Water intrusion is a common problem through any retaining wall that provides the exterior wall to a room lower than the exterior soil level. This is true for homes on sloped lots where retaining walls are used on a lower level, split level homes, homes with basements, and even homes with a sunken or recessed area that is below the exterior grade. The waterproofing systems can fail from deterioration due to age or poor installation. There are excellent waterproofing systems available today that can have a long service life but they require attention to detail that is far too often lacking. Asphalt emulsions were used to waterproof most walls built before the mid-80’s and this product is still being used by contractors today. The asphalt emulsion deteriorates with age and most of these systems will fail at some point when subjected to water.
While structural damage caused by leakage is rare and water damage to interior finishes and materials can be repaired, the biggest risk of any moisture intrusion issue is often mold. Mold needs to be taken seriously because of the high legal liability if nothing else. I do not know what the health risks of mold are and you must be responsible for finding that answer elsewhere. I do realize however, that even small leaks that dampen cellulose materials such as drywall, wood or any wood products for more than a day or two have a substantial risk of mold growth. I also realize that moisture migrating through a retaining wall can continue for long periods without being very noticeable and that the first sign of a problem can be a musty odor. Read the mold statement in your report for more information on mold.
Unfortunately, wet basements and seeping retaining walls cannot be assessed for their severity or frequency during a one time visit. There may or may not be clues that indicate a history of dampness. Even if visible, the clues usually do not give an indication of the severity or frequency, and are usually inconclusive and sometimes even misleading. It is also all too easy to cover or hide evidence of past moisture intrusion either intentionally or unintentionally. Furthermore, moisture problems are intermittent. In some houses, water penetration will occur after virtually every rain. In other houses, it will occur only after periods of prolonged rain or only on exceptionally rainy years.
Waterproofing a retaining wall can be very expensive and often requires excavation to expose the back side of the retaining wall. To avoid that expense it generally makes sense to try less expensive alternatives first. Unfortunately, the less expensive alternatives don’t always work and you need to decide if you are willing to accept the risks involved and/or are willing to accept a less than perfect solution where minor amounts of moisture get in. It will also depend on the sensitivity of the people living in the house to mold, and your assessment of the mold risk. If you are not able to experiment or willing to take some risk the higher cost but lower risk approach makes sense.
Every moisture intrusion problem needs to be evaluated further by a contractor who specializes specifically in solving these types of problems and I do not feel that the vast majority of contractors are qualified to do this. Make sure you ask how much experience the contractor has with these problems and how much of their work is dedicated to solving moisture intrusion problems on below grade retaining walls. Ask them to explain how they have solved similar situations and how successful they have been.
It is possible that water intrusion problems can be solved by controlling rain and irrigation water and making sure that surface water is not allowed to saturate into the soil around the building. This is usually the most cost effective way to start and is almost always part of any solution. Unfortunately, it is not possible to tell if this will be successful until it is tried and passes the test of time. Water from the roof should be captured in rain gutters and down spouts should be tied into a drain that caries the water out to the street or other approved drainage. Take any drainage and grading recommendations in the inspection report seriously and follow the recommendation in the handout for lot grading except that the slope away from the house should be increased to 1 inch per foot for a minimum of 6 feet from the side of the house. I strongly believe that these water control steps should be taken seriously on any house with a lower level retaining wall regardless whether there is any evidence of a problem discovered at the time of the inspection or not.
If water can be collected and discharged far enough away from the house, it will not migrate against the retaining wall and contribute to dampness in lower levels. Reducing irrigation water, installing drip systems or zeroscape planting, eliminating planter areas next to a house, and adding concrete sidewalks and patios are all additional ways to reduce the amount of water saturating into the ground. Water saturation on the uphill side of the retaining wall is the most likely source of any water problem and where much of your attention should be focused. These steps can greatly reduce the risk of moisture intrusion and have been able to solve many of them. However, nobody can guarantee that water control alone will solve a moisture intrusion problem below grade level. Water can occasionally travel horizontally below the surface a long distance and you may not have much control over water saturation on a neighbor properties.
This handout is intended to provide you with general information that I feel will be helpful to you but is not intended as a substitute for further evaluation by a specialist when water intrusion issues are discovered.