It’s very common for me to see floor joists notched improperly. I know it can be tempting to do that if you’re trying to get something installed through your basement, but I can’t stress enough how bad of an idea it is to go cutting away at the structure of your house.
But if you really need to drill or notch a floor joist, here are the rules. These rules are from the International Residential Code 2021, section 502.8. The IRC isn’t enforced everywhere, and if your jurisdiction enforces a different code then you should follow that code. But the IRC is enforced in most places (in the USA at least) and it’s a great reference.
These rules are for dimensional lumber – the standard 2x lumber that you can find at any lumber yard. For any kind of manufactured or engineered products the IRC requires that you follow the manufacturer’s recommendations or the design of a licensed engineer.
The first thing to know is that there should be no notches along the bottom edge of the joist (what the IRC refers to as the “tension side”) except at the ends. This is true for all joists that are 2×4 nominal or bigger – so pretty much anything you might want to use as a floor joist. At the ends the notches are limited in depth to 1/4 of the joist depth.
Along the top edge there should be no notches in the middle third of the span. Notches in the outer third are limited in depth to 1/6 of the joist depth and limited in width to 1/3 of the joist depth. Note that for all these limits the joist depth is the actual depth of the wood, not the nominal depth. For example, a 2×8 is likely to have an actual depth of 7.5 to 7.25 inches depending on how old it is. It’s this 7.25 number that’s used in these calculations.
Holes can go anywhere along the joist, but they’re limited in diameter to 1/3 of the joist depth. Holes must be a minimum of 2 inches from the top and bottom edge, at least 2 inches from any other hole, and at least 2 inches from any notch.