Can Any Part of a Floor Joist Be Notched According to Code?

Center

• Joists bear the weight of the floor above them and must never be weakened to a point where they might collapse under the load. The center part of the joist, in particular, must remain strong for the joist to stay sound. For this reason, the IRC does not allow any notching in the center third of the joist. The code does permit holes in this section, but only within certain limits.

Notches

• You might need to put notches in a floor joist when running plumbing pipes, such as copper tubing, through the bottom of a floor. The IRC allows this practice, but sets a number of requirements to protect the integrity of the joist. You may create notches on either side of the center third of the joist. The notch may not measure any deeper than one-sixth of the joist depth or be wider than one-third of the joist depth.

Hole

• Drilled holes in joists, which provide a convenient way to run wires or conduit under the floor, come under the auspices of the IRC as well. In some cases, you can use holes instead of notches to take advantage of the code's more lenient regulations regarding holes. Holes can only be drilled at least 2 inches from the top, bottom or side of the board. The maximum diameter for a hole is one-third of the joist's depth. You may drill a hole in the center third of the board, if necessary.

Nominal

• When deciding whether to notch a joist, measure the diameter of your pipe, conduit or wire accurately and keep in mind that lumber measurements are nominal measurement. The actual size of a piece of dimensional lumber is slightly smaller than the given size. For instance, a 2-by-8 joist will usually measure 1 1/2 inches by 7 1/4 inches after it is dried. This affects how large a notch you can make in the joist.