Posted: October 1, 2012
First stand in the yard and look downhill. If there is no downhill direction, expect a wet basement. If there is a downhill direction, take a small level and go to the lowest place on the lot. Look at a basement window while sighting along the level. Put a mark on the window pane where your line of sight intersects the window. Go inside the basement, if your eye is now higher than the mark then gravity drainage of water away from the basement is theoretically possible, but if your eye is now lower than the mark then a sump pump is required.
So what constitutes waterproof basement construction? Well, a house rests upon its foundation which is a concrete beam placed under the basement walls. On the outside of the foundation there should be a drain. Often a perforated, corrugated, plastic drainage pipe four inches in diameter is used. In new construction this drain may be incorporated into the form for the concrete. The pipe should encircle the foundation and drain to the ground surface. This drain is the essential element for a dry basement. The basement wall is constructed on the foundation and must be sealed on the outside to prevent moisture from migrating through the wall into the living space. The walls can be made even more waterproof by sheeting them with rubber roofing. After the house is complete the basement walls are back filled. The material covering the foundation drain pipe and placed next to the wall should be crushed stone. This allows water next to the wall to move vertically downward to the drain and be carried away from the basement.
It is fairly common to omit the outlet for the footing drain. Instead the drain discharges to a sump which is usually placed inside the basement. Basements constructed in areas where the soils are deep and well drained may stay dry even without a sump pump, however less well drained soils will need a sump pump. During exceptional rainfall events homes without gravity drainage will be dependent on electricity to keep the basement dry. This can be a problem if the exceptional storm that put water in the sump also causes a power outage.
Some signs of basement water problems include efflorescence or a white powdery substance on the floor or walls. This results from water movement through the floor or wall dissolving minerals from the concrete and then evaporating, leaving the minerals behind as a deposit. Mold growth is also evidence of a damp basement. Molds are fungi and require organic matter for growth. They begin growing from tiny spores and become noticeable when enough growth has occurred to form a colony.
Now you have identified the problem, how do you fix it? Slope the yard away from the house. Divert roof water far away from the house. Install a perimeter drain around the house at least as deep as the basement floor and drain it to the ground surface at a lower elevation. This could be done close to the house rather than right next to the house - exposing the basement wall. Although exposing the wall would allow it to be water proofed. If gravity drainage isn’t possible then a pump can be used. Pumps with battery backup are available. The perimeter drain can be installed on the inside of the wall but now the results are less certain. You are depending on perfectly sealed walls and floor in order for the water to go to the sump without entering the living space.
Within the living space it is important to take the steps noted above to eliminate the source of dampness as much as possible. Dehumidifiers also aid in keeping basements dry by removing moisture vapor from the air. Moldy basements can be cleaned with a strong bleach solution after removing as much as possible of the material that supported the growth of the mold. It is important to use a wet cleaning process for mold removal to avoid spreading the mold spores which are a respiratory irritant for many people.
Along with damp basements often come house centipedes, bristletails and other basement dwellers. Centipedes are scarier looking than harmful, but cleaning and eliminating moisture in the basement can reduce their numbers.
When building a new house you may wish to work with your contractor to get rid of storm water by gravity on the outside of the house if at all possible. An alternative may be to do away with the basement altogether and build more space above ground.
Original document by Tom McCarty.