Understanding the Water Table

The water table is the depth at which the soil is always wet. Call your building dept. to find the water-table depth in your area.

Professional waterproofers often cite a high water table as the probable source for basement water problems. Waterproofers stand to profit from this diagnosis; blaming a high water table means the problem is beyond any self- help remedy and professionals will have to take extensive and expensive steps to cure the problem.

These steps may include excavation and/or trenching, hauling in gravel fill to build leach beds (trenches filled with gravel to disperse the water via ground absorption and evaporation), installing perforated drain pipes and a sump pump, and application of both interior and exterior waterproof coatings to the basement walls. These remedies could easily occupy a work crew for several days and justify a bill of thousands of dollars. However, eliminating ground water problems can be a do-it- yourself job that will offer a relatively inexpensive solution for the majority of wet basements.

Before investigating the source of a basement water problem we should define the term “water table.” The water table is the depth at which the earth is permanently saturated with water. If you are so unlucky as to have a basement that penetrates into this continually wet soil, you must install

drain pipes (either inside or outside the basement footings) and a sump pump to collect the water and move it away from the basement footings and floors.

Before assuming a high water table and calling a professional waterproofing company, consult experts to learn the level of the water table in your area. Your local building inspector can pro vide free information about the depth of the water table. For example, a call to the author’s city building inspector might reveal that the water table in your area is at a depth of 17 feet (5 meters).

Because the basement penetrates less than 8 feet (2.5 meters) into the ground, WE can assume that the water table begins some 9 feet (2.75 meters) below the basement floor, so it is obvious that there can be no water entry from the water table.

Another approach would be to consult excavating contractors who work in the area. These contractors dig basements and install utility piping such as water and sewer lines all over the community, and they can provide expert advice concerning the local water table.

Setting Excavated Soil: To settle soil in a trench, attach a hose to an 8-foot (2.5-meter) plastic pipe. Push the pipe down to the bottom of the excavation and turn on the water. The water will force out air pockets and dissolve clumps of soil. As the fill dirt settles, raise the pipe upwards.

Excavating contractors will have firsthand knowledge of any water table problems or underground springs in the area.

After checking with building inspectors and local excavators, consult the neighbors on all four sides of your house to learn if they, too, are having problems with water in the basement. The water table does not vary widely over a small area, i.e., from lot to lot. If you alone have a serious basement water problem while neighboring basements are dry, you can assume that the problem is not with the water table but is due strictly to runoff from surface water.

Be aware, however, that if a cluster of neighboring houses that historically have had dry basements suddenly develop wet basements, the problem is not with the water table. For example, WE know of one neighborhood in which the basements had been dry for decades. When municipal sewer improvements were done, with extensive new trenching and soil disturbance, all the houses in an entire neighborhood suddenly developed basement water problems. What was the water source? The disturbed soil had allowed rain to percolate in and run through all the pipe trenches. Those excavations included a new trench running from the main sewer route up to the basement of each house, a natural avenue through which surface water could flow. The epidemic of wet basements was caused by the sewer excavations, and the basement water problems were not eliminated until the fill dirt settled in the trenches, so that ground water could no longer penetrate the soil.

Some texts maintain that the depth of the water table varies with the seasons, and with the amount of rainfall. It is obviously true that the amount of water present in the upper levels of the soil varies with the wet and dry seasons, but this is still considered a surface water problem. The true water table, the point at which water is always present in the soil, is constant and does not depend on the seasons. In wet weather you are simply dealing with more surface water.

Finally, modern building codes do not permit contractors to build basements in areas where the water table is high. In some cases you may be permitted to build, but without a basement, on a slab on grade. Decades ago, building codes were not as strict, so houses that are more than 30 years old may have been built on a lot with a high water table. But if your house was built in the

last two or three decades you can be reasonably sure that a high water table is not the problem. Again, your building inspector will be able to advise you whether or not the water table may be the source of basement water.

If inspectors and contractors respond that the water table in your area is an unlikely source of basement water problems, you can assume that the basement water problem is from surface water. A little detective work will help you locate the route through which water is entering your basement so you can eliminate any problems.

Basement construction for wet soil varies depending on the type of soil. Check with building department for the code in your area. Concrete is troweled to form a 45-degree cove between the footings and the basement wall; Parge coat of cement troweled over block wall from footings up to ground level; Drain tile along exterior of footings collects water and directs it away from the wall; Spread gravel 2 inches (5 centimeters) deep under drain tile and 6 inches (15 centimeters) deep over the tile.