Expert: Bruce E. Johnson - 2/22/2009

Question
I'm trying to find out if an addition done to my house prior to my purchase (in 2001) is up to standards.  In the 1990s they added an additional room to the house by pouring a 19x11 foot concrete slab 12 inches thick.  But what was brought to my attention this week was a) it has no footing and b) it has no water lip.  And my question is when building an addition using a concrete slab is a or b required, or just a nice to have? Does the thickness of 12" make a difference?

Answer
Hi Jeremy, chances are that the slab is only 4-6" thick and the exterior edge of it is 12" thick. This is called a monolithic slab because the footing and the slab are poured together.  This is a perfectly acceptable method of construction.  Even if the entire slab is 12" thick it would be acceptable but highly unusual.  As far as a depression along the exterior (I am assuming that is what you are calling a "water lip") this is not necessarily a requirement. It may be a custom in your area.   For years in South Florida we put a depression along the outside of all the commercial buildings I supervised the construction on.  This was a custom but not a building code per se.  Since moving to central Florida ten years ago I haven't seen any architect or engineer use this wall depression in their designs.  As long as the walls are sealed from moisture getting inside the building the depression isn't necessary.  I assume that your addition was permitted and inspected by the local building department.  If so, it is probably up to local standards. I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com