The Wet Basement (and Crawlspace) Blues


By: Robert L. Klewitz, M.S.C.E., P.E.


RLK and Associates Inspection Service


(425) 391-9396




There are times that try men's souls, and dampen an ankle or two as well. The combination of prolonged rains and poorly draining soils is one of those times. The result is water seepage into the basement or crawlspace, and all of the information here applies to both.



A Few Definitions


Probably the most used and abused word in the wet basement and crawlspace business is hydrostatic pressure. Essentially, all this term means is that the weight of a column of water exerts pressure. The pressure exerted is exactly equal to the weight of a one-inch square cross sectional column of water, and directly proportional to the height of the column. In wet basement terms, this means "how deep is the basement and is the ground around it saturated"?


Saturated earth means that the soil particles are wet and the space between them is filled with water. When water saturates to a certain depth, pressure will form. When a lower pressure area exists within a saturated earth zone, the water within the zone flows to the low pressure area, just like with winds. Liken this to an empty bucket with a hole in the bottom being pushed down into a bathtub full of water. As the bucket fills with water, the level of the water in the tub lowers. The same principle applies with wet basements and crawlspaces.


Surface water is water that has landed on the earth's surface in the form of rain. It can also be water runoff from roofs or neighboring hillsides.


Ground water is water that is contained within an aquifer. Aquifers are water-bearing strata beneath the earth's surface that exist above a relatively impermeable layer, and usually within a gravel or sand layer. Ground water can sometimes be seen on the earth's surface, usually in swamps, wetlands, or quarries.


The upper reach of an aquifer is called the water table. The water table will rise and fall with the pattern of seasonal rains. It takes a long time for an aquifer to be recharged because it requires surface water and aquifers tend to extend over very wide areas and contain billions of gallons of water. They tend to rise and fall only with prolonged seasonal weather, not with individual rains. Water tables are discharged and lowered either naturally, or when they are pumped out and removed through, for example, a well.


Perched water tables are miniature aquifers. A hillside or drainage area will trap runoff water in a low area, allowing it to seep into the earth until it reaches a relatively impermeable layer of soil or rock. Here it will tend to form ponds.



Some Basic Facts of Construction


1.    No basement or crawlspace can ever be built to be entirely waterproof for the life of a house.

2.    About 95 percent of surface water will run off a slope of poorly draining soil.

3.    Water that gets into basements was once on the roof of the same house.

4.    Most water problems in basements are really surface water problems.

5.    The most conclusive cause of water seepage into basements and crawlspaces is time.

6.    Water problems are best dealt with from the exterior of houses.

7.    Interior brush-on products will not stop water leakage into basements.

8.    Downspout controls and landscape drainage improvements can usually "cure" many wet basements.

9.   Interior pickup systems are some­times necessary to keep basements and crawlspaces dry.


Waterproofers and Home Inspectors

Waterproofing contractors are often called upon to "cure" wet basement problems. Typically, these folks install a perimeter under-slab drainage system with a sump pump to absorb incoming water and discharge it. Home inspectors not only see this step as unnecessary and expensive, but consider it as being sometimes detrimental to the house itself.

Waterproofers are often called upon to "certify" or "guarantee" that a basement will not leak, which is usually nothing more than an expensive insurance policy. Home inspectors usually pass on information, but provide no guarantees or warranties. Clearly, this appears to be the source of the raging controversy that has evolved between waterproofing contractors and home inspectors.

Home inspectors tend to advocate the removal of the cause of the problem (hydrostatic pressure) by preventing the ground surrounding the foundation from becoming saturated. This is almost always a cure if it can be accomplished. However, sometimes the problem cannot be cor­rected and even the best landscaping efforts fail, but these simpler and cheaper options should always be tried first, before more expensive repairs are considered.

The wise homeowner would do well to approach a wet basement or crawlspace as follows:

1.    Obtain the best diagnosis as to the source of the water.

2.    Eliminate as many sources of surface water as possible.

3.    Use interior pickup systems if surface water controls fail.

Expect conflicts between contractors bidding on a guaranteed job and the usual step-by-step approach of home inspectors. Most waterproofing contractors will point to water seeping in at the floor/wall joint and call it ground water due to hydrostatic pressure. They are correct in attributing the problem to hydrostatic pressure, but the source is usually surface water.


Final Thoughts

Most people do not realize that an average-sized roof will drain between 500 and 1,000 gallons of water every time that it rains one inch. The most direct route to eliminating surface water problems is to make sure the gutters on the house are clean and deliver the water far away from the house. The best way to do this is by having the downspouts drain into solid underground drain piping that gets the roof water well away from the house. Splash blocks are almost never sufficient. Corrugated/flexible plastic piping, while easier to work with, clogs much more easily and is very difficult or impossible to get cleaned out, so solid, non-perforated, “tight-line” plastic piping is far preferred. And make sure to keep those gutters clean!

Grading against the foundation is also crucial. Build up the earth immediately around the foundation with soil containing a lot of clay. Create a slope that will avoid low spots near the foundation and so that water will run away from the house. Try to achieve a drop of one inch for every foot or two from the house, for a total of approximately six feet. The soil should be tightly compacted. Naturally, any voids under stoops and the like should be filled with the same soil.

Sloped yards that drain water near houses should be intersected with shallow angled trenches to carry the water away from the house. Oftentimes, hillside water runoff stops at the base of the hill to create a perched water table. Never run perforated drain pipe near the surface or near a house foundation. This procedure is often used as a way to carry water safely past a house, but it will actually increase the soil saturation and cause greater seepage. A proper “French drain” system that is properly installed and sufficiently deep enough, can often collect most of the water from a hillside or yard area, and direct it into a proper drain, or at least well away, and preferably downhill, from a house. In flat lot and yard areas, a “dry well”, which is a large and deep hole in the ground filled with river rock, can be used to drain water into, as long as it is as far away from the house as possible, if a sewer drain line, or the equivalent, is not available.

Imagine the landscape covered with a thin sheet of plastic. You want to tilt the plastic and wrinkle it to cause the yard and roof runoff water to travel away from the house.

Interior perimeter drain and sump pump systems are expensive and generally require electricity. However, electricity has been known to fail during a storm, just when you need it most. You should keep a battery backup pump, or a generator, on hand for such incidences, especially if you have a serious problem.


Know the facts about house drainage and basement/crawlspace water problems and choose your plan of action wisely and accordingly!