Expert: Dan Griffin - 11/3/2009

Question
we are building a 18feet by 18feet by 10feet high cinder block room. I thought one foot deep and one foot wide would be ok. but this area is down hill and is wet when it rains for maybe a week. what should be done for this room concerning type of foundation. because of wet how deep and how wide should foundation be

Answer
Steve, a normal footing width for 8" cement block is 16" wide and 24" is not rare.  Wetness has nothing to do with it, the issue is the bearing capacity of the soil and the load you will put on it.  You would need to compute all the weights of material and any live or dead loads you will be applying to the structure.  The square footage of the footing must be able to hold the loads once you know the bearing capacity of your soil.  Code books will give you 1,000 pound per square foot for almost any soil other than a swamp.  A soils engineering report may be required if the loads are more significant.


If your soil can hold 1,000 lb/sf, and you use a 16" wide by 72 foot long footing, You will have about 96 square feet of footing that will hold up about 96,000 pounds.

It sounds like your biggest concern should be waterproofing the walls and floor and dealing with water infiltraton.