Q My house suffers from a very wet crawl space. The area underneath the house is always damp, sometimes wet. The moisture causes an oxidation of salts to form along the foundation.

I dug alongside the house not long after the last big downpour we had, and I had water about 2 1/2 feet down, so I put in a sump under the house. I used a galvanized trash can of 30-gallon capacity, with holes drilled in the bottom and the lower section filled with lava rock, with a submersible pump mounted on a piece of marble inside the container. So far, the pump has not turned on .

When installing the sump tank, I did not see any water, but very wet soil. My next recourse is to contact a cement pumping company and ask them to come out and pump about two to three inches of concrete under the house. -- J. M.

A First, do not cover the surface with concrete: It is a porous material, and water simply will seep up through it.

Adequate air circulation and ventilation should be provided in the crawl space; each wall of the foundation should have louvers or vents installed, of a size allowing one square foot to opening for each 300 square feet of ground area.

The bare earth should be covered with strips of polyethylene plastic or moisture- and vapor-proof building paper, overlapping the strips at least six inches and sealing the seams with roofing cement to prevent escape of moisture from the ground. If you prefer, you can also use bricks along the seams to weight the covering down instead of the roofing cement.

You also should do something about the drainage from the hillside. Check the area around the foundation and make sure the ground is sloping away from the house for proper drainage. You may want to install a low concrete wall at the base of the hill, designed to divert water away from the house.

There may be a subterranean drainage from a hillside toward the foundation even though the grade slopes in the opposite direction for some distance. In this case, the water can be diverted by an intercepting drainage line of tile topped with gravel. The lengths of tile are butted together and the joints are covered with pieces of tar paper to keep out sand. The line may discharge into a sewer or dry well, or lead water to a lower level away from the house. It must be laid several feet underground, at the level of the base of your foundation. Use gravel to fill in the trench over the drainage tile. Questions about construction or care of the home may be addressed to Here's How, Copley News Service, P.O. Box 190, San Diego, Calif. 92112-0190. Only questions of general interest can be answered in the column.