How to read IRC joist & rafter span tables

Reading span tables without guessing

Read the span tables free at codes.iccsafe.org. ICC offers free read-only access to the IRC. This page explains how to use the tables; it does not reproduce them. It covers the 2021 and 2024 IRC — verify against the edition your AHJ adopted.

Every span table asks the same five questions: what the member is carrying (floor, attic, or roof, and at what live or snow load), how heavy the materials are (the dead-load column), what the wood is (species group and grade from the stamp), how far apart the members are (spacing), and how much sag is acceptable (the deflection limit in the heading). Answer all five and there’s exactly one cell to read.

How span is measured matters as much as the table. Floor joist spans are clear spans between supports. Rafter spans are measured horizontally, not along the slope. Rafter tables also carry a footnote adjustment when the ceiling joists or rafter ties are raised above the wall plate, because that changes the outward thrust on the walls.

Worked example. A living-room floor is framed with 2x10 Douglas fir-larch No. 2 at 16 in on center, 40 psf live and 10 psf dead. In the 2021 IRC living-area table that cell reads 15 ft 7 in. The measured clear span is 14 ft 6 in, so the joists work. Change to 24 in spacing, or a sleeping-area table with a heavier dead load, and you’d have to look the cell up again.

Engineered lumber is different. I-joists, LVL, LSL, glulam, and trusses are sized from the manufacturer’s tables or the truss design drawings, not the IRC lumber tables. Related: wall framing, headers, and fastening, deck framing (R507), and footing sizes. Build your lumber list with the quantity takeoff template.