Joists, the horizontal-framing members that support the weight of ceilings or floors, are subjected to both dead and live loads. This includes the weight of all materials of construction as well as occupancy loads such as people and furniture. When a load is applied to a joist, the wood fibers along the bottom edge go into tension and those along the top go into compression. As long as the top and bottom fibers resist these stresses, the joist will do its job of keeping the floor strong, straight and solid.
However, when a joist is notched or drilled, the reduction of wood fibers reduces the ability of the member to withstand compressive and tensile forces. A hole or notch that is too big or in the wrong place can seriously weaken the joist, even to the point of failure. The International Residential Code has set forth the following guidelines for the notching and boring of joists.
- Notches in floor joists may occur in the top or bottom of the member but may not be located in the middle third of the span.
- A notch may not exceed one-sixth of the depth of the joist except at the very end where it may be one-fourth of the joist depth.
- The length of joist notches cannot exceed one-third of the depth of the member.
- Holes bored in joists must not be larger than one-third the depth of the joists.
- Holes cannot be located within two inches of the top or bottom edge of the member, or to any other hole located in the member.
- Holes cannot be located within 2" of any notch.
Below is a helpful chart that provides actual notch and hole size limitations for 2x4, 2x6, 2x8, 2x10 and 2x12 joists. It should be noted that limitations on the allowable notching and boring of wood joist are meant to retain the structural and functional integrity of the member.
Qualified Inspection Services provides third party residential inspection services designed to keep builders code compliant. To find out how our services may benefit your company, please contact us today.