Floor joists often contain notches or bored holes to accommodate wiring, plumbing pipes and duct work. Unfortunately, notches and holes are not always done properly – especially in older homes – and in such cases the joists’ structural integrity may be compromised. In this article we will review the modern requirements for notches and holes in joists so you can evaluate your own for stability.
First of all, all joists have a maximum span based on the size, species of wood, and other factors. Span tables are easy to come by, and the architect who designed your home and the general contractor who built it should have ensured the proper joists were planned and used.
Assuming your joists are the appropriate size and wood species for their spans, we will focus on the general rules for holes and notches. In general, notching should not be done in the middle one-third of the joist. This is because the middle section is the most prone to bending under pressure and notching in this area reduces the joist’s strength in a crucial section. Holes are allowed in the middle, but they too should be avoided in the middle third whenever possible. If notching is done in the outer thirds, the notch should not be greater than one-sixth the depth of the joist. For example, if a joist is 2×10 (in which case its depth is actually closer to nine inches than ten), a notch should not exceed 1-1/2 inches, which is one-sixth of the 9” depth. For all notches, the notch should not be carved out in a square or rectangular shape as the 90-degree corners tend to cause cracking in the wood. Instead, the notch should have angled or rounded edges which perform better under stress from load.
Often times, notching must be done at the ends of joists at the top of foundation walls. This is permitted, but the notch depth at the ends should not exceed one-fourth the depth of the joist and the notch length (horizontally) should not exceed one-third the joist depth.
The rule for holes in joists is a bit more lenient than the rule for notching because removing sections of wood from the edges of joists compromises their strength more than removing wood near the center. Holes should have a diameter that is one-third the depth of the joist or less. In the same 9” deep 2×10 joist, any hole should be 3 inches in diameter, at most, which is one-third of the 9” depth. Of course, if the hole can accommodate what it needs to with a smaller diameter, it should be cut as small as possible to avoid compromising the joist’s strength more than necessary. Holes should generally be located near the middle of the joist’s depth and should be at least 2 inches from the top or bottom edge. Likewise, no notches or holes should be within 2 inches of other notches or holes. If you see several holes or notches in a small area and close to others, you know it’s likely a problem.
If you examine the joists in your home and notice any defects that do not follow the rules listed here, it may be possible to reinforce the compromised areas. It’s a good idea, however, to consult a qualified structural engineer in such cases to ensure your flooring system is structurally sound.
*To visit our main website, go to http://www.hillinspections.com