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

Question
Hi Scott,
I came across your answers to someone else and wondered if you can help me.  I'm building a 14'X14'(inside)cinder block root cellar with one wall not covered. All other walls and roof will be buried in gravelly and sandy soil.  I am thinking that the strongest roof would be reinforced concrete.  There will be about 2 feet of soil/gravel on top of roof.  What I can't find information on is the size, distance and pattern of reinforcement (rebar) needed, and if 4 inches is thick enough or if need go thicker?  I don't mind going stronger than needed for safety and also what strength cement to use?
The block walls on good footings are already built to 4 feet and a horizontal rebar layer is being cemented in at this level.
Thanks for any help.
Dag

Answer
Hi Dag, I'm not sure if there is a website that has this information.  But from experience I would say at least a 6" thick slab with #5 rebar at 12" on center 3/4" up from the bottom of the slab and #3 rebar at 16" on center down 1 1/2" from the top of the slab should probably suffice with 4000 pound concrete mix.  However, an engineer should be consulted to verify this information.  Also how the slab is tied into the block walls is important.  You need to have vertical fill cells in the walls and hook bars into the slab from the walls.  I hope this information helps feel free to write again regarding this or other matters sincerely bruce e johnson