January 3, 2013 | 549 views
Ask the Architect
Support your floors properly
Q. I’m really confused about what to do with my home. An engineer was just here, and I showed him the space under my house caused by the flood. It apparently washed out most of the dirt under my concrete floors, and I’m very concerned about the floors collapsing. First, what do you think is holding up my floors, and second, the engineer told me to fill the space with concrete, which is going to be very expensive. What might you suggest?
Your home’s foundation walls are basically concrete beams with steel reinforcing rods in them. These beams go from one pile, driven into the earth, to the next pile. A purist engineer who designed your foundation (and I did look at your original house plans) only solved structural problems, and did not address issues such as foundation wall heaving due to frost or problems associated with rainwater, or even floodwater, from getting under the house. Unfortunately, nobody questioned a professional who signed and sealed the house plans, so anybody else who could have doubted what was planned let it go because the professional took all responsibility.
The concrete floor only has a small amount of thin steel reinforcing in it, instead of properly sized rods. Because of this, open spaces underneath will cause the slab to crack and drop. What is currently holding your slabs up is uneven peaks of earth that remain and the possibility that the metal reinforcing might be cast into the side walls or over the strips of foundation that run up the middle, under your ground floor. The advice that the engineer gave you is incorrect and will even possibly make things worse. Filling areas under your house with concrete is like throwing an anchor, not a life preserver, to a sinking swimmer. Adding more weight, not even tied into the bottom of your floor slabs, will cause the mushy, deteriorating material below your house to sink even more.
I’m telling my clients to use expanding closed-cell foam. This accomplishes supporting the slabs, adhering to the bottom of the concrete (even though the concrete would be better off if it had a clean surface) and providing moisture resistance and insulation, all in one application. I hope this helps. Good luck!