Expert: Bruce E. Johnson - 1/31/2012

Question
QUESTION: I will soon be pouring a slab to build a shop on. I would like to build a cellar/storm shelter under this prior to the pour. I live in Southern Arkansas where there are few cellars. What steps are necessary to make it water tight and prevent "floating"?

ANSWER: Hi bill, basically you need to do a basement foundation with a Foundation drain around it .  The poured concrete walls are painted with a heavy rubber or tar mastic and then before backfilling you put a foot of crushed rock or river rock all around the perimeter, lay in 6" perforated pipe and cover with another foot of rock, top that off with filter paper and then complete backfilling with the soil dug from the initial excavation.  If your building site is on a hill you can empty your drain pipe down the hill by gravity..but if you don't have positive drainage allowing you to do this, you have to install a sump to drain the foundation drain into, the sump is then pumped out as needed.  I hope this information helps feel free to write again regarding this or other matter, sincerely, Bruce Johnson

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QUESTION: Thanks Bruce, it is a help. One more question about the pour itself. Is the floor poured and allowed to set and then the walls or should it be one continuous pour to eliminate joints?

Answer
Hi again Bill, the footings are poured first, then the walls then the floor in most cases however if it is a small room  and you want to cut out one step you can pour the slab with monolithic footings and then do the walls.  If you go this route you can get a water stop which is either a Bentonite material that swells up when it gets wet or a rubber strip that sits up vertical in your slab and the wall is poured on top of that.  In both cases mentioned you must put down a vapor barrier on the ground, ten mil visqueen if you can get it, six mil minimum.  Overlap and seal the seams with tape.  Sincerely Bruce Johnson