2.4.2 Inspecting the Basement Foundation

2.4.2 Inspecting the Basement Foundation

Moisture problems are frequently found in the basement of a home or building. Even in some of the driest of site conditions, foundations are continually exposed to moisture vapor from the ground. When basements are used inappropriately on sites with high groundwater, or in areas with intermittent periods of higher ground moisture levels due to rainfall and water table fluctuations, they are frequently exposed to bulk water. Therefore, foundations must be detailed to deal with the potential for water leakage through cracks and joints, capillary movement of water through foundation materials, vapor transmission of moisture through foundation materials, and condensation of moist air on cool foundation surfaces. The degree of protection required for any given site and selected foundation type is primarily a judgment call by the inspector in determining whether it meets or exceeds minimum building code requirements. The building standards and recommendations in this section are intended to teach enhanced moisture-resistance techniques. The recommendations are collectively applied in Figure 24. Interior finishing and insulation of basement walls are addressed in Section 2.4.3.

Foundations that create below-ground spaces on wet sites are of particular concern. During an inspection, an inspector may find evidence of attempts to remedy moisture problems by painstaking waterproofing efforts that may ultimately have a shorter life than the building. As a rule of thumb, moisture protection of foundations by the owner or builder should err on the conservative side when there is reasonable doubt as to the moisture conditions on site. The moisture-resistant practices presented in this course are for the inspector to use as a resource. They can be used for consideration during the construction of a building. They are quite inexpensive compared to the cost of correcting moisture problems after construction is completed. They also reduce the risk of moisture problems in other parts of the building by protecting a prominent entry point for moisture: the foundation.

ILLUSTRATIONS:

Figure 24 and the following sections highlight details for moisture-resistant basement foundations that inspectors can look for during an inspection. These details can be used in diagnosing moisture-related problems that may be discovered. Most of these details will not be readily visible to a home inspector during an inspection of an existing structure, but may be observable during the construction phase when the building materials and components are exposed and open for inspection.

Inspection of Drainage Slopes Away and Understanding Backfill

Proper grading to provide positive flow of surface water and roof water runoff (or gutter discharge) is one of the simplest and most important features to provide on a building site. When possible, the minimum 6-inch fall in finish grade over a distance of 10 feet from the building (minimum 5% slope) should be exceeded and extended. This is particularly important if backfill practices are not reasonably controlled to prevent settlement. On very flat sites, this may require mounding of the foundation pad, and coordination of appropriate foundation elevations to promote drainage. On sloped sites, excavation and grading in the up-slope direction must provide for sufficient drainage away from the building perimeter and against the direction of natural water flow on the site. For sites with very steep slopes, this may require the use of a retaining wall at the toe of a steep slope.

Backfill soil should be placed in a manner that prevents settlement and potential surface-water flow toward the foundation. This may require that backfill soil be placed in 6- to 8-inch layers or lifts, and then compacted with light-construction equipment, or tamped to prevent settlement over time. A heavy compaction effort (typical for commercial building or roadway construction) should not be promoted, as this may damage typical residential foundation walls. The goal is to compact sufficiently to prevent future settlement from the process of natural consolidation of loosely placed soil. In addition, backfill should not be placed without first installing the floor system (or temporary bracing) to support the foundation walls.

Finally, the upper layers of the backfill should be of moderate, low-permeability soil (e.g., with some clay content) to help reduce the direct infiltration of rainwater adjacent to the foundation. Where only pervious soils are available for backfill, the slope of grade away from the perimeter of the foundation should be increased, or an impervious “skirt” of 6-mil polyethylene may be placed about 12 inches below grade.

Good Backfill

It is notoriously difficult to control grading and backfilling operations in typical residential construction. On many sites, the common practice is to place the backfill with the least amount of effort required to merely fill the hole. Proper backfill practices and grading will ensure that the foundation remains dry to a greater degree than all other recommendations here.

Understanding Foundation Drainage Systems

Foundation drainage serves a number of roles. First, it removes free water from the foundation perimeter, which reduces the lateral (sideways) load on the foundation wall. It also lowers the groundwater level in the vicinity of the building’s footprint, should it become elevated above the basement floor level during a particularly wet season or year. Remember that basements should not be used where groundwater levels are near the basement’s floor level. (See Section 2.4.1.)

Current model building codes require that drains be provided around all foundations that enclose habitable space (such as basements). However, exceptions are made for soils that are naturally well-draining. Unless a site-specific soil investigation is done, or extensive local experience confirms that groundwater levels are consistently deep, soils should not be assumed to be well-draining.

Where the foundation drainage system cannot be drained to daylight by gravity, a sump and pump must be used to collect the water and discharge it to a suitable outfall (e.g., rock pad and swale) a safe distance away from the building foundation. Furthermore, use of a drainage layer underneath the entire basement floor slab, coupled with weeps to a drainage system around the outside perimeter of the foundation, may be a more effective way to eradicate conditions where potential for high groundwater levels near to the basement floor elevation may exist. Experience has shown that trying to seal moisture out of a foundation is not nearly as effective as diverting the moisture with a drainage system before it gets inside the living space.

