Tradespeople often need to cut notches or holes in floor and ceiling joists to run pipes, ductwork, or wires. Sometimes joists are notched at the end where they bear on a wall or sill. If the cut-outs are too large or in the wrong location, the joist can be weakened and unable to support the load it was designed for. These joists may need reinforcing to reduce bounciness, sagging, or excessive deflection in the floor or ceiling.
The illustration and table below shows where cuts are permitted and the maximum size allowed, based on D, the depth of the joist (top to bottom) in inches. For example, a hole should not be greater than 3 in. in diameter if the joist measures 9 in. deep, the approximate size of a 2×10 after normal shrinkage.
Click here to download a full notching and boring guide for joists and studs from the WWPA.
In general, never make notches in the middle third of the joist, where the bending forces are greatest. Other rules, in compliance with the International Residential Code (IRC R502.8 and R802.7) are:
- Don’t make any holes with a diameter greater than 1/3 the depth of a joist.’
- No holes closer than 2 inches to the top or bottom edge.
- No holes closer than 2 inches to any other hole or notch.
- No notches in the middle 1/3 of the joist, but holes are permitted here.
- No notches deeper than 1/6 the joist depth.
- No end notches (where the joist is supported) greater than 1/4 the joist depth.
- The length of a notch should not exceed 1/3 the joist depth.
- Do not make square or rectangular cutouts. Also avoid square cuts in notches — angled cuts are better, as shown. Square cuts tend to start cracks.
- No notches are allowed in the top of a large beam (greater than 4 in. thick), except at the ends.
Read more on Fixing Bouncy & Sagging Floors