Plywood instead of concrete for slab on grade
robertbob2 | Posted in Building Code Questions on
Will a plywood slab meet North American building code requirements?
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Projects with plywood sub-floors rather than slabs have been approved, built and featured here on GBA. But your question is too general. Building codes aren't continent wide, and getting something approved in one jurisdiction doesn't mean it will fly everywhere. Your best bet is to look at both the building code you are under, and run it by the inspectors where you are before proceeding.
I have researched it pretty thoroughly and I'm not aware of anything in the IRC model code that would prohibit a plywood slab with a concrete perimeter beam, but there are a lot of details to understand and some of them are different than standard practice. I agree with Malcolm that a good place to start is your code official; if they say it won't work, ask for specific code references that prohibit it. Code officials are justifiably leery of assemblies they haven't seen before and that aren't explicitly allowed for in the codes.
The one pertinent grey area in our code is the requirement to maintain a 6" distance between untreated lumber and grade. Outside that is usually interpreted as only applying to landscaping, not hard finishes, so garage and entry slabs can be close with the tops of foundations. In crawlspaces and basements it applies to framing where there is no slab, so it seems like there is some room for interpretation once you get to plywood on foam.
Malcolm, the IRC has a comparable requirement but they have an exception when using "an impervious moisture barrier": https://codes.iccsafe.org/content/IRC2015/chapter-3-building-planning#IRC2015_Pt03_Ch03_SecR317.1
Interesting. I went back to see if maybe a similar amendment had been added to our codes and can't find anything comparable. That doesn't mean it wouldn't be approved here, just that it isn't as straightforward as it appears to be where you are.
Thanks for the replies I will contact the local inspector. I should have phrased my question differently, what I meant to ask was, has anyone had any experience with plywood slabs vs concrete in regards to Building code requirements.
The code we are governed by applies to British Columbia.
That's the code I was looking at. If you want to take a stab at interpreting it you can find it here:
The relevant sections are:
I would add 9.16
Yes, good catch. It includes the requirement that wood floors at grade: "constructed of wood shall conform to CSA S406, “Permanent Wood Foundations for Housing and Small Buildings.”
I've done 3 slabless slabs now here in N WI and use 8-10" of 3x4 no fines gravel with 3" screenings on top. Then 6" of EPS with Stego plastic barrier on top. I like a dimpled matt above that for drying potential before laying Advantech or 3/4" Plywood down in 2 layers for sub flooring.
Sounds like a great system. Do you run the second layer of plywood perpendicular to the first?
I hope these cement-free "slabs" become accepted in more jurisdictions. I'd like to use them in my design projects. For now, unfortunately, I have to stick with what inspectors are familiar with.
I do, second layer gets cut so seams are covered as you would insulation layers.