Will this wall collapse? 1920s subfloor replacement

Hi, I'm removing subfloor in order to level joists and install tile. However, it looks like this wall is build on top of the subfloor, and I'm not sure what's holding it up now - have I made a mistake? Ignore bad plumbing, it's coming out.

 

I assume that is not an exterior wall. Yes, walls are built on top of the subfloor. Removing the subflooring removes the support for the wall. Even though it is an interior non-load bearing wall in old homes they all seem to end up carrying a load and it appears that wall has dropped. It looks like the top of the old floor boards is about flush with the top of the joists.

You've got a difficult situation to fix. You need subflooring to go from joist to joist to support the wall. Ideally you would open the floor on the other side of the wall. Jack the wall up and install new subfloor between the joists on either side of the wall.

Another common problem is when joists are completely or mostly cut to make room for plumbing. Over time things sag as the load gets re-distributed causing floors to sag and what should be non-load bearing walls to end up carrying load through the subflooring.

I hope the knob and tube wiring is no longer being used. If it is now is a perfect time to replace it.

 

...not because I think you're wrong, but because I'm praying there's an easier way to deal with this situation. Here are my appeals:

(1) The wall hasn't actually dropped, it just looks that way from the photo. The small lip of subfloor remaining didn't move down when I removed this side of the subfloor.

(2) There's a diagonal joist running under the wall at about the middle of it, adding critical support...?

(3) Can I just put blocking between the joists directly under the subfloor and wall and hammer in shims to level? If it HAS dropped, it's a negligible amount.

(4) The people in the tile forum told me I should remove the subfloor to install new subfloor. I blame them.

 

Unless you open up the floor on the other side of the wall to attach the other end of cripple joists I don't know if you can attach anything well enough from this open end to provide structural support. The wall is probably/hopefully not load bearing so the only load it would carry is from the building settling and moving with age and it it's not it by now hopefully it won't in the future.

The bigger deal is supporting the edge and aligning the elevation of the new subfloor. Easiest would be to cut away all the old subfloor to be flush with the wall. Then you can install new sheeting over the entire floor in rectangular pieces. Or, you can cut your sheeting to whatever is needed to fit around the remaining tails of subflooring and shim as needed to get the thicknesses to match.

 

Second opinion....it WILL fail if you don't take measures to support the full length of the subflooring. Do you have access to this area from below? Adding blocking is one way, albeit incomplete as far as longitudinal support goes, and that is what you need. Blocking, exactly 3/4" below your existing joist tops will allow you to possibly slip in some 3/4" plywood to help span it all. How will you attach the far end od the blocking? Do you have access?

 

On the other side of that wall, there is hardwood flooring, so I want to avoid tearing out the floor and replacing the subfloor on the other side.

It is indeed an interior wall on the second floor. Below it on the first floor is a room (not a wall holding it up.) The ceiling has a stain, the type that peels and falls in—a plumbing leak prompted this whole project—so I'm OK with attacking this problem from underneath. The first floor was going to need some new ceiling and paint anyway.

Weight consideration: All the lath-and-plaster on this side of the wall is going to be removed and replaced with drywall, then 1/3 of it tiled.

Knob and tube is indeed still in use and feeding most of the house's receptacles and overhead fixtures. I hope to not disturb it for now.

 

How did the framers originally justify crippling that joist and going all diagonal around the soil pipe?

What if I open up the ceiling under the wall and put blocking between the visible crippled joist and the next joist further away?

It seems the wall to the left side of the sink DWV has done reasonably well over the years with that crippled joist, so if I simply restore support to the right side of the sink DWV with blocking, we'll be OK. yes?

It doesn't seem to be sagging, despite the pictures. Could it be that the plaster is keeping it the wall square and under a lot of stress?

As far as blocking goes: a half dozen +/- 15 inch lengths of 2x8s placed perpendicular to and between the joists that run parallel on both sides of the wall, accessed through an opening made in the ceiling below, banged tightly into place and attached with 16d nails from the outsides of the joists directly into the blocking?

 

I would take this opportunity to rip out every bit of knob and tube wiring you access. I have a number of rental houses and many of the older homes had knob and tube. About 8 years ago I started a program of insulating the attics and I can't tell you how many homes had evidence of a fire that had luckily burned itself out. The old knob and tube was OK when there was one light bulb in the center of the room and maybe a table lamp and radio. You plug a space heater or fridge into that wiring and it's just an accident waiting to happen.

Many years ago my insurance company wanted disclosure of homes with knob and tube wiring and fuse panels. They would not cover either. The knob and tube had to go and the fuse panels replaced with modern circuit breaker panels.

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Cripple joists installed from below like you described would be fine. It would also make it easy to jack so any load from that wall gets transferred to the joists. The cripples would also help support your new subflooring. To properly nail through the joist into the cripples (not toe nailed) will require opening up the ceiling below larger to allow room to swing a hammer in the adjoining bay. Joist hangers on the cripples will allow you to do everything from within the one bay and keep the hole downstairs as small as possible.

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I assume your house is balloon framed. The exterior walls are load bearing and the floors attach to the outer walls. Any interior walls are added on top of the floor. So, even though the interior wall may not be technically load bearing they often end up supporting some weight as the house settles. Even the weight of the wall itself could be a thousand pounds resting on the overhanging end of subfloor that was cut.