Basement Information
Many "how-to" magazines run articles touting miracle products that stop water from permeating through foundations or basement walls and floors. While some products may be effective at mitigating water migration through cracks or flaws in a wall, blocking groundwater can be as tough as keeping water from leaking through split seams of a wooden boat. So long as any part of the boat is in the water, a certain amount of water is bound to find its way inside. Never wait until water enters the basement or permeates the foundation before treating the root cause. The resulting dampness is both distasteful and potentially dangerous, even if there is no noticeable standing water.
There are four ways that moisture can permeate foundations or enter and collect in basements: High groundwater table Surface (rain) water leaking through openings or flaws in the walls Condensation Broken plumbing and/or irrigation runoff 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. Wrong Culprit Years ago I did a job for a homeowner who insisted that her basement became wet only after a rainstorm. After lots of digging and waterproofing, (and continued water in the basement) I was left with a serious question about the real source of the water. Finally I started checking the house plumbing and discovered a leak in the water heater! I've learned to more be suspicious. 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. Current building codes require 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. Perimeter Drains 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. 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 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 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. This same procedure is excellent for drying out existing wet basements where conditions permit. Ensuring Effective Drainage 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. 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. Filter Fabric: Synthetic cloth-like material that is used for several different types of construction related applications such as erosion control, road stabilization and soil separation. Can consist of either woven or non-woven fibers in varying thicknesses or weights. Available in 12 to 15 foot wide rolls several hundred feet in length. Woven fabrics (usually black) resemble the stuff that modern day grain bags and weed control fabric are made from while non-woven fabrics can resemble a range of materials from soft felts to the stiff shiny house wrap (to which they are closely related) usually seen enveloping homes under construction. Maintenance It is essential to keep the drain outlet running freely. The rodent guard must be kept clear of any debris. The area below the outlet must be kept cleared of leaves and debris, especially if the drain empties into a ditch. Sometimes drains that have clogged up internally can be cleared by the use of a pressure washer (or even a garden hose) to break up the clog. I've cleared obstructions from perimeter drains and had water drain out of the basement just like water leaving a bathtub. Properly installed perimeter drains create an "island" out of the house site by lowering the water table completely around the house. They keep water from entering under the footings and, in my experience, provide the surest protection against any type of dampness in a basement resulting from groundwater. There are some situations where perimeter drains are not necessary, however. A thorough knowledge of the site, or an exceptionally low water table indication from a septic system test pit, can provide such a determination. If you happen to be one of those folks already blessed with an unwanted indoor swimming pool each Spring, whether in a new or existing house, rest assured that there usually is something that can be done. However, since soil conditions vary so greatly, it is wise to consider your circumstances carefully before deciding on a course of action. Also see the University of Minnesota Extension Bulletin Moisture in Basements: Causes and Solutions Filter material: According to the American Iron and Steel Institute's Handbook of steel drainage and Highway Construction Products, 1967: "Early subdrains consisted of a trench filled with coarse rock ("french" drains) which quickly silted up. Extensive research by the U.S.Waterways Experiment Station at Vicksburg, Mississippi, shows that a graded material roughly equal to concrete sand (AASHO Specs) has been found most suitable. Such material gives better support to the sidewall of the trench and thereby reduces erosion and silting. Filter material should be placed in layers and tamped." Leaks in Foundation Walls Another condition that can sometimes appear to be high ground water leaking in may simply be surface water entering over or through the foundation at bulkheads or window openings, seams between walls, and/or, flaws in the foundation. Poor surface grading can direct water toward the lip of a bulkhead or towards basement windows where it can spill over into the basement. By not paying attention during heavy rains, a homeowner can sometimes end up blaming the wrong culprit. Surface water leakage Surface water leakage is reasonably easy to locate once you get on track. The solution often involves regrading to drain water away from the foundation. In the case of foundation windows, however, it is not always possible to shed water away since the bottom of the windows are often too far below the adjacent ground, requiring the installation of window wells. These often consist of