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I have two distinct problems I found after removing a replacement piece of subfloor that a previous owner had cut and put in to place.

First is there is a very large notch (12") created in a joist so that a shower drain pipe could be set on it which causes a lack of support at one edge of my replacement piece of subfloor.

The second problem is, there's about a 2.5' section of the subfloor that will overhang the closest joist about 3", right at the edge of the wall. The problem is, the shower water supply and drain pipes run right under the wall base plate and along the closest joist keeping me from being able to sister the joist to add support.

The questions are:

1) Is having a 3" overhang at the edge and corner for that 2.5' (which will be under the new shower and up to about where the adjacent toilet sits, but not behind the toilet) be a problem, especially since I'm going to be laying tile over it?

2) If #1 is a problem (I suspect it is), is there any way to add support to this for the subfloor?

3) How do I fix such a large notch? Can I just sister a joist (with a smaller notch in for the pipe to it to add support?

I'm not opposed to having a structural engineer come in to devise a way of fixing it if needed, but with the current situation, they are not available around my area and probably won't be fore a while. I'd like to fix it myself if possible (and legal) otherwise I'll just have to close up the bathroom and wait until I can get someone.

Here's a pic of the issue.

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  • Question, the floor joist, does it move much when you stand on it? Bounce on it? It'll be 'soft' feeling before the strength limit is reached.
    – Ack
    Commented May 2, 2020 at 19:23
  • This was under the shower, so not something I would have stood on. My concern mostly is that I'm swapping carpet for tile, and also want to use tile for the shower, and with tile being less forgiving, I want to protect it from flexing enough to crack or make the tile come loose.
    – Gary Schreiner
    Commented May 2, 2020 at 19:28
  • Does the notched joist carry any weight? That installer really made a mess of that.
    – Harper - Reinstate Monica
    Commented May 2, 2020 at 19:29
  • No, I mean stand on it now, and report back
    – Ack
    Commented May 2, 2020 at 19:33
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    @Harper-ReinstateMonica it's hard to tell how you intend your message and I'm going to error that it's legit rather than sarcastic. The joist depth will be sized and spaced (16" oc vs 12" oc or even 19.2") for the greatest load demand for the entire level (floor), and that is likely not even based on strength but stiffness (deflection). If deflection based, that critical joist is over-strength. The rest of the floor joists are definitely 'over-strength'. We don't value engineer and change the on-center spacing or size the depth of each joist based on it's individual load demand
    – Ack
    Commented May 2, 2020 at 21:54

2 Answers 2

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1) Is having a 3" overhang at the edge and corner for that 2.5' (which will be under the new shower and up to about where the adjacent toilet sits, but not behind the toilet) be a problem, especially since I'm going to be laying tile over it?

For tile, probably since any movement could crack the tile or grout

2) If #1 is a problem (I suspect it is), is there any way to add support to this for the subfloor?

I would attach a metal angle iron to the -face- of the wall bottom plate with the top of the bottom piece at the right height to support the sheathing where it hits the wall. Strength isn't much of an issue, you're just stiffening things up so the size of the angle is not really that important, anything that you find at a local hardware store that you can't bend by hand will be fine. Normally, we'd frame in with wood but that is very tricky here

3) How do I fix such a large notch? Can I just sister a joist (with a smaller notch in for the pipe to it to add support?

Gaa, dang plumber, they just don't care. That joist is not supported the normal load as the others are, because it is located so close to the wall so isn't supporting the full dead load as if it was fully space, also there is no way that the live load can occur right next to the wall. The issue is compression, since the cut is on top. This is good, better than if at the bottom where the wood is in tension. Unless the joist and floor feels bouncy, it's probably ok. The joist will move (deflect) before the load causes a failure. If you replace the plywood tightly, it will provide a lot of the resistance to compression that was removed when the joist was cut.

FWIW, I'm a structural engineer.

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    Thanks, I never thought of the angle, I like this idea. I could cut a piece off the top part of either end so it slides under the subfloor and screw the existing subfloor to it to add a bit more support. To your last point, I would assume this has been in place for at least a decade and didn't fail, but the bathroom was carpeted (yuck) and the shower had a shower pan, now that I'm moving to tile last thing i need is to have to replace it due to cracking.
    – Gary Schreiner
    Commented May 2, 2020 at 19:50
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    For tile cracking, you're more concerned about 'localized' deflection, in that you want the tile to 'bend less' and a 1" deflection at 6" away is much worse than 1" deflection over 10 feet. The angle solves the floor sheathing issue. The other issue is the floor joist. Deflection issues occurs before strength for longer members like this joist. We solve the deflection issue then the strength issue is also dealt with. Did you do the bounce test yet? and what is the distance between the top of pipe and top of joist (bottom of sheathing)?
    – Ack
    Commented May 2, 2020 at 19:55
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    I did the bounce test, i can't feel any bounce or squishiness when I stand on it (~240lbs), it feels solid.
    – Gary Schreiner
    Commented May 2, 2020 at 19:58
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    great, then you're good to go. I would infill the top of the joist with a block 1-3/8" thick which is the space you indicated was between the top of pipe and the top of the joist. Cut such that the grain is the long way, like the joist, and fit it tight. No need to glue the ends, but perhaps a screw near each end, down from the top, if you want. It won't go anywhere. This will make it even stiffer and stronger. Install the plywood floor sheathing tight too will also help. Doing both of these improve the situation a lot.
    – Ack
    Commented May 2, 2020 at 20:00
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    Awesome, thanks for your patience, help, and providing your knowledge. Makes me feel a lot better about getting this completed.
    – Gary Schreiner
    Commented May 2, 2020 at 20:04
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Maybe you needed to cut the subfloor back as far as you did. Usually you only want to cut it up to the front edge of the joist making it easier to sister-in another joist piece. Given what you have I would sister-in a double joist between the current joist and the back wall butting it up against the drain pipe on each side. It would be best to notch them out around the pipe to give support underneath if you can get them in but it's so close to the wall that may not be necessary. It might be tough to get in there but that should take care of the 3" overhang. The problem is going to be on the left in your picture. That edge needs to be well supported especially if you're laying tile over it. If there is a cross-piece between the joists a little further back to the left, cut the subfloor back to the crosspiece and sister-in another to that cross-piece to give support for the subfloor. You need support under the perimeters of all subfloor sections.
It may be a bit of overkill but for max support I like to use Timberloks for the sistering.
enter image description here

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  • I didn't cut this, unfortunately it was done by a previous owner, I removed the piece of subfloor that was previously replaced and found it. "Given what you have I would sister-in a double joist between the current joist and the back wall butting it up against the drain pipe on each side." For this, i might be misunderstanding what you're saying so feel free to correct me, but the wall doesn't have a joist under, but the next joist is about another 13" past the wall's base plate, so there's no way to sister to the other side of the wall without removing subfloor in the other room.
    – Gary Schreiner
    Commented May 2, 2020 at 19:30
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    I was saying to sister a joist in at the top of your picture between the existing joist and the wall. However, ACK's suggestion with the angle iron is probably the way to go. As an SE he understands these stresses better than I do. For additional stability I would probably sister a joist in and do the angle iron. My second point dealt with the subfloor needing support on the left side of the pic you posted.
    – HoneyDo
    Commented May 2, 2020 at 20:07

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