# Adding a footing to an existing slab on grade



## jklingel (Dec 21, 2008)

(1) It looks like you are inviting a swimming pool or at least potentially wet earth under that patio area, which will lead to water under that end of the slab. How will you avoid that? (2) If for some reason you should not dig under the slab and re-fill w/ concrete (I'd guess you could; have thought about doing this to my garage some day), then digging next to it (if that won't disturb the soil, too) and putting a foundation wall next to the slab should work. You'd then drill the slab in several places, install/epoxy rebar about 8" in, and leave the rebar sticking out a foot or so. Pour the new foundation wall to embed the rebar. CHECK W/ AN ENGINEER ON THIS. That, I am not.


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## Nailer (Jun 2, 2010)

In Vancouver Canada the winters get pretty cold. I would imagine foundation depth is 4' or greater. The reason I bring this up is that if you place a footing and it is not deep enough the frozen ground will lift it and do real damage to your slab.By not having a footing the slab is currently "floating" on the surface.


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## jklingel (Dec 21, 2008)

As I think about this more, why even bother w/ a "footer" (foundation wall, whatever)? Why not just let the slab do what it has been doing for 83 yrs, shore up the earth (exposed when the digging down happens) w/ wood or concrete NOT ATTACHED to the slab, and call it good. Let everything float. Since the new-cut edge will now be exposed to cold, I'd insulate it. Perhaps install 2" XPS vertically along that exposed edge and also 2' horizontally under the patio to (hopefully) avoid more freezing under the slab than it is used to. 1" of XPS is apparently worth 1' of dirt.


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## jascox (Mar 16, 2010)

Thanks for the responses. Vancouver doens't get too cold. But we typically have a couple of weeks of pretty cold weather (-5C sort of thing).

But pouring a footing and risking all the potential stresses seems like a bad idea, even if permafrost is unlikely in Vancouver. I like what jklingel has suggested as a mini-retaining wall (wood or concrete to keep all the support soil under the slab, and just keep the two completely separate.

Again, thanks for the responses.
JC.


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