DEAR TIM: I recently started a room addition project that has a crawl space. My builder insisted on installing a drain tile pipe on top of the foundation footer. Since there is no basement below my room addition, I feel that the pipe is a waste of money. Would you do this on one of your projects? B. F.

DEAR B. F.: Absolutely! The money you are spending for this pipe is a very wise investment. I feel that you are extremely lucky to have selected a knowledgeable builder.

Foundation drain tile systems serve a very important function. They are generally installed to provide a means to channel excess water away from your footing and foundation. However, if equipped with a simple optional pipe, they can also allow you to inject water along your footing and foundation. The value of this second point is frequently overlooked.

Most houses or room addition projects are constructed on soil, not bedrock. Soil is made up of small deteriorated pieces of rock. However, these solid pieces only make up a portion of the total space that soil occupies. The remainder of space is comprised of void areas. These empty areas can be filled with a combination of gas, air, or water in varying amounts. The closest thing I can compare it to is a sponge.

Because of these characteristics, some soils can cause tremendous problems if allowed to become alternately dry and wet. Certain soils expand when wet and contract when they dry. Some clay soils react dramatically to moisture variations. They actually shrink and swell. You can demonstrate this very easily with certain types of sponges. These sponges tend to shrivel up when dry, however as soon as you wet them they 'puff' up. Can you imagine how your house or room addition reacts when this happens to the soil surrounding your foundation and footer?

There is an interesting thing about these soils. When subjected to an even, constant state of moisture content, they usually don't shrink or swell. They have a strong tendency to stay the same size. The trick is to maintain this delicate balance. Geologists have a fancy name for this condition -- dynamic equilibrium.

For those of you who have decided to stay with me on this, here is the payoff. Your drain tile system allows you to easily maintain this state of dynamic equilibrium. Here is how it works.

During dry spells or drought conditions, you can inject water into your drain tile system. This allows you to trick the soil in contact with your footing and foundation. It will have no idea that it hasn't rained in months.

To create this illusion, you must have your builder install a 'T' fitting in your drain tile system. A pipe will extend to the surface from this 'T' fitting. This simple pipe allows you to easily inject the necessary water to saturate the soil surrounding your foundation footer. I usually recommend that people insert their garden hose, turn the water on low, and let it run overnight. If you do this once a week you generally will not have any problems. You must monitor the water flow carefully if you have a basement. If your foundation is not waterproofed, too much water might possibly cause a leak into your basement.

You can check your soil characteristics by using a soil map. These maps and descriptions are usually available at your county offices. Often they can be found in the agricultural department or the engineering and building department. These maps are prepared by geologists who specialize in soils. Often they are free. I highly recommend that you obtain one prior to building your next home or room addition.