How to Avoid Flooding Problems
Avoid flooding in your new home.
Here are some items to consider:
High Groundwater Table
Wet basements are a very common problem, particularly in low-lying areas or mountains where high water tables are common. This means that the ground water level has risen up to its high point for the year due to melting snow, spring rains, lack of water pickup by the trees and plants, and the absence of sun to dry the ground during the winter. Some soils such as clay bind water tightly, keeping the water table high for extended periods. Porous soils including sands and gravels pass water quickly, yet often have high springtime water tables, especially in low areas due to excess springtime runoff.
Unfortunately, many homes are built with inadequate protection from groundwater, even though site conditions may be well suited for the proper installation of a simple and cost-effective drainage system. Once a water problem is discovered after a house is built, it is very difficult and expensive to remedy. If left unchecked, structural and health-related problems may persist. Excessive moisture not only results in a musty smell or damage to carpet and wood flooring materials, but it promotes growth of mold spores, which can result in asthmatic symptoms in many people. For this reason, the use of vapor barriers under slabs as well as other drainage techniques are essential to keeping your home dry year-round.
To help illustrate the high groundwater problem, imagine a 12″-high pile of sand in the middle of your bathtub — with six inches of water in the tub around it. If you were to dig a six-inch-deep hole in the sand, you would find water at the bottom of the hole. By adding more water to the tub, the water level in the hole would rise as the surrounding water reached equilibrium due to hydrostatic pressure. This simulates the effect of a higher water table acting on a foundation. Now, inserting a watertight box into the hole would cause it to float. If there were holes in the box, water would seep inside. In this analogy, the box is the basement or foundation, and the tub water is groundwater.
Keeping Water Out.
Ironically, making residential basements absolutely watertight can lead to other problems if not properly engineered. For example, basements of many commercial buildings in highly developed areas are built watertight. However, their construction costs are extremely high compared to typical residential basements because their foundations and floor systems are built to far more stringent specifications. This is not only to keep water out, but to keep the high pressure of outside water from causing structural damage. Consider this real-life situation that occurred a few years ago:
During one extremely rainy night, a family awakened to a strange sound coming from their basement. Apparently, excess rainwater had saturated the ground around their home given its location in the lowest section of their development. The extreme pressure from the accumulated ground water actually buckled up the basement floor, causing the soil from under the floor to erupt six feet, covering their washer and dryer. The sudden soil displacement undermined the overall foundation, causing one of the walls to fall over a foot. The house was abandoned for massive structural repairs.
Although this is an extreme example, it demonstrates the powerfully destructive force of water. This foundation was built in a manner similar to many residential basements. Yet, by allowing the groundwater pressure to rise without relief, failure resulted in the weakest part of the structure which, in this case, happened to be the concrete floor.
Ironically, if homes were absolutely watertight, they might even float like the box in the bathtub, resulting in an unstable structure. However, many homes have sump pumps to discharge out any water that might leak in which could otherwise damage the basement. Fortunately, as water is pumped out of the basement, the water table around the house is lowered, (like pulling the plug on the bathtub drain) relieving the “hydrostatic” pressure. This is why floating houses or erupting floors are rare.
One effective way to check the water table near a house which has a dug well is to uncover the well and measure the depth from the ground surface down to the water. Compare this depth to that of the basement floor below grade and the difference equals the depth to which the basement would flood if the water were not removed. Even if the house is located on sloping terrain, the water table remains approximately the same distance down from place to place as groundwater tends to follow the contour of the surface.
In many situations, particularly in mountainous regions where the land is seldom flat, it is possible to drain water away from most foundations and basements by gravity rather than by pumping. The traditional method specifies a perimeter drain around the outside of a building during its construction. The drain outlet slopes downhill away from the house until it reaches the surface of the ground where it can freely discharge by gravity flow. The advantage of this method over pumping water out of a sump is that it lowers the water table around the house, preventing water from ever reaching the cellar in the first place.
In many instances where experts have been asked to troubleshoot wet basement problems,a common finding is drain pipes that were installed above the foundation footing, which allows water to rise to the level of the floor (Figure A, level Y).
If the builder made the additional mistake of installing the drain pipe with the weep holes facing up, then the level of water has to rise to the top of the pipe (level X) before the pipe can pick up any water. Other times there is little or no porous bedding material and pipe inlet holes can become plugged. Another problem occurs with large bedding stone. Migration of the original soil into the bedding plugs up the stone, leading to failure. Often a combination of problems may coexist.
