What causes a wet basement?
Wet basements can be caused by several water related conditions. These include:
- The most common cause of a wet basement is when the basement was cut into and placed below the elevation of the local watertable. This is equivalent to placing the basement of your house under (or partially under) water; your basement becomes a well that fills with groundwater anytime the watertable rises above the elevation of the basement floor.
- Another common cause of wet basements is that the downspouts of the house direct the roof runoff underground along and near to the outside of the basement wall. Thus, every time there is roof runoff this water is directed to a location next to the outside wall of the basement. During the hours to days required for the soil to absorb this excess water away from the basement wall, this excess water can and often will find a path through the basement wall into your basement.
- Another common cause of wet basements is when the soil next to your house has not been properly graded. The soil/lawn next to your house should always be graded so surface runoff cannot flow towards or come near your house (or basement wall).
- Another common cause of wet basements is when the house is built near the base/toe of a steep sloping hillside. The most common location for springs is at the toe of steep hillsides or mountains. These springs may not flow during periods of normal precipitation. However, when an unusually large amount of rain occurs saturating the soil, springs often develop near the base of steep sloping hillsides. These springs develop because the local watertable has risen upwards toward the land surface. These high watertable conditions often cause springs to begin to flow from the toe of the hill. These flowing springs may be a source of water that will try to enter your basement, either up through the basement floor or through the basement wall. The high watertables also create a condition where the water will rise up through the basement floor.
What To Do
If free or gravitational water accumulates in the soil outside your basement walls or rises upward below your basement floor, this free or gravitational water will begin to leak through the basement walls or come up through the crack between the basement wall footings and the basement slab that makes up the floor of the basement.
The Basement Floor
In most homes, the basement floor is not sealed or waterproofed in such a way that water rising up under the basement floor slab will be kept out of the basement. In addition, most basement floors have a drain installed in the floor so the homeowner can dispose of excess water under the house. This drain will also allow water to come up through the basement floor if the local watertable rises above the elevation of the basement floor.
From a practical perspective, this simply means that anytime water accumulates under the house or the local watertable rises above the elevation of the basement floor, water will come into the house and onto the basement floor.
If your house is located where these under basement floor watertable conditions exist, either permanently or during periods of wet weather, you need to find a way to reduce or eliminate this problem.
As implied in the assessment section above, the easiest way to eliminate this problem is to not build a house in a location that will have these under basement floor watertable conditions. This means do not build a house near the toe of a steep sloping hillside. These sites are very susceptible to rising or high watertables, especially during periods of above average rainfall.
If this high watertable condition is anticipated when the house is being build, contractors may place a "Footing Drain" around the outside of the basement footer as shown in Figure 1. This footing drain must be placed well below the basement floor elevation. This footing drain must also be constructed with a slope so any water entering the drain will flow to a specified low-elevation location where it can be piped to a stream, swale, storm drain or other acceptable receiving location. In many cases these collection points are underground and there is no convenient way to discharge this unwanted water away from the house. In these cases, a sump may be required where this unwanted water can be pumped away from the property. These sumps are described in the next paragraph. Retro fitting a footing drain to a house is a very difficult and expensive project.
Figure 1. Cross section of a home foundation
If you already own a house located at the toe of a slope, solving this high watertable problem will not be easy or cheap. The only alternative is to create a sump into the high watertable and place a pump in the sump so that when the watertable rises to a level near the elevation of your basement floor, the free or gravitational water in the soil will enter the sump and then be pumped away from the property. The sump will, if properly located, drawdown the rising watertable and hopefully keep this unwanted water from entering your basement from below. Usually the best place to locate the sump is either on the outside of the house, upslope of the house or in the basement on the upslope side of the house. If you decide to construct a sump upslope and outside of the house, it may be useful to not only dig a sump to a depth well below the basement floor elevation, but also dig a trench at this elevation across the uphill side of the property to intercept the rising watertable before it reaches the basement floor elevation. In some cases, this sump and sump pump can be located in the basement floor near the upslope side of the house. This sump must have an open bottom to allow groundwater to enter the sump so it can be pumped away.
The Basement Walls
If excess water is allowed to accumulate along the outside of your basement walls, it will find its way through the basement walls into your basement. In cases where roof runoff is directed downward along the basement wall or where the surface grading near the house directs surface runoff toward the house, water will penetrate the basement wall without creating an under-basement-floor drainage problem that causes water to enter upward through or around the basement floor.
If you have wet basement walls, the wetness problem must be solved by correcting the situation outside of the house. Note: You cannot solve wet basement wall problems by painting the inside walls of your basement in spite of what the paint-can may suggest.
To solve the wet basement wall problem, start outside the house. First make sure surface runoff water cannot flow towards or come near to the basement wall. If the soil has been graded so surface water can and does flow to the basement walls, add soil and regrade the area so water cannot flow or come near the basement walls. Keep this flowing surface water at least 10 to 15 feet from the basement wall.
Second, if your downspouts direct your roof runoff downward along the basement wall, change this. The downspouts must be cut and this roof runoff must be piped away from the house. In some municipalities, there are ordnances that forbid the dumping of roof runoff on to the soil surface or into the lawn. They will tell you to direct all roof runoff to a "dry well", which is simply a big hole filled with large rock located some distance from the house. The rocks in these dry wells will, over time fill with soil and become non-functional. This is often when the wet basement walls begin to occur. Either rebuild the dry well(s) or re-direct the roof runoff into the lawn some safe (10 to 15 feet) distance and downslope from the house. If your municipality allows you to pipe your roof runoff into the local storm sewer or even into the street, this is a great idea. Take advantage of this.
If regrading and re-directing your roof runoff does not solve your wet basement wall problem, it may be necessary to excavate along the basement walls and seal the outside of the basement walls. This is a very expensive alternative, but in some cases it is the only solution.
Careful siting of the house so it is not located in an area where the local watertable is likely to rise above the elevation of the basement floor is very important. To correct a raised watertable under your house, a drainage system or sump is needed to remove the gravitational water from under the house.
Careful landscaping near the house and grading to direct surface runoff water away from the house are very important approaches to correcting or eliminating wet basements. To be more specific, the most important approaches to prevent wet basements are:
- site the house away from the toe of steep hillsides
- grade the area so surface runoff never comes near the house
- direct all roof runoff away from the basement wall and the house.
If there is no free (or gravitational) water in the soil outside of your basement wall or above the basement floor elevation, you should never have a wet basement or wet basement walls.