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Community and Q&A

Slab on grade flooring options

bobhol | Posted in General Questions on

I have a cottage that is being replaced by a retirement home .It is on bedrock in eastern Ontario and I plan to build slab on grade unless there is a better option .It will be a 1200 sq. foot 1.5 story.I am hoping to build with advanced framing style and sheath it with 4 inches of exterior foam. My question is about the concrete floor.I realize I can polish it and etch it but I am not sure about comfort of the hard surface and the temperature of cold concrete on your feet .I wonder if I could put foam board on it ,then an underlay plywood to lead to a normal house floor covering ? any thoughts would be appreciated …regards,Bob

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  1. GBA Editor
    Martin Holladay | | #1

    It's possible to install any type of flooring you want on top of a concrete slab, including hardwood, softwood, or carpeting. Any competent flooring installer can help you with the details.

    Some types of flooring require the installation of strapping or sleepers, so don't make your ceiling too low.

    Your slab will need two types of insulation: vertical rigid foam at the slab perimeter, and a continuous layer of horizontal rigid foam under the slab. You won't need any more insulation above your slab unless your contractor makes a mistake and forgets to put the insulation under the slab, where it belongs.

    Don't forget to install a layer of 6-mil polyethyelene (or thicker) between the top of the horizontal rigid foam and the slab.

  2. kevin_in_denver | | #2

    Martin's comments are all good.

    I'd like to add that I've been living on a stained slab for eight years and cannot be more enthusiastic about its durability and beauty. Its hardness doesn't bother us, we put down some gel mats in the kitchen work zone.

    The main reason NOT to cover it with foam and plywood and a flooring system is cost. You would be paying extra to have a floor that needs more maintenance. Our maintenance cost has been zero so far and I estimate another ten years before we'd need to do some.

    The concrete floor is always first-time visitors' favorite feature of the home.

    The foam and plywood also will thermally "decouple" the slab from the living space. That can be a comfort and efficiency disadvantage during seasons with >75F days combined with <70F not.

  3. bobhol | | #3

    Thanks for that insight of living on a concrete floor...I was concerned it would feel cold in the winter but you are saying that it is not an issue?...thanks,Bob

  4. kevin_in_denver | | #4

    The coolness isn't an issue for us. If the slab is also in the bedrooms and bathrooms, a few well-placed area rugs can fix any issues.

    Don't let any heating experts convince you to heat the slab, either. It's a waste of money in a well-insulated home, because the heat comes on so rarely. The slab is nearly always cool.

    Another money-saving possibility is using a monolithic slab. Just follow the design requirements for a Frost-protected shallow foundation.

  5. bobhol | | #5

    Thanks...I will read up on a monolithic slab ...I am not familiar with it ...regards,Bob

  6. user-1087436 | | #6

    One thing I've always wondered: how do you keep a slab from cracking? Maybe it's just me, but I seem to see a lot of cracked slabs. In a home with no other flooring, that would be a major bummer.

  7. GBA Editor
    Martin Holladay | | #7

    Q. "How do you keep a slab from cracking?"

    A. Don't add too much water to the mix; encourage slow curing by preventing rapid evaporation once the concrete is placed (using wet burlap, sprinklers, or curing compounds); and establish an appropriate number of control joints so that any cracks that do occur look good rather than random.

  8. homedesign | | #8

    Q. "How do you keep a slab from cracking?"
    A. In North Texas,(highly expansive soil)most slabs come with the Concrete Guarantee
    .... "The Concrete is Guaranteed to Crack"....

    Some Insulated Slab Insights can be found at "Slab Happy"

  9. kevin_in_denver | | #9

    Fiberglass strands added to the mix will pretty much eliminate those micro-cracks. They don't prevent big cracks, though.

    Post-tensioning is a fairly inexpensive way to eliminate large cracks, even in North Texas.

    The best looking control joints are made by cutting the concrete a few days after pouring. Tooled joints aren't as clean-looking.

  10. user-1087436 | | #10

    Thanks for the interesting replies. To Kevin Dickson: Post-tensioning is something I first heard of as an English teacher in a Turkish technical university. I know how it works for beams, but how on earth does it work for a slab?

  11. user-1087436 | | #11

    No pun intended. Please.

  12. user-659915 | | #13

    There are those who love concrete floors and those who can't abide 'em. They will certainly be hard under foot and as Kevin points out they will always feel cool to the touch. And as he also comments the thermal mass will help stabilize indoor temperatures in certain climate conditions.

    The good news is that if you decide to try the finished concrete and later decide you're not happy with it you can always add warmer, more forgiving finishes later if you choose: we've had great success with floating floors of cork, bamboo and hardwood for example, laid directly onto the concrete. But if you take this approach be sure to cover your bases by setting countertops high so that the dishwasher will still fit after you add additional material below it - say 3/4". This can be temporarily furred down for a neat look. Whatever you do though insulate below the slab not above it. The thermal mass in the slab will still have value even after you add a light covering.

    By the way, floating floor materials generally have extremely tough factory-applied UV-cured silicon dioxide finishes which are at least as durable as an etched concrete finish.

  13. dankolbert | | #14

    Polishing a slab will be almost as much as laying a floor. But you don't have to polish; we built a house several years ago, added dye in the truck (I think it was well under $1000 for the dye), clients decided not to polish and have been quite happy with the results since then. I think we put a sealer over it but can't remember.

    And as others have said, there are plenty of floors that can go right on top of the concrete (once it is fully cured) - glue-down wood, floating wood, cork, marmoleum, etc etc.

  14. bobhol | | #15

    I am very happy with all the information that is being given to me regarding slabs and flooring ...thanks to all who have answered ...this is a great forum and I learn something everyday ...regards Bob

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