Conventional perimeter drains are usually constructed as shown in Figure B with 4″ pipe laid at the base of the footing. Most contractors use 1 1/2″ crushed stone around the outside of PVC or thinwall perforated septic system pipe which has 1/2″ or 5/8″ holes every few inches. Usually this is entirely satisfactory, though in some cases capillary action (wicking) within the soil can make the basement floor damp even though it is a few inches above the artificially lowered water table.
Due to the severe conditions encountered when building a home several years ago, the following example illustrates an improved method. Just as sitework started, the property was deluged with rain. The rain aggravated an already difficult groundwater situation, threatening postponement of construction. However, with patience (and a good pair of rubber boots), perimeter drains were installed four feet outside of where the house was to be located. Within days of installing the drains, excavation for the basement was able to proceed as if it were a dry summer.
The home is built into the ground about six feet deep on a gently sloping hill. Several years have since passed and the home has never been bothered by moisture in the basement. This is in spite of the original springtime water table just two feet below the surface of the ground. Figure C shows how the drains were installed. This same procedure is excellent for drying out existing wet basements where conditions permit.
The first consideration is setting the drain pipe deep enough to lower the water table well below the floor in order to reduce the effects of capillary action (wicking). Another way to reduce this effect — generally limited to new construction — is to install a layer of crushed stone entirely under the basement floor. Water cannot “wick” through this coarse aggregate. Although this is more costly, it is good insurance for a dry basement.
Alternatively, installing the drain pipe a little lower may be a very cost-effective alternative, assuming there is sufficient side hill grade to ensure gravity discharge well away from the structure. For new construction and retrofit situations, place the drain pipe a foot or more lower than the footing whenever possible and far enough away to avoid undermining the foundation. Generally 2′ to 4′ or even more is necessary to protect landscaping or porches and similar structures.
In retrofit situations, installing a drain on the upgrade side of the house can yield a significant improvement, as it is usually only necessary to intercept water moving through the ground toward the house to eliminate the moisture problem. This type of drain is considered a “curtain drain” rather than a footing or perimeter drain, especially if it is several feet from the house. Its job is to lower the water table downgrade in order to protect the house. The curtain drain method is an important consideration particularly for retrofit installations as it can reduce site disruption and the resulting cost of excavation.
At times it is impossible to place a pipe at or below the bottom of a footing due to immovable obstructions such as boulders or a rock ledge. At times these may actually be cast into the footing. In this case, it is a good idea to pressure wash the rock surface and grout (seal) the rock to the wall with a strong, well bonding masonry product. This masonry seal keeps water from entering under the footing, and is formed into a shape that gradually slopes away to divert water from the rock or ledge to a point where it can be picked up by a conventional perimeter drain system. This technique requires patience and a little luck to be entirely successful, but it is often easier than trenching through solid granite!
The pipe around the house can be laid level and needs only the slightest pitch to vent successfully, unlike a sewer line which is designed to carry floating solids. 1/16″ to 1/8″ per foot is more than enough pitch, though steeper is perfectly acceptable. Use perforated or slotted pipe only in the area that needs to be kept dry, especially if the outlet pipe runs near a septic system (see local codes) or there are trees anywhere near its path. An inexpensive tool ideal for ensuring the proper slope is the WatrLevel, now available to homeowners and contractors through Builders Websource. The WatrLevel simplifies the process of setting pipe invert elevations, including around corners — which is typical in most residential situations.
Water flowing through a pipe toward the ground surface provides an invitation for tree roots which can enter and eventually block it completely. It is best to use solid pipe with root proof joints to prevent root penetration whenever possible. Schedule 40 pipe (the heavy stuff) should be used wherever there is a possibility of any type of vehicle hitting or crushing it. The outlet should be placed high enough in the outlet ditch or sidehill to let water freefall several inches to keep small amounts of buildup from blocking the pipe. It is helpful to make a header of stone around the outlet for protection.
The outlet must be located in accordance with regulations regarding distances from septic systems and the neighbor’s lot lines. The end of the pipe should have a rodent guard to keep critters from nesting during the dry season in this ready-made hotel. Use 4 pieces of brass wire (which doesn’t rust or corrode) spaced about ¾” apart to discourage the critters. Anything finer can plug up with algae and silt that might occasionally wash through the line.
Whenever possible, install a tee or an elbow on the drain line at the origin of the pipe (the opposite end from the outlet) and extend a riser pipe to the surface of the ground next to the house. This riser serves as both an inspection point and an emergency cleanout if ever necessary. Cover the cleanout with a 4″ plastic screw-on cap and clearly mark the location. Often it is possible to make the cap flush with the surface of the crushed stone that is used as a splash guard around the foundation, leaving it nearly invisible but easily accessible. It is helpful to make diagrams or take photos to record all such locations. In many cases it is also helpful to place a tall stake next to the outlet (particularly in snow country) in the event it may need to be checked during winter.
