A hole is required for piping 4 inches from the bottom of a 2x10 joist that's 12 feet long, the hole is 2 feet from the end of the joist. The maximum allowable size for the hole is what?

• A. 2 inches
• B. 4 inches
• C. 3 inches
• D. 5 inches

Answer: (C)

Holes in a joist are limited to maintain bending strength. When a hole sits in the middle third of the joist’s depth, its diameter cannot exceed one-third of the joist depth.

A 2x10 has an actual depth of 9.25 inches. One-third of that is about 3.08 inches. The hole is located 4 inches from the bottom, which places it in the middle third (roughly between 3.08 and 6.17 inches from the bottom). So the largest hole allowed is about 3 inches. A larger hole, like 4 or more inches, would weaken the joist.