are often moved.

Knowing the spacing helps when you add insulation, hang things from the roof framing, or plan new roof sheathing, since the sheathing's span rating must match or exceed the spacing.

Rafters or trusses?

• Trusses are engineered and built in a factory. The spacing, usually 24 in, is part of the design. Do not cut or change a truss without the designer's approval.
• Rafters are cut on site. They allow vaulted ceilings and attic rooms, and suit small roofs, additions and sheds.

Spacing in sheds and small buildings

Garden sheds and small outbuildings are often framed at 24 in on center to save lumber, with roof panels rated for that spacing. In snowy regions, 16 in on center or deeper rafters are common even on small roofs, because the snow load does not shrink with the building. Local rules for small structures vary, and some need a permit above a certain size, so check before you build.

Code and safety notes

• Rafter size and spacing for a given span come from span tables based on wood species, grade, snow load and dead load. In the US these are in the IRC; Canada and Australia have their own tables. Check local code.
• Rafters must be tied together at the bottom, by ceiling joists or rafter ties, to stop them pushing the walls out.
• Wind uplift connectors, such as hurricane ties, are required in many areas.
• Roof work is work at height. Use fall protection and brace rafters as you set them.

Frequently asked questions