Where the drain makes a turn to conform to the foundation (such as a 90″ corner), use a gentle sweep as opposed to a sharp 90″ connector. The sweep helps to eliminate clogging and simplifies use of a plumbing snake in the event of failure. In addition, periodic cleanouts should be installed after every few 90″ turns.
To prohibit 1½” stone from silting up, the stone should be wrapped in filter fabric to hold back the original soil. Water still passes through the fabric but the soil stays in place. Better yet, use a different bedding material that readily passes water but is considerably less expensive. Washed concrete sand, the coarser the better, allows water to flow through readily but keeps the trench more stable than stone while reducing silt buildup in the system. I’ve used it for 25 years in all but the wettest conditions (where I occasionally use ¾” stone wrapped in fabric) with outstanding results and no call backs. This technique requires using pipe with smaller holes since the half inch or larger holes in standard perforated pipe will not hold sand out.
Since no one makes pipe with 5/16″ holes like they used to 25 years ago, you can produce your own slotted pipe by setting a radial arm saw about 3″ above the table and slicing through ¼ the depth of solid pipe every two to three inches or so. Wear goggles to prevent flying chips of hot plastic from striking your eyes. Use either rigid polyethylene or good quality PVC pipe that resists shattering when the saw blade passes through. The 1/8″ slots that result seem to let in water just fine and yet keep the concrete sand from entering the pipe. Pipe is installed with the slots facing down or turned slightly toward the direction of incoming water.
Proper backfilling is important to keep moisture away from the foundation wall. Using clean, porous sandy backfill against the wall will keep water from lingering and finding its way through imperfections. To conserve the amount of sandy material that may need to be brought on site it is possible to add sand directly against the wall and original material (with the large rocks removed) towards the outside of the excavation using alternating buckets of backfill (and compacting these layers) as the excavation hole is filled. The backfill should be topped off with nonporous material (clay or loam) and sloped away from the foundation to prevent surface water from entering the ground next to the wall.
If the drain pipe cannot be run to a safe outlet area, it may be possible to run the outlet to a dry well on the property that is downhill from the house but high enough above the water table to “relocate” the water on site without it ever being brought to the surface. If this is not possible, it may be necessary to connect the perimeter drains to a collection basin outside of the house. From there, a sump pump can lift and discharge water away from the house. It is helpful to find a place to send the water that is far enough away so that it does not recirculate through the ground into the basement.
Of course, the disadvantage of using a pump is that the dewatering process is entirely dependent on the reliability of the pump and the supply of electricity.
I can remember receiving letters from home years ago telling of the recurring disasters to my aunt’s house caused by excess water in her basement when her sump pump failed. Freezers full of spoiled food, ruined furniture and the general nuisance of a flooded basement were becoming too much to bear. On one trip home I did some checking and told the folks that a gravity drain could probably solve the problem.
They went ahead and had one installed and have had no more trouble with unexpected flooding or pump maintenance since. What is surprising is that it took a college education and several years in the construction field before I was able to make such a simple and obvious recommendation. Yet I know of similar situations that exist today, and houses that are being built where sump pumps are expected to do what could have been done with a few pieces of pipe and some planning.
Many basement drainage companies still rely on sump pumps for all installations, regardless of whether or not gravity could be used to discharge the water. This creates a long-term dependence on a mechanical device.
Sump pumps can get plugged up, wear out, fail to come on after months of non-use, or stop because of a power failure. Unfortunately, it is often during the worst weather conditions when power is lost in rural areas while, at the same time, excessive rainfall is causing the water table to rise. About the only assured way to protect a basement is to install a redundant pump and a separate circuit coupled with a battery-powered back-up system that automatically kicks in when all else fails. Such systems are now on the market but represent additional investment and occasional maintenance to keep them in working order. They use an automotive-type battery for power, similar to safety lights in public buildings.
If there is no choice but to install a sump pump, selecting the right pump should be based on the severity of the situation. There are many types market, but the extra money spent for a high quality submersible style with a built-in switch is well worthwhile, especially in extreme situations. Excessive dampness can quickly corrode unprotected motors on pedestal pumps and a power failure that allows water to completely cover an unprotected motor can require replacement of the entire pump. Also, there are automatic switches now available that allow sump pumps to draw water down to within a fraction of an inch of the floor and to turn on when water is only slightly deeper. This can be helpful for occasionally dewatering a basement without digging a sump hole through the concrete floor.
Original Copyright © 1997-1998 Russell H. Lanoie