corrugated galvanized steel half circles but can also be made of pressure treated wood or masonry. The goal is to make the top of the window well high enough to be able to slope the surrounding ground surface away from the house. A problem arises when the window well is directly below the roof drip, especially in cold, wet climates where rain gutters are seldom used (because snow and ice fills them, making them useless, or ripping them from the building!) The solution is either to cover the window well with a clear plastic dome which also keeps out leaves and critters or to create a mini-drywell under the window well to allow rainwater to soak away into the ground before it can rise up and find its way through the window. This leads to another issue: porosity or flaws in the wall itself. If a foundation wall is totally watertight there is little problem with letting water run down against it all the way to the perimeter drain. If the wall has had adequate damp-proofing and/or the water drains quickly through the soil, there is little to be concerned about. The problem is that many foundations are have defects and hairline cracks which allow water infiltration. Sources of Leaks in Foundation Walls Concrete block foundations often crack in the grout joint between blocks. Even poured concrete or ICF walls can develop shrinkage or settling cracks in addition to water channels from anchor bolts and form ties. Honeycombing due to improper vibration and consolidation is another source of leakage, wherein the concrete aggregate became separated leaving voids in the wall. In addition, poured walls often have seams between different stages of the pour. Another problem exists with many of the older foundations that are constructed of various sizes and shapes of cut granite and round fieldstone. With or without mortar between, these foundations typically exhibit numerous passages for water to follow. Water can enter walls around utility penetrations such as sewer, water, gas or electrical conduits. There are several instances where I've been engaged to dig around existing buildings and to waterproof walls and/or install drainage systems only to discover that the actual problems were unrelated to groundwater. Having been under the direction of an architect in some cases, I've had to follow his recommendations rather than follow my instincts to determine the problems. Consider this example that involved an old house with a loose stone foundation. The original work that was done under the direction of the architect only aggravated the wet basement problem. Digging a shallow perimeter drain actually allowed more water through the porous basement wall. This was the same mistake many people make when they dig out next to their foundation wall and replace the original backfill with crushed stone either for appearance and splash protection. Putting crushed stone directly over the original soil, even with the installation of a perforated drain pipe often causes more water to enter the ground. The crushed stone becomes a dispersion system in the same way it does in a septic drain field. Usually little water enters the drain pipe, except in a deluge, and more water soaks into the ground next to the foundation where it makes its way into the basement. What I did to solve the problem was to seal the outside of the wall with a coating of concrete that directed surface water away from the foundation as it soaked into the ground, keeping it from streaming through the loose stone foundation. The addition of a bituminous asphalt coating or a Bituthene membrane can also help to keep water from penetrating the foundation wall. The success with this project has prompted me to use this technique in many other situations. Rather than using concrete, however, in most cases I rely on a layer of rubber or plastic to direct water away from the wall. Usually I excavate just a few inches below the surface next to the building and three to four feet lower at a distance of four to five feet from the wall, sloping the soil down on a 30 to 45 degree angle away from the building as I go. I rake the earth smooth, remove all rocks and sticks, and tamp it firm. Then I install a layer of rubber or a couple of layers of 6 mil polyethylene over the slope, right up to the wall. I don't usually worry about sealing against the wall, as my primary concern is to direct the majority of the surface water away from the wall and out into the earth where it can soak harmlessly away, leaving a "dry zone" beneath the membrane next to the wall. I use this same technique under a window well if there is any question about the integrity of the wall below the window. I dig out a few extra feet, slope the soil, install the plastic membrane on the slope, place crushed stone as a mini drywell for any rainwater that may enter, install the window well structure and then backfill to the surface as discussed. This technique of installing a membrane just below the ground surface is similar to installing "flashing" around a chimney or other structure on a roof. One of its biggest benefits is that it can eliminate the need to completely excavate a foundation wall all the way to the footing and attempting to waterproof it. It is somewhat similar to installing a full length "splash block" mentioned in other basement drainage articles on the Internet except that it is out of sight. I've also had success with this technique even when attempts to make the wall watertight below have failed.| Barrie Home Inspector | Office Phone: (705) 795-8255 |
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