Flattening plywood sub-floor on a slope

9 years ago

I'm prepping a plywood sub-floor, which will get floating cork flooring. The entire room slopes maybe 1.25" from end to end (19 feet), and maybe half that from side to side (13 feet). Also quite uneven in flatness, which is the issue I need to address. The slope isn't going to get fixed, since there are 3 doorways -- 2 tile and one hardwood -- with which the floor needs to maintain height. Also another doorway and 2 closets.

What is the best way to flatten floors on a slope? I don't think that I can use self leveler, as it will just flow down the slope. I can fill in the lowest spots with 1 to 2 layers of underlayment plywood (1/4" nominal, but about 0.21" actual thickness), but still need to taper down the steps at their edges. Lots of #15 felt layers and work, if I go that route. If I use some sort of patch/leveler/cement product, what should I use, and how best to apply it?

-Gary

Featured Answer

attofarad
Original Author

9 years ago

I'm not eager to shim joists -- that's over 1000 screws and nails to pull out. Also, I think the plywood is glued to the joists.

I've already sanded down high spots. Something like Mapei Planipatch, or something else that sets a bit slower? Any specific product recommendation (widely available) would be appreciated. It will take a few bags of that in total, maybe 6 or 8 25# bags.

Comments (9)

weedyacres
9 years ago
Is your unevenness mostly from high spots or low spots? I'd sand down the high spots and spread floor patch on the low spots.
PRO
Joseph Corlett, LLC
9 years ago
Shim your joists. It's not that big a deal.
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gregmills_gw
9 years ago
1.25" over the course of 19' is hardly anything to bat an eye at. now without seeing the layout of the floor, I can only assume here, but any chance you can start at the highest end of the floor first. my guess, is as you go down the slope you wont even notice the change.
attofarad
Original Author
9 years ago
Greg,

What exactly do you mean by "start at the highest end of the floor first" ? Start with patching? With SLC?

thanks,
Gary
gregmills_gw
9 years ago
no start you cork at the higher end of the floor, the part that raises up to 1.25" as opposed to starting at the bottom of the slope.

the amount of span that 1.25" is insignificant in my opinion. surely that might change if I were to physically be there, but being that its cork you're putting down, I don't think you need to do anything as far as shimming or messing with self leveler.

you don't need to get it level, just flat. so if your ply wood seems are jagged then sand those flush. if you have a major dip, that isn't accounted for in your measurement of 1.25" over 19' then shim that with underlayment if possible.
attofarad
Original Author
9 years ago
Yes, I'm not worried about the overall drop, in either direction, just trying to get it as flat as possible (not level).

The drop isn't uniform, with dips. For example, along the long wall, the drop toward the middle of the wall is nearly 5/8" lower than the straight line drop. Needs to be very flat along the periphery, so that base molding doesn't have gaps underneath. Other dips are also too much for the floating floor to lie flat, so also need to be fixed.

So, I am planning to shim with underlayment (5mm=0.2") in the areas that are dips of that thickness or more. Some areas get a second layer.

After shimming with underlayment, can I use floor patch (or some other cement compound) to taper the edges from underlayment to no underlayment, and to flatten other areas? If so, specific type recommendations appropriate for 1/4" thick down to zero thickness would be greatly appreciated. Application tips also.

Thanks for the help,
Gary
weedyacres
9 years ago
I wouldn't mess with underlayment shim + patch, I'd just patch. Though gregmills is a pro and I'm not. :-)

I can't remember what specific products I've used in the patch. Just read the labels on the stuff at Lowe's and found what was suited for the application. As long as it's made to stick to wood and won't delaminate afterwards, you should be good.
attofarad
Original Author
9 years ago
All done. Took me a lot of work at my non-pro rate of progress. After sanding high spots, I filled in whatever odd-shaped areas that were more than 5mm low with underlayment plywood (as many as 3 layers at one spot), stapled down. I then used patch -- twelve 25# bags of Henry Unipro, and 2 of Mapei Planipatch -- mixed in small batches, which I applied by screed and trowel. Not a pro, so I often had to repeat or touch up. Eventually got flat (not level) to under 1mm in 8 feet for the entire floor, except along one line near the door where the subfloor took a bit of an abrupt dive, but got that down to under 1/8" in 6 feet, which even I don't notice, now that the flooring is down.
I did get some spider cracks in the patch, but it stayed well adhered, and didn't seem to crack any more in any section after the first cracks appeared. I've seen other photos and references that this is fairly common over plywood subfloor, and not a problem.