We've just had the house re-wired (UK), and were wondering whether a hole and notch the electricians have made will significantly weaken the joist?

It is the end joist in the room.

Joist dimentions are 175mm height, 45mm width/thickness and 3350mm length (assuming it is length of room).

Hole has an 80mm diameter. Notch is 70mm in height (some wood still together in middle).

What would be our best course of action to ensure nothing happens (if it will)?


Picture of joist and hole/notch

  • Hello, and welcome to Home Improvement. You probably should be concerned, but what's underneath the joist? And, you should probably take our tour so you'll know how best to participate here. Nov 7, 2019 at 19:33
  • Hi thanks, the room below this is the living room. This is at the end of the house so behind the joist pictured is an end wall. I checked a few websites and I believe the hole should have been no bigger than 1/3rd of the joist, and the notch 1/6th.
    – dirtmcgirt
    Nov 7, 2019 at 19:37
  • Yeah, that big notch is almost halfway through the joist, which is (or should be) verboten code-wise. Nov 7, 2019 at 20:01
  • Yeah, is just below half the height of the joist. Do you think this would be something remidied with extra support such as an additional piece of supporting wood attached along the side whole joist or steel plates over notch and or hole? Cheers
    – dirtmcgirt
    Nov 7, 2019 at 20:17
  • Were permits pulled and will this job be inspected? If so make sure you point this out to the inspectors. You might want to run it by the electricians too.
    – JACK
    Nov 7, 2019 at 21:06

2 Answers 2


Fortunately, the abused floor joist still looks straight. If it happens to be sitting on a foundation/basement wall, it shouldn't be an issue. If it is not supported underneath, it would be a good idea to place a post under it. I wouldn't let the same electrician work on the house anymore.


Generally speaking, holes are usually OK, and notches are not. Notches are like ripping the edge of a piece of paper, which creates a weak point for potential fractures. As noted, the notch on the far right is nearly halfway through the beam, which means you'll have to sister it to repair it (assuming it can be done at all, might want to consult an engineer).

The other notches and holes look decent. The notches on the left are on top, small, and will likely be secured by the subfloor (I would insist on a 3"/76mm screw on either side of the notch when the subfloor is installed)

If you want peace of mind about the big hole, add a joining/mending metal plate above and below (cheaper to do it now)

  • What is a “joining/mending metal plate”?
    – Lee Sam
    Nov 8, 2019 at 5:27
  • @LeeSam Search "joining mending plate" - it's a piece of metal with screw holes drilled in it that you can screw to the joist to reinforce it. Nov 8, 2019 at 11:50
  • It would probably be enough to cut the notch a bit wider and a bit cleaner (time to get out the Fein), and then cut a piece of wood very slightly longer than the gap, and hammer it in. Nov 8, 2019 at 11:52

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