I have a 100 year old city rowhouse and thinking to finish the basement. I have cleared all the dangling electric cables and rerouted the water lines the attached to the ceiling beams. All I have left on the basement ceiling is the gas line that runs perpendicular to the basement ceiling beams. It is held with hangers against the beam but I was thinking to cut 1.5" notches on the bottom of the beams and fit the gas lines in. The beams are a true 3"x8" and the notches would one side not in the middle. If it one beam, I would think it is ok being 1.5" is less than 1/3 the size of the beam, but does cutting this notch on all the beams change things? I am not sure I want to run CSST so I am looking to all options here. Also, concerned notching all the beams will compromise the structure. Any strong opinions against notching? I want to clear the basement ceiling to make it appear more spacious and possibly put a ceiling and I only have 78" height.

  • 2
    Gas lines? More than 1? And are such large holes necessary? For notching it's 1/6 beam height. For holes 2" or more from top and bottom, it's 1/3 beam height. See codes.iccsafe.org/content/IRC2021P2/…. That's by today's book, though. What's the current condition of your floors? Is there much sagging already?
    – popham
    Jan 31 at 2:45
  • 1
    Have you built walls yet for this finished basement? You might route through them instead. Drilling the holes in uninstalled 2x4s sounds a lot easier than messing around over your head, too. That's if the walls aren't built yet.
    – popham
    Jan 31 at 3:39
  • CSST = Corrugated Stainless Steel Tubing
    – D Duck
    Jan 31 at 20:01

1 Answer 1


Notch depth is 1/6 of joist depth, maximum. No notches at all in the center 1/3 of the joist.

notches and holes in joists

Image source, no endorsement implied.

Holes, of 1/3 joist depth or less in diameter may be placed at least 2" from the edges. The center of the joist is the least impact, structurally.

I would strongly suggest getting over whatever objection you have to CSST. You're messing with the gas line at all, you might as well end up with it at a modern standard of installation; And you can put it through holes.

  • 1
    I just noticed that the illustration's left hole is right next to the notch. Technically there's a busted constraint there. The constraint isn't shown in the IRC's illustration either (but the illustrator didn't explicitly break the constraint).
    – popham
    Jan 31 at 20:36

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