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Community and Q&A

Slab on Grade in Climate Zone 6

jberks | Posted in Energy Efficiency and Durability on

Hey all,

So watching a bunch of Matt Risinger, he’s always talking about slab on grade builds, and how we have basements in the far north etc. I know these topics have been brought up before.

I’m in Climate zone 6, Toronto. I’m designing a 100sqft satellite building, or you could call it a shed with all the bells and whistles of plumbing, elec, heating etc.

I’d like to do a frost protected slab on grade for this to incorporate infloor radiant hydronic heating. The slab will essentially act as the radiator, and be the base for the Insulated perimeter wood stud walls.

Most general knowledge for small structures like decks and sheds are all about pier foundations. The frost line here 4′ deep and its all about getting lower to avoid heaving or else….

Does anyone have any wisdom with building a slab on grade within frost lines?



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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure what you mean by "building a slab on grade within frost lines."

    Did you mean to write "frost walls" instead of "frost lines"?

    In Toronto, the frost line is probably about 3 feet below grade. Most slab-on-grade buildings have frost wall perimeter foundations (with short concrete walls that extend 3 or 4 feet below grade to a footing that is placed below the frost line).

    For more information on this type of foundation, see Insulating a slab on grade.

    A frost-protected shallow foundation is different. I'm not sure whether your local building inspector will accept a frost-protected shallow foundation -- but if you want this type of foundation, you can always ask for permission to build one. Here is more information: Frost-Protected Shallow Foundations.

  2. Trevor_Lambert | | #2

    If you're using the slab as a radiant heat source, an insulated floating slab is probably the way to go. 8-12" of EPS underneath and around all the sides may seem like overkill, but the incremental cost is not all that much. The slab needs to be specially reinforced to spread the load. You also need to make sure the sub soil is suitable. In a lot of municipalities, out buildings below a certain size (typically 10 square meters, around 107 square feet) require no building permit. An electrical permit would still be required.

  3. Expert Member

    The Canadian NBC says it doesn't apply to structures under 10 sq m, but that is followed by "that do not create a hazard". Because the caveat is so broad, inspectors can require a permit under a number of conditions, including what the use will be (no bedrooms), whether it contains electrical service, and on and on. It is a mistake to construct outbuildings thinking they are exempt solely on the basis of their size.

  4. Trevor_Lambert | | #4

    One should always consult the local building codes. Where I used to live, the rule was without caveat. Having an ambiguous caveat like that makes it pretty difficult to determine after the fact whether a permit was required and not obtained, or requested and deemed not required. Unless there's actually a written form requesting the exemption, which kind of defeats the purpose of the rule in the first place.

  5. Expert Member


    Not wanting to worry about it down the road, on my own property I ended up getting permits for both outbuilding. The building inspector jokes that I may have the only two permitted sheds on Vancouver Island.

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