The concrete in my basement is, as most concrete is, very uneven. I was building up my subfloor to match an existing floor, so I got about 200 board feet of PT 4x4 and ripped 1/2" off using a table saw followed by a planer. Then they kinda sat for a month while I trecked across the trans-Canada railway. Anywho, the combined effect of the planing and the sitting was warpage. I put anchors in the concrete and tightened them down real good to flatten them, but some pieces are really far gone, where some of them snugged right up.

Here's a photo of how it is right now:

Here's a photo of how it is right now

It has a lot of high corners because of the warpage, and I sanded a good bit down with my angle grinder and some 36 grit.

Every time I move the level though it seems like something else is too high or too low. It's been hours of back and forth and it's starting to seem impossible to make this flat. The floor isn't actually level, it's 2-3 degrees tilted outwards, but flat is what matters to me. I'm starting to think if I put the plywood down that things just might go well, although at some points the plywood seems like it would be resting on the edge of the 4x3's because of the warpage.

Is there a reasonable tolerance that I should abide by in making these flat? Is there a better way? Is it ok if not the whole sleeper is in contact with the floor board? (bonus points: my framing down there looks structurally sound enough, right?!)

(and because I know somebody will call me out on it, there's no plastic under there but the epoxy coat I have down there is one of the moisture barrier variety, and I also put PT on to just to be safe - I know already! I know!)

  • What flooring are you intending to apply? – Matthew Oct 15 '12 at 17:54
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    Granite tile, intended to be on top of ditra – kavisiegel Oct 15 '12 at 17:55
  • You should definitely include your finished floor in the original question. – Matthew Oct 15 '12 at 18:00

Perhaps you're thinking about this in the wrong order.

What if you applied the plywood to the almost level sleepers and then leveled a surface over the ply?

I think you''re going to have a very difficult time leveling sleepers as you have in that photo.

EDIT: Per your comment that you intend to install tile atop ditra.... you have a very forgiving tolerance with the subfloor, then! Glue and screw the ply into the sleepers. Then apply the Ditra. You can level out imperfections in the leveling with thinset below the tile depending on just how bad things are.

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  • Actually, that's the simplest most genius thing I've heard all day. I have half a bag of self-leveling concrete and a whole bottle of primer left over. That could happen, but I'd be worried about the stresses - might it crack a tile if the plywood is tightened somewhere that causes a hill or valley and walking makes it "bounce?" – kavisiegel Oct 15 '12 at 17:56
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    Your plywood shouldn't have deflection. Glue and screw to the sleepers. If you're doubtful, using a thicker Sturd-I-Floor ply. If you're nothing doing too large of a surface, spring for the 1-1/4" ply... maybe ten bucks more per sheet... but how much peace of mind do you want, and how much will you pay for that? – Matthew Oct 15 '12 at 17:59
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    Use the leveling compound below the ditra if you need it. Use the thinset layer as the final leveling layer. – The Evil Greebo Oct 15 '12 at 23:37
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    Yes, glue and screw. Use construction adhesive to glue the plywood down, it has a high level of solids and will fill significant voids. Plywood glued and screwed this way will never move the slightest bit over the sleepers under any kind of load. It will be fully equivalent (better actually) to perfectly flat, fully supporting sleepers. You don't need to "suck" the plywood down into the sleeper "valleys" with screws, only drive enough to get full contact with the glue and bring the sheet into level. This will minimize internal stresses. – bcworkz Oct 16 '12 at 19:37

You could try a plane as a tool for shaving down the high spots. A good quality plane with a sharp blade can remove a lot of wood in a short time.

I do wonder though....if you get them all honed down to a satisfactory "flat" will they stay that way? Seems like they could warp more.

I also see a small problem with how you have laid out the wooden members on the floor. You have pieces going up to the studded wall at 90 degrees. When you come to lay the floor there is no support for the floor edge at the wall between these 90 degree pieces. It is recommended to have all edges supported.

In regular house construction the joists go in first, then the plywood subfloor comes next and then after that bottom plates and studs are installed.

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  • Actually I should have mentioned, I tried a plane first. I didn't like the lack of control though, the tool did it's own thing and I just placed it where appropriate. It left lots of edges. Anyways I suppose you're right, they could warp more, but they've dried for about 3 months now, and if they try they'll have a hell of a time pulling the 600lbs concrete anchors out and pulling the rest of the subfloor with it. About the floorboard, I was just going to notch them around the studs. I realized that mistake when it was too late. – kavisiegel Oct 15 '12 at 17:50

A belt sander can take off a lot of material fairly easily. But be careful, for the same reason.

As to level, knock down the ridges and check again. Use a long level (at least 4'). If there are significant irregularities after shaving the ridges, you can tack down shims in the gap spaces. Dry fit the plywood and test for rocking or springing. If you find some, lift and add/move shims. When it seems solid, nail and glue the shims, then glue and screw the plywood.

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Instead of a wooden subfloor, you could fill it with self-leveling concrete.

Of course, you could also dig up the existing concrete and lay all the new plumbing in the ground under a new concrete floor, which is, I believe, the best method if not the easiest or cheapest.

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  • I would venture to say that even if I went though 500 sandpaper discs trying to flatten these sleepers, it would still be easier and cheaper than starting over, because even when I start over.. I would still have to level it. There's no real benefit to starting over, really – kavisiegel Oct 15 '12 at 17:41
  • You can get "self leveling concrete" which is liquid enough to find its own level. But for the most part I agree with you. – Brian White Oct 15 '12 at 19:40
  • I agree with Brian... Dig it up, put the plumbing in the ground and level/flatten the new concrete... While you are at it, add in some radiant floor heat, and tile. The "right" solution is always the best solution. (Your existing PVC can still be used with a few careful cuts and some couplers) – user9842 Dec 31 '12 at 12:38

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