GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Concrete-Free Slab for Garage

[email protected] | Posted in Green Building Techniques on

With the greener building practices turning up around concrete-free slabs, I have not found anything around using the same practice for a detached garage.

To be clear (and if you’re not familiar), I’m referring to a general floor assembly of crushed/compacted rock, then foam, then poly, then staggered sheets of plywood subfloor.

The immediate concerns that come to mind are:
– the finish layer: what would you lay on top of the plywood to protect it from the elements? tile? epoxy?
– the compressive strength of the foam and plywood: would it hold up with a vehicle driving and parking on it?
– garage door transition: would you use the same one you’d use for any other door opening?

What do you think?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Akos | | #1

    I don't think you can as wood won't meet non-combustible requirement unless you get expensive wood. At that point, might as well pay for the concrete.

    You also have a pretty large point load you have to deal with, works out to around 100PSI, which is well above the capacity of most rigid insulation.

    The concrete topping again helps with this as it spreads the point load out a fair bit. You can do the same thing with wood by adding more layers, but now you are adding even more of the expensive fire rated wood.

    I think concrete free detail is best left for houses.

    1. [email protected] | | #3

      Excellent points. I hadn't thought of non-combustible requirements.

  2. Expert Member


    Not a particularly good alternative, but maybe consider pavers instead - much as you would use on a driveway? However I have no idea how their embodied carbon compares to a slab.

    As the alternatives represent such compromises, for the small amount of concrete used in a garage, perhaps the best strategy is to offset the carbon footprint in other ways.

    1. Expert Member
      NICK KEENAN | | #6

      Pavers at least are reusable and can be taken up and put back for repairs. Concrete is single use. So it's not quite a fair comparison.

      I wanted to try pavers for a garage, the issue I ran into was they were quite expensive.

    2. [email protected] | | #11

      Pavers are definitely one alternative. And as pointed out, reusability/repair is one benefit.

      But due to higher cost, questionable embodied carbon savings (if any), and loss of a smooth, easily sealable surface, this alternative is less attractive than just sticking with concrete.

  3. Expert Member
    PETER G ENGLE PE | | #4

    You also need to consider the impact of both flammable and non-flammable liquids. Garage slabs are pitched to the door for this reason. Consider what would happen if gasoline spilled and soaked your floor sandwich. Or pesticides. Or just the constant dripping of meltwater from the vehicles. Go with concrete here and save carbon elsewhere.

    1. [email protected] | | #10

      I guess I think of concrete as porous - so it would need to be sealed to prevent gasoline and other liquids from soaking into it as well, right? If I can seal concrete, why not other surfaces?

      If we're not sealing the concrete, then we're in the same categorical design as the gravel / paver routes in my mind.

  4. user-6623302 | | #5

    There is many a garage out there with no finished floor, just compacted gravel/stone.

    1. Expert Member
      NICK KEENAN | | #7

      Many basements in this area from the 1800's are floored with pavers.

  5. JC72 | | #8

    It's a bad idea for a multitude of reasons.

    1. user-6623302 | | #9

      What is, pavers or no floor?

  6. charlie_sullivan | | #12

    I'm really late to this question but I've been thinking about it and thought I'd revive the discussion.

    As far as just a gravel floor, my concern would be how to manage moisture. With no vapor barrier underneath, moisture from the ground would make the garage humid all the time, leading to rust, mold, etc. With a vapor barrier underneath, moisture from wet cars would pool Perhaps it's possible to have clean coarse gravel that allows that water to flow through easily, and have a layer underneath it to shed the water, that's still above grade so it can run out. Maybe an EPDM roof membrane.

    If you can source cobblestones locally, that's an option that's lower CO2 than concrete paver blocks. Either reclaimed or newly quarried. I got to wondering about the CO2 impact of shipping rock long distances, and calculated that on North American diesel-powered freight railroads, you could ship rocks 41 thousand miles before the CO2 impact would reach that of concrete. Or maybe 4000 miles by truck. So that's not a bad option from a carbon perspective even if there's some trucking involved, but from a cost and climate perspective local sourcing is of course best.

    But is that really better than just gravel? If you could grout the seams, you could, to some extent, allow water to run off instead of infiltrating below, and you could use a vapor barrier underneath without water pooling on it. So that's what I like about it. And maybe cutting blocks uses less energy than crushing gravel. You'd want blocks cut as square as economically feasible.

    I've also thought about putting steel diamond-plate sheets on top of crushed gravel, with welded or even just bolted seams. It's hard to know how thick it would need to be. You'd want the load to transfer to the gravel underneath, and if it flexes a little that's fine, but you won't want it to crumple and rip. It might be that a sandwich of steel on top of plywood would be more robust with thinner steel and still meet requirements to be non-flammable.

    For rust protection of the steel, galvanizing would probably do the trick. I might add tung oil and/or active galvanic protection. Or use aluminum instead of steel--these thin diamond-plate aluminum sheets are only $4/sq ft. They might not be thick enough over crushed gravel, but would probably work with a plywood layer underneath.

    I'd probably use "glavel" instead of crushed gravel to get some insulation and avoid needing the foam your design has--with either the steel or the cobblestones.

    Finally, I've thought about reverting to a car port with a gravel floor, so it's open and the humidity is just the humidity outdoors. But that allows mice and squirrels easier access to the car. Perhaps a car port with 1/4" wire mesh walls?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |