Notching and Drilling Joists

This piece explains the building code rules for cutting notches and drilling holes in floor joists, the wood beams that hold up your floor, so plumbers and electricians can run pipes and wires through them. These rules trace back to 1930 and are now part of the International Residential Code. The point is simple: cut a joist in the wrong place or make the hole too big, and you weaken the floor, sometimes enough to cause a permanent sag or a bouncy feel underfoot.

The rules boil down to location and size. Notches, cuts into the top or bottom of a joist, are not allowed in the middle third of the span, since that's where bending stress is highest. Outside that zone, notches can't be deeper than one-sixth of the joist's depth or longer than one-third of its depth. At the ends, where joists rest on a wall or beam, deeper notches are allowed because the stress there is different. Holes drilled through the middle of a joist can be bigger and placed almost anywhere, including the center of the span, as long as they stay at least 2 inches from the top, bottom, and other holes or notches.

None of this applies to engineered products like I-joists, LVLs, or trusses, which follow the manufacturer's own instructions instead. For an older house, this mostly matters if you're ever having plumbing or wiring rerouted through the floor framing during a renovation, since it affects whether existing cuts are sound or whether new ones can be made safely.