Why do suspended garage floors have problems?

expert EXPERT April 26, 2007

Suspended (or structural) garage floors are supported by the foundation, instead of the ground. Most slabs simply rest on grade and have no structural function. They are often not supported by the garage foundation..

Garage floors that are suspended require special attention. Loads are considerable and distress in the floor may be difficult to detect. Because of the weight involved (many cars and vans weigh over 4,000 pounds) and the point loads involved (all of that weight rests on four tires) it is difficult to get a sense of whether the floor is strong enough to support the car over the long term.

In many cases, you can observe some deterioration to the suspended floor. It may be difficult to determine whether or not this has significantly weakened the floor and further investigation by a specialist is the best recommendation. Cracks may or may not suggest structural problems.

Suspended concrete garage floors take much of their strength from the steel reinforcing bar (rebar) embedded in the concrete. Road deicing salts dripping off cars parked in the garage can attack the steel reinforcing rods as well as the concrete. If the reinforcing is badly corroded, the strength of the slab is greatly diminished, although there may not be much visible evidence of this.

Some suspended garage floors are wood. The moisture carried in with the car can lead to rot, especially since garages are usually not well ventilated and the wood may not be able to dry quickly or completely. Rot at critical endbearing points, for example, can render a suspended garage floor unstable even though 99 percent of it looks great. Again, inspection by a specialist is strongly recommended.