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It was suggested by a plumber, but is it common practice? The I joists are 11 7/8" but a 3.5 or 4" toilet drain line seems like a large hole to be cutting even if it's in the middle of the joist's depth.

  • Engineered wood, I take it? (I'm sure steel joists exist, but probably don't see much play in residential work) – ThreePhaseEel Nov 23 '15 at 3:40
  • Yes, Engineered I-Joists. Looks like the TJI joists allow a 4" hole as long as it's at least 2' from the end. – Nic Nov 23 '15 at 3:42
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    Totally common. The manufacturer will have a cutout table on their website. – Aloysius Defenestrate Nov 23 '15 at 4:58
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Yes, this is common. The load in any joist is carried in the top and bottom surfaces (the flanges) of the joist. That's why I beams/I joists work. The thinner web between the flanges basically serves to keep the flanges at the same distance from each other. There is no load in the center of the web. Thus, the center of the beam does not need to be the same width as the flanges.

The manufacturer of your I joists has a guide for cutting holes in the joist, indicating how big the holes can be, where the holes can be located and how close to each other and to the ends of the joists the holes can be. I included an example below, but of course you should get the guide or template for your specific brand of joist.

Never notch the top or bottom flange of a prefabricated joist. If you do, you will compromise the joist.

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    Good answer in general (in terms of what is allowable), but in terms of the engineering I have a small point of correction. The web isn't just to keep the flanges apart, it does carry load, but a solid web isn't required. A joist with holes in the web is much like a truss: the top flange in compression and the bottom flange in tension take the bending moment, while the web takes the shear force in alternating tension and compression. – AndyT Nov 23 '15 at 9:22
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    @AndyT Thank you! I was going to mention something very (very) brief about the lateral/shear force on the web, actually, but figured I'd keep the answer short(ish). Obviously, if there was no load of any kind on the web, there'd be no point in its existence. – Craig Nov 23 '15 at 16:54
  • Now how would a drain line even fish through floor joists if it runs perpendicularly? I'm really tempted to run my plumbing under the floor joists and use a lower ceiling....even though this is the basement bathroom. – Nic Nov 25 '15 at 12:59
  • Well, you only need an inch of drop for every four feet for a pipe 2-1/2" or smaller. That's only 4" over 16 feet. And you can cut holes in the joists within the permitted range. So just measure carefully to maintain the minimum slope, following the joist manufacturer's rules, and you'll be fine. Or you can fix the pipe under the joists and install a drop ceiling later like you're talking about. – Craig Nov 26 '15 at 2:07

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