Q: How big of a notch can I cut from a floor joist before it weakens the joist?
A: Floor joists occasionally need to be notched or drilled during construction, but you should take pre cautions not to weaken the structural strength of the framing member. First rule: Avoid notching a joist along the middle third of its length. However, drilling holes for wiring, etc. along this middle third is acceptable. The holes must be located a minimum of 2 in. from the top and bottom of the joist and from other holes. The maximum hole size is 1/3 the joist’s depth. You can cut a notch at the end of the joist to 1/4 of the joist’s depth (maximum). Along the outer third of a joist, you can cut a notch a maximum of 1/6 the joist depth and 1/3 the joist length without compromising its strength.