Expert: Dan Griffin - 5/29/2008

Question
My property slopes toward my house.  Whenever it rains my carport (attached to the house) collects a certain amount of drainage.  I am enclosing this carport to make a shop/man cave and wish to pour a 5" thick, 18" high and 7 foot long retaining wall on top of a section of the slab to keep the water out.  A base plate for a studded wall will be attached to the top of the wall.  What do I need to do to keep the joint between the wall and the existing slab water tight?  Is is simply calking after the curing or is there a weatherstrip of some kind that is installed before the pour?

Answer
John, the best approach would be to cut the slab back.  Dig down to frost depth in your area.  Form and pour a reinforced concrete wall with j bolts installed to receive your base plate.  I would dig slightly under the slab so the new footing/wall would carry the existing slab to prevent any settling.

YOur existing slab was not designed to carry a load and creating a water tight seal is almost impossible.  Waterproofing would be done on the outside at any rate.  If you will be inspected or permitted, they will demand a footing under an enclosed space anyway.