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jascox 07-05-2010 12:32 AM

Adding a footing to an existing slab on grade
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I am looking for some advice on a little summer project that I am planing out. I have owned an 82 yr old house in Vancouver Canada for about 1 year. We are looking to build a ground level patio and a deck extending off the second floor on the south side of the hose.

The house foundation is a slab on grade. I also found out that there is no footing to the foundation. Not a huge deal since it has been standing for 82yrs and there are only a few small cracks. I had the entire interior slab exposed for the past wet winter and saw no water at all.

But one of the design requirements is that I want the area under the deck to be a usable area. And since the second floor is only 7ft above the slab (low basement), that means that beams for the deck will be perfect head smacking height. My solution will be to dig about 18" to 24" along the south side of the house for a patio area.

The problem here is that I will need to dig below the bottom of the slab. So my solution will be to pour a retrofit footing to support the south edge of this slab.

So my big question is if anybody sees a problem with any of this? I could see arguments that a slab like this will want to move as a whole. By supporting one edge with a a footing, this could cause stresses near the south edge? Is there anything else that I should worry about? Does it make any difference that this is on the south edge of my house, while all the joists run technically the south wall isn't even load bearing.

I have added a few photos and a couple of drawings. Any advice would be greatly appreciated.

jklingel 07-05-2010 12:47 AM

(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.

Nailer 07-05-2010 01:05 AM

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.

jklingel 07-05-2010 02:24 AM

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.

jascox 07-05-2010 02:20 PM

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.

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