Expert: Dan Griffin - 8/1/2008

Question
I want to enclose my front porch which has an existing roof and is a large concrete slab. The building inspector said the slab had to go down 24 inches, below grade but ours only goes down 6-12 inches. When I told him this, he said the only way to fix this and make this project permittable is to tear out the whole slab and start again. Is this true? Is there any way to add footing that would meet this code without tearing everything out?

Answer
Enclosed habitable space requires a footing.  I doubt your existing porch is 5 to 12" thick all the way across, I'm sure you are seeing a thickened edge.  It would be possible to cut the outside edge off the slab, excavae to proper frost depth, rebar, and pour.  It should also be possible to pin or shore the existing porch, excavate a portion to frost depth, clean and rebar pin an addition to the bottom of your thickened slab.  It is a good situation for plasticizer in the concrete to help ensure full bearing.  

Be aware before you consider this method that any exterior concrete was deliberately poured out of level so it could shed water and that it has no vapor barrier under it.  The floor will be pitched 1/4" per foot or more.  Many floor goods will not adhere well if there is more than a pound or two of vapor pressure under the slab.

Your inspector is not really wrong to suggest remove and replace.