Wavy subfloor and expansive soil

[Kirby-bo]

My wife and I just bought a house in Colorado, and have discovered that the uneven floor is due to expansive soil. At least, that's what the structural engineer says. The house was built in '72, so they didn't float the non-bearing walls, as they would have done in the 80's. My luck. Also, the subfloor consists of a layer of plywood and a layer of 3/4 particle board. The floor isn't that bad, really -- in fact, the structural engineer said he would just live with it -- put carpet down and just forget about it. But we want wood floor, so... what to do. Here's one solution suggested by a local carpenter: rip out all the p-board, and replace it with plywood. Shim up the top layer of plywood to make the floor level, and then finish it the way you want.

I'd be grateful for any opinions on this approach, and/or suggestions for a fix or anybody else's experience with this problem.

I should say this is a minor case of expansive soil -- no structural issues, no significant cracks anywhere. The structural engineer wrote "no structural issues" in his report. As I understand it, the soils swell up under the slab and push up on the non-load bearing walls. But this is all new to me.

Thanks!

 

[jay surfs]

Not a pro. Renovating my second home.

So I just bought a new two story house also, on expansive soil in CA. Half the house floors are wavy and or sagging a little bit. I am currently demoing the upstairs, including the sub-floor 1x6's and all the wall board. I will be replacing the subfloor with 3/4T&G osb plywood, and #10 2 1/2" screws.

My neighbor is a contractor. he came over and we discussed my options.

Upstairs floor is noticeably tilted. The first floor slab below is cracked the width of the house, and the south side wall of the house has sunk/pulled out aprox 1" 1/4" inches. So it's very noticeable in the master bedroom. I'm going to install NuCore floating floor boards. So he suggested I make the floor plum, and not level. As in, a continuously flat surface, but not level to the earth. The Nucore flooring will set together well without gaps at least. And to sister in new joists where a joist is sagging, and notch or plane down high joists. I bought a 96" level to help.

I think that will be far less difficult to do than make the entire upstairs perfectly level. So I'm going to live with the slight tilt. The tilt spans over 15 feet so it's livable.

The first floor living room has a large hump in the middle of it, maybe it rises about 3/8 of an inch, across 3 feet or so. I'm going to just notch the joist or two involved, to be level with the other joists in the room. boom. Neighbor contractor dude said that will work fine.

Good luck to ya! Hope my ideas may give you some insight.

 

[Sponsor]

 

[Jadnashua]

THere's more than one way to resolve this. First, get rid of the particle board...it does essentially nothing good for you and most of the fixes wouldn't work with it in the first place. It has essentially no structural benefit, blows up if it gets damp long-term, and can wear away if there's any abrasion say from grit getting through existing carpeting. You need to determine the actual extent of the variations from both flat and level. Your finished floor will want flat, level is nice, but optional.
1. You could have self-leveling concrete (SLC) installed. You can do this yourself, but it's a bit tricky and in fact, it doesn't really self-level without some help. Note, most of them actually level things, so that could make a transition messy. There are some thixotropic versions that can be made into a ramp, but are harder to install.
2. Trying to shim on top of the existing ply would be messy. If you remove the existing ply, you could sister some cleats onto the sides of the existing joists so that you have something nice and flat to then put new subflooring onto. Screws and construction adhesive would let you use something like a 2x4 along the side.
3. If the structural engineer approves, you might be able to plane the high joists down, and add some shims to the low ones to make things level. I did this on my entire first floor. A laser level, long straightedge, and a good power planer let me get things level across the entire first floor to install an engineered wood finished floor. While messy, it didn't actually take all that much time.

 

[Kirby-bo]

THere's more than one way to resolve this. First, get rid of the particle board...it does essentially nothing good for you and most of the fixes wouldn't work with it in the first place. It has essentially no structural benefit, blows up if it gets damp long-term, and can wear away if there's any abrasion say from grit getting through existing carpeting. You need to determine the actual extent of the variations from both flat and level. Your finished floor will want flat, level is nice, but optional.
1. You could have self-leveling concrete (SLC) installed. You can do this yourself, but it's a bit tricky and in fact, it doesn't really self-level without some help. Note, most of them actually level things, so that could make a transition messy. There are some thixotropic versions that can be made into a ramp, but are harder to install.
2. Trying to shim on top of the existing ply would be messy. If you remove the existing ply, you could sister some cleats onto the sides of the existing joists so that you have something nice and flat to then put new subflooring onto. Screws and construction adhesive would let you use something like a 2x4 along the side.
3. If the structural engineer approves, you might be able to plane the high joists down, and add some shims to the low ones to make things level. I did this on my entire first floor. A laser level, long straightedge, and a good power planer let me get things level across the entire first floor to install an engineered wood finished floor. While messy, it didn't actually take all that much time.

 

[Kirby-bo]

Thanks for the replies-- good info. I'm definitely taking out the particle board. Now I wonder about floating the non-bearing walls in the basement. Don't I have to do this to prevent problems in the future?

 

[Jadnashua]

A partition wall, running in between joists, that is built tight between the subflooring and the joists above will continue to flex the subflooring beneath it as seasonal dimension changes and loads from above are applied and removed. Whether it will end up causing your tile installation to fail has a higher probability.

I have one like that in my place. A water bed (long since removed) caused the subflooring to literally bend into a z-shape, almost 1/2". I didn't install tile, so can't tell you if that would have created problems, but did end up shoring that section up with an additional joist to try to help.

I'd ask over at www.johnbridge.com.

 

[Kirby-bo]

Thanks for all the help. I'm still wondering about one thing: if I go down to the basement and float the non-bearing walls now... will the floor above move? That would be bad -- if I make the subfloor level and then the floor drops when I float the walls below. So... what's to do? Float the walls and wait a couple of years for the floor to stabilize?
Aargh.

 

[Jadnashua]

That's a tough question. From a long-term standpoint, wood acts more like wax than a spring. IOW, over time, it will bend and stay there, verses springing back like a spring. Short-term, it is springy, dependent on the amount of weight applied and how long it stays. Think of a bookshelf over time...remove the books, and it often won't return to being straight, it will have taken on a set. WOod tends to expand across the grain more than along its length, so the height of the wall won't change all that much between seasons. OTOH, tile doesn't like movement much. I'd consider discussing it with a structural engineer, familiar with your area and the soils involved. AN hour or so of consultant fees could be cheap. It gets more expensive if you want him to design something and certify the plans, but educated guesses from an expert may be better than what you'll find on the internet.

I'd still discuss it over at www.johnbridge.com ...lots of tile pros there that have probably been exposed to this situation.