Is there a way to level a wood floor?
February 8, 2012 8:46 PM   Subscribe

Leveling a wooden plank floor to install tile, for dummies.

Thanks for everybody's help on my last bathroom design question. I've decided to do it the easy way, and just tile the floor and then install a clawfoot tub. I was hoping to get a little help with the floor tile.

My subfloor is wood planking. It's something like this, except without gaps between the planks or holes in them. It slopes toward a center point, losing about 1" in height over a 30" distance.

What I want to do is to use some sort of self-leveling concrete to fill this, but can you use self-leveling concrete over wood planking? For instance, it sounds like this product (pdf) could be used over wood, with some mesh. Then, this product recommends I put down plywood first, then use the leveler on top of it. But what about little dips underneath the plywood? Is there no risk of bending and cracking there? Then, elsewhere I hear that any self-levelling concrete over wood will crack when the wood expands and contracts. This house was built in the 1890s, so I'm guessing this wood planking is pretty old. I would prefer not to tear it out and put down plywood or somehow level the joists.

My plan was to level the floor with some sort of self-levelling compound, screw down 3/4" plywood, then Ditra, then tile. I have about 1" from the high point of the floor to the surface of the adjacent room's floor. Ideally, I'd use something that could be brought to a feather-edge at the floor's high-point, but at this point, I don't care if the floors are slightly different elevations. StickyCarpet's approach here is my new backup plan.

Thanks for any suggestions.
posted by slidell to Home & Garden (8 answers total) 2 users marked this as a favorite
Products like the Quikcrete Leveling resurfacer that you show can be hard to use if you haven't done it before. They set up so fast that parts are hardening while you're trying to trowel it smooth. I've actually used tile mastic or thin set that I've had around to level portions of floor before tiling. They're easy to work and dry rock hard. The premixed is really easy to use but expensive.

Maybe try the leveler in two shots, doing a first rough coat to get it close and not worry too much how smooth you get it, then feather out with a second light coat over that. With 3/4 ply screwed down over that you should be good to go, plenty ridgid.
posted by PaulBGoode at 12:07 AM on February 9, 2012

Thanks, Paul. A couple quick follow-ups, for you or anyone else who comes along.

1. I'd love to believe that I could use either thinset or some sort of self-leveling compound over the 1 x 6 planking, but I've heard some say that the planks will expand and contract so much with humidity changes that it would break up the concrete. Do you think that's a risk? Does it matter, if I've already put down 3/4" plywood and ditra?

2. Would you put down some sort of felt paper in between the planking and the thinset / self-leveling concrete?

3. You say "screw down" the plywood, so you'd screw the plywood through this layer of concrete? I hadn't thought about that until now, but will this be hard to screw through?
posted by slidell at 1:30 AM on February 9, 2012

I've done exactly this sort of job, including the SLC, the plywood and the Ditra. Even one better, I embedded electric radiant heating wires in the SLC.

But what about little dips underneath the plywood? Is there no risk of bending and cracking there?

No, this is where you're going wrong. Put the plywood down first.

Wood planks absolutely do expand and contract enough to fracture thin layers of cement products. Would these cracks harm your tile installation through a layer of plywood AND a layer of Ditra? Maybe not. But you don't need to worry about that because that's not what you do. You have nothing to gain by putting the SLC down first. You need the plywood and the SLC both, and you should do it the way it says to do it on the box: plywood first. The plywood you'll be using will be plenty flexible enough to conform to the slightly irregular shape of the existing subfloor.

If you head over to the John Bridge Tile Forums and do some reading, you'll find that the first thing you need to do is figure out whether your floor joists are rigid enough to support tile. This will involve measuring the height of the joists and their length between structural supports, and entering them into a calculator that gives you a thumbs up/thumbs down indicator of whether the framing is sufficiently rigid. After that, the next step is to ensure that the subfloor is also rigid enough. The standard recommendation is 1 1/8" - 1 1/4" of wood thickness. Your existing planks are probably 3/4" thick, so you need to add 3/8" or 1/2" of good quality plywood (few voids), properly fastened down. It's been years, and I can't remember what the recommended plywood fastening method is - whether you're supposed to use glue or thinset between the planks and plywood, what sort of fasteners at what spacing, whether to drive those fasteners into the joists or just into the plank subfloor. Get all of those details straight before you start, because they are important. The better contributors at the John Bridge forums will cite best practices from the TCA handbook.

