Q imported from our old site, Face Lift Floors: I recently installed a plywood subfloor over a slab. There is a cement “lip” bordering the entire room which is uneven and ranges from 1″ to 3″ in different spots. It is impossible to make the plywood meet this lip all the way around the room.
Is there anything that can be used to lay over the subfloor to help in the uneven areas? Maybe some kind of moisture barrier that will also help uneven spots? We are putting down a laminate floor in the room.
A: I wouldn’t be thinking of putting any sort of “vapor barrier” over the plywood you have fastened to the concrete. If anything, the vapor barrier is to be placed between the concrete and plywood, and not on top. I am not sure how you can deal with this concrete lip, especially given that it is not even in height, and ranges from 1-3 inches. How far out from the wall does it protrude? Is it close enough to the wall so that 5/8 baseboard and then 3/4 quarter round would cover it?
Keep in mind if you are installing laminate flooring: the subfloor has to be flat. If there are slight dips of, let’s say 1/16 or 1/8 inch, you can use black tar paper to level the area, or perhaps even a quick dry concrete patch. Perhaps you may have been better off in the long run to use Dri-core rather than plywood. It gives you a bit of elevation off the concrete with a plastic vapor barrier already attached and allows you to ship certain areas with these spacers that snap onto the underside.
Hindsight is great, isn’t it? If this lip around the perimeter is large, perhaps you will need to build a small box around it. I hope some of these ramblings might be helpful.
Original / moved link https://faceliftfloors.com/q-and-a/cement.php