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Mogily 03-05-2012 09:27 AM

holes in floor joists
Hi there, I just bought a home. I pulled down the existing ceiling in the basement and noticed that someone had drilled 3/4/' holes within the bottom two inches of the floor joists (joists are 12x2). It is for PEX tubing. There are quite a few holes and I'm concerned. I have asked a few people and no-one seems concerned (they believe the code is not for structure but so that someone doesn't put a screw into the PEX).

should I be concerned?

Daniel Holzman 03-05-2012 09:41 AM

This topic has been repeatedly discussed on this forum, do a search for holes in joists or similar, you will find dozens of threads discussing exactly where to put holes and notches. Just for your future information, the center of a beam or joists (in the vertical orientation) is called the neutral axis of the beam, because there is no stress when the beam is loaded in bending, which is how joists and most beams are loaded. This is the place you want to drill holes, because the center of the beam or joist does not add any bending strength to the beam, so cutting out a relatively small hole (like a 3/4 inch hole) through the center of the beam does not reduce the strength of the beam significantly.

The worst possible place to drill on hole is at the top or bottom of the beam or joist, because the edge of the beam carries the highest stress. So they put the holes in the wrong place. Also, the middle of the beam or joist (viewed along the beam) carries greater bending stress than the end points of the beam, so if you can avoid drilling holes at the midpoint, it is a good idea. The diagrams of allowable notching and hole positions essentially show the same thing, but there are some differences in allowable size and location depending on which code you operate under.

All that said, I would not worry too much about 3/4 inch holes in the 2x12, but now you know where to drill if you need to add more.

Mogily 03-05-2012 01:07 PM

Thanks for the response, I did notice the other threads about where to drill the holes in joists but was not sure about the holes because there are so many. It sounds like I soundn't worry. Thanks!

Mogily 03-05-2012 09:11 PM

Hi Dan, sorry, I just measured my joists; they are 2x8 on 16' centers, with 12" span. does that change your answer?

thanks again, your input is greatly appreciated.

Daniel Holzman 03-06-2012 07:47 AM

I assume you mean 2x8 on 16 inch (not feet) center, with 12 foot (not inch) span? And the holes are 3/4 inch? Assuming this is correct, the installation was not correct, but I would not get overly concerned over it. I have been in dozens of houses where the plumbers hacked up joists in totally inhumane manners in an effort to install their piping, and the houses did not fall down. However, now that you know the proper way to do it, no reason to repeat the errors of others in the future.

Just for fun, you may want to look at,00.html

where there is a nice diagram showing allowable notching and holes. Remember, this diagram is for ONE specific code, yours could be different, but probably not very different, at least in the U.S. Note that the maximum hole size is 1/3 the actual joist depth, so for a 2x8 that means not large than about 2-3/8 inch, so you are good there. However, note that the edge of the hole is not supposed to be any closer than 2 inches from the edge of the joist, so that is the problem.

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