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Community and Q&A

Monolithic Frost-Protected Slab Foundation Code

olandsns | Posted in General Questions on

We’re looking to build a single story outbuilding (32×16) at our new home in BC, Canada. Climate zone 4 marine. I don’t really want to engage an engineer for such a basic structure, but when I speak to our building department about reducing footing depth and frost protected shallow foundations, I just get a “don’t know of anything in the code that allows it,” response.

Do most places require an engineer to sign off of these designs in order to get approval? Should I just submit my plans with calculations and reference ms to the formulas I’m using and hope for the best?

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  1. Expert Member


    The BCBC does address this:

    In it says : "The foundation depths required in Sentence(1) are permitted to be decreased where ... the foundation is designed for lesser depths."

    There are also opt-outs in that section for accessory buildings.

    What that leaves you with is finding out from your local inspectors what documentation they require for the design.

    What is y0ur local frost depth? In most parts of Marine Zone 4 it is shallow enough that what governs is a sufficient depth to bury the perimeter drains and rock, not freezing.

    1. olandsns | | #4

      Again, thanks Malcolm. I totally missed this section of the code.

      I'm not sure I fully understand how the chart here applies to a monolithic slab. Our soil here would be considered silt and to have good drainage to below the level of frost penetration (18") at about 2.5-3' we hit clay.

      My plan is to go down 14", 12" wide integral footing. 2" insulation on the perimeter. considering whether or not to put 2" under the slab.

      Does this make any sense?

      1. Expert Member
        MALCOLM TAYLOR | | #5


        Your plan sounds good. I would suggest going with the under slab foam. For the work and expense involved it will appreciably improve the comfort of the floor.

        You still may run up against the confusion of your inspectors trying to get approval for a foundation that doesn't quite go down to the frost level. it might be easiest to form up your thickened slab to reach the 18" mark.

        Good luck with your build!

  2. olandsns | | #2

    Thanks so much.

    1. Deleted | | #3


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