Hole diameter: one-third of depth
2018 IRC R502.8.1 limits bored holes in sawn lumber joists, rafters and beams to a diameter of one-third of the member depth. For a 9 in joist: 9 ÷ 3 = 3 in.
Keep the fractions straight on the same 9 in member:
- 9 ÷ 6 = 1½ in is the interior notch-depth limit, not a hole limit.
- 9 ÷ 4 = 2¼ in is the end-notch limit.
- 9 ÷ 2 = 4½ in is no limit at all; a hole that big leaves too little wood above and below to keep the required edge clearance.
The 2 in clearance
R502.8.1 also requires every bored hole to be at least 2 in from:
- the top or bottom edge of the member,
- any other hole,
- any notch.
1 in, 1¼ in and 1½ in fall short; 3 in is more than the code requires.
Check both limits every time
A hole must pass the diameter limit and the clearance limit. Example: a 2½ in hole in a 9 in joist, with its edge 1½ in above the bottom edge. The diameter is fine, since 2½ in is under 9 ÷ 3 = 3 in. But 1½ in of edge distance is short of 2 in, so the hole does not comply.
The trap: answering "yes" because the diameter is within one-third checks only one of the two rules and forgets the edge clearance.
I-joists and trusses: manufacturer's rules
The R502.8.1 cutting, notching and boring limits apply to sawn lumber joists, rafters and beams. A manufactured wood I-joist or a truss follows the manufacturer's rules. The trap: the one-sixth and one-fourth notch limits do not transfer to I-joists. BCSI-B4 sets truss stack heights and says nothing about cutting I-joists.