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Old 01-16-2014, 07:14 PM   #1
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Addition over sloped patio


I am looking at building a 25' x 20' addition over an existing concrete patio which is about 3" thick. The slab is in good condition, it seems to stay dry and doesn't seem to be shifting/cracking. I am planning on adding a continuous spread footing to the patio since there is currently no footing. The existing house is raised foundation and the slab ranges from 3" to 11" below the existing interior floor height.

Does anyone see any issues with this? Also, is there a way that I can do this without using a vapor barrier? Even though the slab seems to stay dry (I am located in California), I am skeptical about using a vapor barrier. It seems like if water found a way under the barrier, it could cause issues with mold so I would prefer to add foundation venting instead if that is an option. I will be working with a contractor, so everything will be done correctly, to code, and with permits. I am just looking for some advice on which direction I should head.

Any advice is appreciated.
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Old 01-16-2014, 07:17 PM   #2
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Your local building code, as interpreted by your local building inspector, will no doubt be very explicit about how deep the footings have to be, what size, whether a vapor barrier is needed etc. Since you are working with a knowledgeable, local contractor, and you are pulling a permit, you can presumably leave the details to the contractor, or of course you can visit the building inspector with the contractor when the contractor presents the plans for approval.
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Old 01-16-2014, 10:05 PM   #3
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I do most of my design work in California, that being said, I have never seen an instance where a Building department has ever let someone build living area over an existing slab where it is not known if there is a vapor barrier UNDER it.
People have submitted plans that show a product like DryLok (spelled right?) on top but that idea has been a non-starter.

You would need to have proper footings under it anyway so why not just demo the entire slab and start new.
Or perhaps better yet, leave the slab in place and build a new raise floor over it?

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Old 01-17-2014, 07:01 PM   #4
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The few times I have tried to leave something like an existing slab in place to try and save the customer money it has cost me more. Almost always cheaper in the long run and more importantly the project gets built right from the very beginning. Foundation is not the place to try and save money.
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