Expert: Dan Griffin - 3/6/2011

Question
I am thinking of purchasing a property in upstate New York with a single-wide mobile home on a slab foundation. I don't know yet whether or not the slab is insulated or reinforced, although considering the purchase doesn't qualify for financing, I'm inclined to think it isn't. What I have in mind is to get rid of the single-wide, and enlarge and, if necessary, insulate the slab in order to build a new site-built home on it. If the existing slab isn't reinforced, will that pose problems in the future, say when the soil under one slab shifts and the other doesn't? I've read that when enlarging slabs, it is recommended to drill vertical holes in the existing slab and insert rebar that will grab onto the new slab when it's poured. Is that an accepted practice? In general, I'm hoping to find out what you think of the feasibility of the whole idea. Thanks.

Answer
Aaron, we need to get some common language between us.  A foundation is a reinforced concrete structure that extends down to frost line which varies all around the country.  This concrete foundation is often called a footing.  I would expect upstate NY to be around 4' depth.  A foundation is sized to carry the load imposed on it and varies based on the weight of structure and the soil bearing capacity of the soil.  A slab is flat work concrete of fairly uniform thickness that may or may not have any reinforcement in it.  A patio or a driveway would be examples of slabs.  There are hybrids in which the slab and the footing are poured at the same time, though with your deep frost depth I would guess this to be unusual.

A site built home or any structure that is subject to inspection will require a footing, most AHJ inspectors will require a full perimeter footing.  I assume you are talking about buillding a house where the concrete slab is the floor of the house which is very common here.  It would be possible to extend a slab with integral footing.  This is done by doweling the edge of the slab and the exposed end of the footing with rebar dowels.  The holes will be horizontal, not vertical.  

It would be unusual for your single wide to be on a real footing.  It would also be unusual for the elevation of the slab to work out well for a house floor.  It is normal for duct work, electrical, and plumbing to be installed before pouring the slab.  Trying to work around the existing slab may be quite problematic.  One idea that I have seen done quite often is to move on a mobile home to use and live in while "the real house" is being built and removing the trailer, etc once the house is ready.  The slab might work out well for a gazebo, carport, tractor shed, etc.