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I'm moving my sink. And I have to run my drain pipe through my floor joist or cabinets. My floor joist are 2×8's. Can I run a 2 inch pipe through them. And if i did run them through my joist would it make them weaker?

  • Just to clarify, do you need to run across (i.e. drill holes through) the joists, or along the joists (i.e. between 2)? If drilling, will you be drilling near the top, middle, or bottom 1/3 of the joists, and how far from the ends of the joists? TL;DR - yes, drilling joists will make them weaker, the question is will there be sufficient strength left, and we need those details to tell you. – FreeMan Oct 27 at 10:48
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It depends on the size of your drain. You can do it with a 1.5" PVC but a 2" really won't work.

Allowable hole size in joist

The maximum allowed hole you can drill in a joist is 1/3rd the depth of that joist. The actual depth of a 2x8 joist is 7.25" allowing only a 2.4" hole.

A 2" PVC pipe is 2.375" outer diameter (2-3/8") and you'll probably never get a 2.375" pipe into a 2.4" hole. Milwaukee is the brand plumbers use in my area and Milwaukee's hole saw kits don't even come with 2-3/8" saws because they're not practical.

If your drain is 1.5", Family Handyman has a helpful article showing you the right way to plan where to drill the hole in your joist. Note you need to plan on a 1/4" slope per foot of drain line, too; so don't figure on going through very many joists in series with this drain, or you'll be too far from the center of the joists and compromise them.

1.5" drain capacity

A 1.5" drain is only enough for 3 drainage fixture units (DFUs). That's sufficient for your kitchen sink & garbage disposal (DFUs per fixture table) but it is not enough for an additional dishwasher to meet code.

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