With the plywood down, you'll have your rigidity fully established. Plywood is far more dimensionally stable than solid wood planking, so you can bond the SLC to it without fear of the thin areas turning to powder over the course of a few years. Note: the SLC I used required a special primer to ensure a good bond to wood.

As Paul indicated, SLC is not idiot-proof. It's self-leveling in the same sense that pancake batter on a hot griddle is self-leveling, i.e. not very. In my case, I set up a couple of temporary guide strips of wood shimmed level as StickyCarpet did with his plywood strips in the thread you linked, and I used a straight board to sweep the SLC more or less level between those strips. As the SLC started to firm up, I gingerly removed my guide strips and filled in the gaps with more SLC. It was an awkward process but it worked reasonably well. I did not get the surface of my SLC as smooth as hoped, but this was easily fixed; after it cured, I troweled on a skim coat of the same thinset that was to be used to adhere the Ditra.

If you possibly can, have a helper on hand when you tackle the SLC part of the job.
posted by jon1270 at 2:11 AM on February 9, 2012 [4 favorites]

Hi again, and thanks for the advice. I looked at the joists beneath, and it does look like they could conceivably be leveled as part of the framing work we're doing now. If so, would it make sense to pull off the floorboards above? The people at John Bridge seem to think so. I'm nervous that removing them will suddenly necessitate whole realms of work that I didn't anticipate or remove one part of what has generally held the house together so well over this past century.
posted by slidell at 9:45 AM on February 10, 2012

Your subfloor is not holding the house together. Or, rather, if it is then you have bigger problems than how to install tile. The only structural issue I can think of that might come up with removing the subfloor is if one of the walls that's parallel to the joists is positioned between joists, such that cutting the floorboards along that wall will leave the wall itself unsupported. That was the case in my house, so cutting out the original subfloor out wasn't feasible.

From here, where I can't see anything, I can't say for sure what I'd decide to do. I would certainly explore the possibility of straightening the framing and putting in a more stable subfloor, because it would make for a cleaner and maybe more durable job.

I'm nervous that removing them will suddenly necessitate whole realms of work that I didn't anticipate

Doing a tile floor correctly is a lot of work regardless of which approach you take. Quick and easy tile installations are the ones that look like hell and fall apart when they're 8 months old.
posted by jon1270 at 11:08 AM on February 10, 2012

That was the case in my house, so cutting out the original subfloor out wasn't feasible.

More accurately, cutting out the original subfloor would've required some extra bracing that seemed, in my mind, contrived, and which I didn't feel like messing with. In retrospect it wouldn't have been all that hard, but I'm a lot less green than I was then.
posted by jon1270 at 11:12 AM on February 10, 2012

jon1270, actually, we are both right -- as I climb around more, the subfloor IS holding a portion of that floor together, AND I DO have bigger problems than an unlevel subfloor. In fact, there's at least one floor joist that is resting on nothing on one end and held at the level of the others by the floor planks! Not surprisingly, that's the area where it's least level. I've been spending a lot of time reading "how to correct your deficient framing" guides. I don't know why the engineer said it was fine, but he didn't climb up on the ladder, so maybe he was not able to see that certain joists are floating in thin air.
posted by slidell at 11:50 AM on February 19, 2012 [1 favorite]

Good for you for finding the source of the trouble. Also, thank your lucky stars that you didn't just build the tile installation on the sloped floor. If you had, it would've cracked and leaked in short order, and you'd have to tear it out and start over.
posted by jon1270 at 11:58 AM on February 20, 2012

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