I am running some new romex for a couple of electrical outlets. One of the outlets is on one side of a wall in one bedroom and the other outlet is on the other side of the wall in another bedroom. When I went in my crawl space to see what was below the wall cavity, I found two floor joists running right underneath it. Based on my measurements, I'm fairly certain I have no room underneath on either side of the double floor joist to drill up into the cavity. If I were to drill right next to the joists, I'd be going up into my button board rock lath walls.

Do people drill vertically through floor joists? If so and if there are two floor joists together, does one drill the hole in one of the joists or right at the seam between the two? Or do I drill a hole diagonally through one or both? In case it would affect the hole size, I'm using 12-2.

  • is there any possibility of going up into an attic space instead of going down? Commented Jul 10, 2011 at 14:47
  • shirlock: Originally, outlet wiring in those rooms went through the fixture wiring and came from the attic. I didn't like that so was going to come from the crawl space for the outlets.
    – Ryan
    Commented Jul 10, 2011 at 18:50
  • I wound up drilling diagonally up from the crawl space starting at the corner where the floor joist met the floor. The drilling angle enabled me to avoid going through the button board. Essentially, I was wrong in saying "I have no room underneath...to drill up into the cavity."
    – Ryan
    Commented Jul 21, 2012 at 22:51

1 Answer 1


Disclaimer: We bring in outside contractors for our electrical installs, so I can give you some general tips, but I'll defer to others that may know more about this specific problem.

In general, joists, and any lumber supporting a load on its side, will lose much of its carrying capacity when it is drilled or cut on the top or bottom. For passing through the joists, you should make the hole in the center (between the top and bottom) of the joist, and make it the smallest hole that allows the wire to pass freely.

Having a double joist running beneath a wall means that the wall and joists are designed to be load bearing. So because of this, unless someone with more structural knowledge says it's ok to drill a hole, I'd avoid it.

While I can't think of a good way to get the wire into the wall, you could install your outlet on an adjacent wall, or you could go for a floor mounted outlet as a last resort:

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  • 4
    +1 Drilling vertical holes through the total depth of timber joists are an absolutely no, no. Unless you can access either side of the joist(s) and fix some appropriately sized and fastened steel stiffener plates to the faces of the joist(s) ie The steel plates will ensure the integrity of the timber joist(s) are not compromised due to material being removed...
    – Mike Perry
    Commented Jul 10, 2011 at 14:03

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