How far apart are roof rafters?
Roof rafters are most commonly spaced 16 or 24 inches on-center (o.c.); 12 and 19.2 in o.c. are also used. There's no single required spacing — the IRC sets the allowable span for a given size, species/grade, spacing, and roof load (including snow) in its rafter span tables (§R802.4), so the spacing you can use depends on the load and the distance spanned.
Source: 2021 IRC §R802.4 (rafter spans) · Verify against your AHJ and the adopted code edition.
On-center (o.c.) spacing is measured from the center of one rafter to the center of the next. Common choices:
- 24 in o.c. — typical for rafters, with sheathing sized to suit.
- 16 in o.c. — closer spacing for longer spans, heavier snow loads, or thinner sheathing.
- 12 or 19.2 in o.c. — used where the span or load calls for it.
As with floor joists, the spacing isn’t a fixed code number. The IRC gives the allowable rafter span for a given size, species and grade, spacing, and roof load — and the live and snow loads matter a lot here — in its rafter span tables under §R802.4 (we cite the section and link out rather than reproduce the tables). Heavier snow country drives tighter spacing or deeper rafters for the same span. Section numbers shift between code editions, so confirm against your adopted edition and local amendments.
Work it out: the rafter length calculator gives the common-rafter run, rise, line length, and plumb/seat cut angles from a span and pitch — the geometry of each rafter, once you’ve set the spacing.
Sources
- ICC Digital Codes — 2021 IRC — §R802.4 (allowable rafter spans)