Expert: Dave Johnson - 9/11/2009

Question
We are connecting 12ft. of new concrete slab to old concrete slab for a 2 story addition to a cabin.  We normally would not heat it in the winter, but we want to put in floor heat in the new slab (boiler).  Will this pose a problem of the unheated existing slab heaving and causing problems with frost?  The ground underneath is mostly sand and rocky clay (but not real heavy clay).

Answer
Hi Jerry,
Always a guessing game with this kind of question.
If the drainage is good and you've never experienced movement of the old slab you're probably in good shape.  Anytime you join 2 slabs there is a risk of problem.

Also, if you do it, I'm thinking you should have a heavy footer under the new slab, considering this is a 2 floor addition. Another must is drilling a series of holes into the existing slab and insert 1/2" re-bar so that at least 8" protrudes into the new slab. Ideally a piece of re-bar welded or tied across the stubbed out pieces will help.

Best of luck, and if you need more, mail me again.

Dave