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Adatwist 02-12-2013 10:34 PM

Weakening Joists with holes

I currently have 2x16 16" oc solid wood joists in the basement. There are five to six 3/4" holes already drilled in each joist to run the original electrical, all of them only 1" from the bottom instead of the 2" required for structural integrity. So far after 6o years the floors still haven't caved, but now I need to run new electrical for new lights. I will need to drill two 3/4" holes on each joist.

Does anyone know how many holes I can drill in a joist without further weakening it?


Adatwist 02-12-2013 11:42 PM

oops.. i meant 2x20 joists

GBrackins 02-12-2013 11:51 PM

you are talking about solid sawn lumber that is 2x20?

Adatwist 02-13-2013 01:44 PM

Sorry.. type again!! i meant 2x10

GBrackins 02-13-2013 03:33 PM

whew! I was going to say that's some custom framing......

here is the 2009 International Residential Code limitations for notching and drilling of floor joists (your code may vary). This will indicate where (outer 1/3 of the joist span) and sizes allowed in those areas. Just be aware the more holes/notches placed in an area the less strength there will be in the joist.

Hope this helps! Good luck! :thumbsup:

RickyBobby 02-15-2013 11:27 AM

Can you use any of the existing holes that have already been drilled?

You can always gusset with 3/4 ply/glue/nails over any empty holes to provide some more strength.

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