Expert: Bruce E. Johnson - 1/26/2009

Question
I am putting on an addition to my house.I would like to know if there is some kind of membrane that goes between the old foundation and the new one so it does not leak. I am in the western suburbs of chicago ,ill

Answer
Hi Gary, are you doing a basement foundation, a crawl space foundation or a monolithic slab foundation?  For the first two types of foundations, the basement and crawlspace, I recommend that you allow for a good caulk/expansion joint.  A half inch is the minimum or you can chamfer the edge of the new foundation concrete enough to allow you to pack the joint with backer rod and then caulk it with a good urethane caulk.  We use eucolastic or vulcum for this application. There are other more complicated and expensive expansion joints available but the urethane is the most user friendly.  There is a clay expansion material made of bentonite that expands when it gets wet, it is used more for elevator pits and that sort of thing.  If you are joining two monolithic slabs together it's basically the same material but it comes is thinner self leveling format that you pour into the joint.  Be sure to specify the urethane is for vertical application if you are doing the wall joints, the self leveling type will just run down the wall and make a mess..I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com