Three fractions of depth
2018 IRC R502.8.1 sets the notch limits for sawn lumber joists, rafters and beams. Every limit is a fraction of the member's depth:
- Notch depth away from the ends: one-sixth of depth.
- Notch depth at the ends of the member: one-fourth of depth.
- Notch length: one-third of depth.
An 11¼ in joist: 11¼ ÷ 6 = 1⅞ in interior notch depth, 11¼ ÷ 4 = 2 13/16 in end-notch depth, and 11¼ ÷ 3 = 3¾ in notch length. A 9 in rafter's notch may be 9 ÷ 3 = 3 in long.
The trap: one-fourth applies only at the ends of the member; out in the span, one-sixth governs. Half the depth is not an R502.8.1 limit.
Length is a fraction of depth, not span
The trap: notch length is one-third of member depth, not of the span. The span thirds only set where notches may not go.
Keep out of the middle third
R502.8.1 prohibits notches in the middle one-third of the span, where bending stress is highest. The outer thirds are allowed, subject to the depth and length limits. The trap: 2 in is the bored-hole clearance, not a notch-location rule, and no 6 in hole separation exists.
Thick members: tension side only at the ends
For members 4 in or greater in nominal thickness, such as a 4x12 beam, the tension side shall not be notched except at the ends. The trap runs both ways: "anywhere outside the middle third" ignores this restriction, and "never notch at all" overstates it.
Check all three limits together
A 2x beam 12 in deep spanning 12 ft: depth limit 12 ÷ 6 = 2 in, length limit 12 ÷ 3 = 4 in, and the middle third runs from 4 ft to 8 ft. A 2 in deep, 4 in long notch 2 ft from the support passes. A 6 in long notch fails on length, a 3 in deep one wrongly uses the end limit, and a midspan one sits in the prohibited third.