Waterproofing and Damp-Proofing

The terms “waterproofing” and “damp-proofing” both relate to moisture protection. However, they are often misapplied and incorrectly used by inspectors. The term “waterproofing” is often used when “damp-proofing” is more appropriate. Damp-proofing does not provide the same degree of moisture protection as does waterproofing.

Model building codes typically require damp-proofing of foundation walls that retain earth and enclose interior spaces and floors below grade. The damp-proofing shall be applied from the top of the footing to the finished grade. Damp-proofing for masonry walls involves a Portland cement parging applied to the exterior of the wall. The parging shall be damp-proofed using one of the following materials:

• bituminous coating;
• acrylic-modified cement;
• surface-bonding cement;
• a permitted waterproofing material; or
• other approved methods or materials.

The use of waterproofing measures is reserved for conditions where a high water table or other severe soil-water conditions are known to exist. Strictly speaking, “waterproof” does not mean “watertight” (as with a boat hull). It simply involves the application of a more impermeable membrane on the foundation wall. With some exceptions, walls shall be waterproofed by one of the following methods:

• 2-ply hot-mopped felts;
• 55-pound roll roofing;
• 6-mil polyvinyl chloride (PVC);
• 6-mil polyethylene;
• 40-mil polymer-modified asphalt;
• 60-mil flexible polymer cement;
• 1/8-inch cement-based, fiber-reinforced waterproofing coating; or
• 60-mil, solvent-free, liquid-applied synthetic rubber.

The waterproofing method is recommended as a best practice, especially if the basement is intended to be used for storage or living space. Waterproofing could involve the simple application of damp-proofing, plus a layer of 6-mil poly on the exterior below-grade portions of a basement’s foundation walls. Other single-ply or built-up membranes may also be used.

Damp-proofing is the application of one or more coatings of impervious compounds that prevent the passage of water vapor through walls under slight pressure.

Waterproofing is the application of a combination of sealing materials and impervious coatings to prevent the passage of moisture in either a vapor or liquid form under conditions of significant hydrostatic pressure.

Look for Foundation Cracks and Water

It is important to realize that all concrete and masonry construction will develop cracks due to the effects of shrinkage. As these cracks widen over time (usually due to small amounts of differential settlement in the soil supporting the foundation), the pathways for water intrusion through the foundation increase. Visible cracks may become a concern to homeowners even though these often have little relevance to the structural integrity of the foundation. The question becomes how to best control these cracks.

The optimum location for reinforcement to control cracking and prevent differential settlement is at the top and bottom of the foundation wall in a horizontal direction. Horizontal reinforcing of this type should be considered, in addition to adhering to code-required vertical reinforcement. By placing horizontal reinforcement, the wall acts as a deep beam, even after cracks initially form due to shrinkage during the concrete’s curing process. If the wall is adequately tied (or doweled to the footing), then the reinforcement in the bottom of the wall may be placed horizontally along the length of the footing. The reinforcement at the top of the wall is known in masonry construction as a bond beam. Alternatively, truss-type reinforcing wire may also be used between horizontal courses of masonry block.

Epoxy Sealant for Masonry Cracks

An epoxy sealant can be injected into cracks in masonry foundation walls. Inspectors often find cracks in poured concrete foundation walls. Typically, these cracks are shrinkage cracks and not an indication of major structural problems. The only problem other than the cosmetic appearance of shrinkage cracks in a poured concrete foundation wall is water penetration through these cracks. Epoxy sealant is an easy and relatively inexpensive solution for this potential moisture problem.

The process is simple. First, the crack is first cleaned and exposed, and maybe even enlarged in some areas. A sealant is then applied over and along the crack. It serves as a barrier for the injection material, and also holds the injection nozzles in place so that no drilling of the foundation is necessary. The resin compound is a moisture-activated, flexible material that provides a permanent seal, even if there is future movement in the foundation. Next, the epoxy is injected. The epoxy is a two-part material consisting of resin and a hardener. When mixed, these liquids create a Super Glue®-like bond. Injecting a crack with epoxy resin is ideal in cracks where there will be little or no movement in the foundation.

Epoxy sealant can be applied at block foundation walls and at concrete floors of basements and garages.

Sealants for Through-Wall Penetrations

Utility penetrations through foundation walls should be carefully sealed on the exterior face of the wall prior to placement of waterproofing materials and backfill. High-quality urethane caulks are most suitable for this application. (Refer to Section 2.3.4 for additional information about sealants.) In addition, the wall construction should be inspected for penetrations due to voids or other problem areas (such as form ties) and appropriately repaired and sealed.