Expert: Bruce E. Johnson - 11/9/2008

Question
I have a concrete pad at the end of my 57 stone driveway and want put a 12'x20' addition on the pad(which is 4" thick), then connect it to my house.  i wanted to put the room on the slab to cut cost.  what steps would i have to take to do the foundation?

Answer
Hi Robert, first of all you will have to reinforce the edges of the slab that will be bearing the weight of the addition.  A 4" slab won't support a room this size without help.  You will have to undermine the edges of your slab going back under the slab at least a foot and go at least eight inches deep and fill this void with concrete reinforced with two bars of #5 rebar.  The rebar should sit on chairs that keep it in place and 3" above the ground.  The concrete would need to be pumped into place using a concrete pump.  A key to a successful footing expansion like this is to have a form around the outside edge so that you can overfill the footing void you have dug to make sure that the concrete pushes up tight to the bottom of the extisting slab concrete.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com