Footing/Foundation Importance

DEAR TIM: I have a concrete patio slab which is in excellent condition. I would like to construct a screened-in porch on this slab. The screening materials as well as the other building materials are lightweight. Do you think the slab will be able to handle the weight of the structure? G. Y.

DEAR G. Y.: The slab will probably handle the weight, however I feel that you have a more serious problem. Based upon your information and sketch, your slab has no foundation beneath it. It simply is not a good idea to build a structure on such a slab.

The reasons for this are numerous. Some of the primary reasons structures require foundations are as follows. The foundation of a building is designed to withstand the loads placed upon it. Without a foundation, a wall or column supporting a concentrated load could actually crack the slab and sink into the soil. Believe me, that could really ruin your day.

Foundations also protect structures from frost heaving. I have seen certain clay soils heave a slab 2-3". Frost is a powerful force in certain climates and must be respected. Foundations and the footers that support foundations should be poured below the frost line. Frost lines can range from inches below the surface to several feet. Check with your local building officials to determine the frost line in your locality.

If you build your screened porch on the slab several things could happen. Frost action could lift the slab and pull part of the porch away from your house. The frost action could also crack the slab and cause differential movement by lifting one part of the slab higher than the other.

My advice is simple. I suggest that you remove the slab and install a properly designed foundation system. Reinstall a new insulated slab so that you have the potential of using this room on a 365 day-a-year basis should your space requirements change in the future.