DEAR TIM: I am about to begin construction of a very large room addition. This room addition will have a finished basement. I've seen a black compound sprayed on new house foundations. Is this the product I should use to 'waterproof' my new room addition? Is there anything else I should do to make sure that my basement will not develop a leak? H. H.

DEAR H. H.: Both of your questions are excellent. I hope I don't run out of room! Let's start with the first one.

The product that you described to me in your letter was an unmodified asphalt based "damproofing" product. It is commonly mistaken for "waterproofing." Many, many consumers think they have waterproofed basements, when in fact they simply have been damproofed. Damproofing is a low cost method of retarding water infiltration. It is by no means a true waterproofing membrane.

Damproofing products are generally asphalt or tar based compounds. They are sprayed onto foundations prior to backfilling. Because concrete and other masonry products will easily transmit water vapor, these products are designed to block water vapor transmission. This, basically, is the extent of their capabilities. Damproofing compounds generally become brittle after they dry. They also do not have the ability to bridge cracks which may develop at a later time. Ground water can actually dissolve them over a period of time.

Water-proofing compounds, on the other hand, are much different. There are a wide variety of products on the market. Some are urethanes, modified asphalts, clay based, or rubber based. These products can either be spray applied, or are available in sheets. Water-proofing membranes have several distinct characteristics. Ground water has little or no long term affects on most of the products. They can bridge small foundation cracks. Usually these products retain their elasticity and some actually have self-sealing properties.

Waterproofing compounds are able to resist a hydrostatic head. A hydrostatic head develops when water builds up against the side of your foundation. The pressure can be enormous. I have personally witnessed a stream of water squirt two feet out onto a basement floor.

Now, let's talk about your second question. There are many things that you can do to help insure that your basement stays dry. First, be sure that your foundation is constructed to be as strong as possible. Install as much reinforcing steel as practical to insure that in the event a crack occurs, it will not widen or displace. Consider increasing the thickness of your foundation walls. Remember, no waterproofing compound can bridge a major structural crack. Your first line of defense is a strong crack-free foundation.

Install an excellent foundation drainage system. Use plenty (three feet minimum cover) of large washed gravel over the drain tile. Make sure that you cover the gravel with 4" of straw or tar paper before backfilling! This is a commonly overlooked step. These materials prevent silt from the 'fluffed' backfill dirt from clogging the gravel and the drain tile system. Be sure that the ground slopes away from all points of the foundation at least 1/2" per foot for a distance of 10 feet minimum. Pipe downspouts and sump discharge pipes away from the house. Do not let this water collect or discharge at the base of the foundation. If you follow all of these steps you probably will have a dry and enjoyable basement for many years to come.