What percentage of a joist's depth is allowed for notches?

• A. 15%
• B. 25%
• C. 10%
• D. 33%

Answer: (B)

Notches in a joist weaken the member where bending stresses are highest, so the allowable size must balance passing through services with maintaining strength. The standard rule is that a notch can be up to 25% of the joist’s depth. This limit preserves enough material to keep bending strength and stiffness adequate under typical loads. If you notch deeper than a quarter, the remaining cross-section is too small to safely carry the load without risk of excessive deflection or failure, unless additional reinforcement is provided. The other options don’t align with common practice: 15% or 10% would be more restrictive than necessary for typical service needs, and 33% would exceed safe limits without reinforcement.