I am building a room in the basement where the slab has a pitch. I have already installed had a foam and magnesium board subfloor and my intention is to have a carpet installed on top of that. The room is about 17 x 10 and the pitch is on the 17 foot length. It is roughly 2 to 2.5 inches lower on one side that the other end.

I didn't "fix" the pitch on purpose because it is there to allow any water leaks to flow towards the drain.

So what are my options when I finish the walls and install a basement and a carpet in this room. Will I need to live with the slopping floor (which I guess will be most pronounced in the baseboard) or is there a trick to fix this where it is unnoticeable? Is there a "carpet padding shim"?

  • I'm in the beginning stages of finishing my basement too and I'm going with xps board with mag board over the top. Did you find foam with drainage channels? I was thinking this would be perfect (as opposed to spending the money on delta-fl or another product). Please share with me the foam product you are using for the subfloor. Thanks!
    – user30285
    Jan 3, 2015 at 5:56

3 Answers 3


I would stop and make your floor level before you proceed.

Home Depot/Lowes/etc stores in my area sell 2'x2' subfloor 'tiles' that is basically tongue & groove OSB with dimpled foundation membrane on the bottom side. However, you can do this yourself using the foundation membrane and sheets of plywood or OSB. The flooring looks like this:

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As I see it, you have two choices:

  1. Leave your subfloor as is, but install sleepers and plywood or OSB on top (will reduce ceiling height a bit).
  2. Rip out the subfloor, and install something like the dimpled foundation membrane, then the sleepers and the plywood or OSB. This is better, in my opinion, because it would allow any water to run downhill instead of being trapped through capillary action.
  • The subfloor I have installed has drainage channels underneath. After a lot of research, this was the best product I found in terms of allowing the floor to breath, water to drain with the pitch and also provide the most R value. I am using the dimpled membrane (DMX 1-Step) in the rest of the space where water is less of an issue. I am trying my very best to not have any wood product in the basement, so can't go with OSB now after specifically going with an OSB-free subfloor product. FYI, I properly measured the pitched and it is about 1.5" over 15 feet.
    – 0pt1m1z3
    Dec 29, 2014 at 14:01
  • Okay, I understand your desire. I had assumed you were putting plywood or OSB over your subfloor. Have you thought about how the carpeting will be installed - will tackless strips be used? If so, what will they be fastened to? 1.5" is a lot of space to fill, so I would suggest the rigid pink styrofoam (it comes in various sizes and thickness). I have read of people using asphalt roofing shingles to shim subfloors -- that might work for you. It is important to figure out how you are going to end up with a smooth flat surface for your carpeting. Do you have a dehumidifier?
    – Mark
    Jan 8, 2015 at 4:40

The normal fix for this type of installation is to install treated lumber sleepers that are cut to the slope of the floor to level out the base. It sounds like you have proceeded beyond the point so this may not be a solution for you unless you look for a similar solution installed over your foam and base material.

  • Because of high humidity and other moisture issues, I am trying to keep it as wood-free as possible. Hence the magnesium board subfloor.
    – 0pt1m1z3
    Dec 28, 2014 at 1:27

I would leave the pitch for your room. I think doing any sort of subfloor is a waste of time if you are laying carpet and could in fact cause long-term issues.

As you have already mentioned the baseboards are the only place you will see this. This is actually pretty easy to deal with unless you have cupping, which it sounds like you don't. Throw on another question for how to deal with the baseboards and I will help out there.

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