How to cut hole larger than 1/3 of joist?

Question

I need to run a toilet drain (3”) through three joists (9 1/4”). Code states holes in joists can not exceed 1/3 the size of the joists. Catch is for 3” pipe I need a 3 5/8” hole — which is larger than 1/3 the height of the joist.

Now what? I assume I could reinforce the joist? But I can’t find any specific info on this. Is there an official way to handle this?

EDIT: one workaround I’m seeing is that I could instead drop below these joists in a closet...which would reduce the height of the closet but maybe the best option?

Answer 1

I have seen these before. https://www.metwood.com/joist-reinforcers/ . I do not know if they are code approved, but it looks like a good start.

This pic is from their website

It looks like they are not graduated lower to account for the fall of the pipe unless they over sized the hole to allow for some slope.

Answer 2

So in summary, by you are 5/8" over. You would have to consult your local building dept for those answers. I would "imagine" ( not know ) that if there was some steel plating you could affix to the sides of it that were 20 inches long x 3/16" thick, ran on both sides of the wood ( 90° away from the 3-5/8" hole) that it could possibly make up for the loss of structure the wood had prior to drilling the hole. That's a scientific wild ass guess.🙂

Answer 3

The “one-third rule” is for general construction. For shorter spans, (like a 3-4’ span across a closet,) it may be perfectly fine.

We’re not all structural engineers, so those “rules” are for general construction practices. However, we do prefer holes through the center of the joists (halfway from the top to the bottom) where there is neither tension nor compression. And not located in the middle third of the span where the joist “bends” the most.

Your “notch” may be fine and the joists across the closet may be stronger than the span across your bedroom. (We prefer a footing under both bedroom walls, but is not mandatory, depending on loads, spans, etc. )