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Community and Q&A

Slab on grade floor repair

user-4053553 | Posted in Green Building Techniques on

About 20 years ago two rooms were added onto this house, both have a slab on grade and the floor tiles are cracking, i had to repair a wall in one of the rooms, and it looks like the floor was the slab with a styrofoam sheet on top (2-3 inches thick), with 1/4in plywood, black adhesive (tar?) and kraft tiles on top (i have no idea what these tiles are called, they are 12×12 in, used in many schools and older buildings, are less then 1/4 in thick and are the same fibrous material all the way through (and i’m hoping not asbestos, they were installed about 10 years ago i am told).
What is a better way to replace this with a durable floor, i have no experience with slab on grade (and i’m a DIYer not a construction worker). I don’t have ceramic tiling skills, but i think Styrofoam and such thin wood are not good ideas.
Both rooms have floor flex and cracking, i am assuming the second was built the same way, though i have not torn it up to find out, they were done by the same contractor (the tiles were done a decade ago when other repairs were done, i don’t know what the tiles would have been before the current ones but i doubt that matters).

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  1. GBA Editor
    Martin Holladay | | #1

    There is nothing wrong with installing a layer of rigid foam on top of a slab, but your builder should have chosen a more durable (and thicker) subfloor than 1/4" plywood. A better choice would be 3/4" tongue-and-groove plywood, screwed through the foam to the concrete below.

    If you want, you can leave the existing flooring in place and simply install new 3/4" T&G plywood on top of the flooring. Once this plywood is secured to the concrete with screws, you can install any type of finish flooring you want on top of the plywood.

    If this work sounds daunting to you, you should hire a contractor to perform the work.

  2. user-4053553 | | #2

    Is white styrofoam rigid enough or should i replace it with something different? I'm happy to remove whats currently there to get rid of the squeaks and unevenness, from home depot's canadian website i see 3/4 plywood but not tongue and groove, is this critical? They do have 3/4 tongue and groove OSB, i assume plywood is preferable?
    In store they have more options then online so i'll look for 3/4 tongue and groove plywood the next time i am there

  3. Expert Member
    Dana Dorsett | | #3

    The white EPS foam board is fine, but needs at least 7/16" OSB for a subfloor, ideally with the seams staggered by a foot or more from the EPS. Hopefully the 1/4" goods had the seams staggered with the foam?

    Standard 13/16" -3/4" subflooring will work, but it is a bit overkill, since it isn't spanning joists, but is supported everywhere by the foam. Adding ~1/2" goods over the existing tiles with the seams of the new subfloor staggered relative to the 1/4" sheets will be stiffer than 3/4" subfloor over 16" o.c. joists. Using a bit of construction adhesive between the tiles and overlaid subfloor and some 1" ring-shank nails every 24" along the edges (and a couple in the middle) to keep the corners down and waves from forming you'll be just fine with half-inch OSB/plywood, no t&g needed.

  4. user-4053553 | | #4

    I'm thinking of ripping it up in the spring and going from there, thanks for the advice Martin and Dana :)

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