andy998

I am not a mason so I need some advice on a job if it is feasible engineering wise.
I have a client with an existing patio 12 x 20. They want to make it 14 x 22 & 5" higher before I add the Sunroom addition. Can I dig and pour a footing around the outside edge of the existing slab and then pour the additional 5" of concrete over the existing slab patio. We are in the north east and am concerned with frost heave of the old patio slab. The client does not want to have to have the existing patio slab torn out.

stacker

if they dont want the old slab tour out,that the only thing you can do.i would make sure i pinned the new slab to the old slab wtih rebar.

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Tscarborough

I am vauge on frost heave, but if you dug the new footings to prevent frost heave of THEM, wouldn't that prevent the interior (old) slab from heaving?

As a side note, I would pour the footings and slab monolithically, tied to the old slab. That 9-10" of new/old slab will give them a very nice thermal mass, BTW.

jvcstone

Andy,

That's exactly the way I poured the slab for the house I'm building for myself. Extended and raised an existing slab. Been in for 7 or 8 years now, has a two story cmu structure on top of it, and to date, no problems. Of course we don't have frost heave here, but expansive clay soil which gives the same net result.

If the existing slab was designed for the frost heave, and shows no damage or movement, then your plan should work if you make your new footings deep enough.

JVC

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Glasshousebltr

I'd pour the foundation and footing first, back fill with rock, add an expansion joint, then pour the slab over. This way if the old slab presents a future problem, the floor can be replaced without disturbing the foundation.

Bob

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maj

Yep, seperate footings & foundation for northern climates. Use an isolation membrane between the two slabs, so any current cracks or heaves in the existing slab do not transfer to the new one. Also consider using 2" rigid foam around the perimeter of the foundation to help keep thermal transfer from occuring.

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andy998

Thanks for all the great replys to my question.