Cutting/drilling hole in joist - How big?

curls00 · 07-13-2007, 02:20 PM

Hi there.

I am looking to access some plumbing directly behind the first floor joist, via my attached garage.

The kitchen floor joist is sitting directly on the top of the concrete wall in the garage (whose floor is about 18-20" lower than the kitchen/main floor). Above the joist is obviously a 2x4 insulated stud wall.

Can I cut a square out of the centre of the joist to get access back there? If so, how big can I go? I'm assuming since the joist is NOT a hanging joist (it's sitting on concrete), that I can get a pretty large section out of there without worrying? Once I'm done I fully plan to fasten a new piece of plywood to the joist to cover the hole.

I'm definitely no carpenter (obviously!) but can someone please let me know if what I am looking at is feasible and what limitations I have?

Thanks.
Eric

Bud Cline · 07-13-2007, 05:26 PM

Typically a hole cut through a floor joist should not exceed 33% of the joists dimension and even that isn't a good idea. In this case if the joist is totally resting on a ledge I'm not seeing a problem but it should be repaired as you said after your done.

crecore · 07-13-2007, 11:53 PM

really hard to say with such little info.. but I would personally never cut a square hole for one thing.

troubleseeker · 07-20-2007, 10:56 PM

If I am visualizing you description correctly... the joist is running parallel on top of the wall, so it is completely supported, thus really carrying no load. I don't see any problem with cutting a temporary access hole, but agree that a round hole is much better than square. Are you sure there are no wires or pipes strapped to the other side of this joist? Be sure to slightly angle the blade of your reciprocating saw so that you are cutting a "tapered" plug that will be removable from your accessible side. When finished, glue the plug back in, and apply a 3/4" plywood sister at least 18" past the hole in both directions. This is actually more cosmetic than structural , but should you happen to sell the house, it will help prevent a notatation from some annoying home inspector about a cut floor joist.