The diameter of any bored hole shall not exceed what fraction of the joist depth?

• A. 1/4
• B. 1/3
• C. 1/2
• D. 1/5

Answer: (B)

The maximum allowable hole diameter is one-third of the joist depth. This limit helps preserve enough remaining material to keep the joist strong in bending. Holes are typically placed in the middle third of the depth to avoid the top and bottom fibers, where bending stresses are highest, so smaller or more deeply located holes would weaken the joist more than this rule allows. Holes larger than one-third would significantly reduce stiffness and could lead to failure under load. Smaller diameters (like one-quarter or one-fifth) are permitted, but they aren’t the maximum allowed—one-third is.