Keeping water out of basements is a major concern in many regions. The source of water is primarily from rainfall, snow melt, and sometimes irrigation on the surface. In some cases, the groundwater table is near or above the basement floor level at times during the year. There are three basic lines of defense against water problems in basements: (1) surface drainage, (2) subsurface drainage, and (3) waterproofing on the wall surface (see Figures 2-3F, 2-3S, and 2-4).
The goal of surface drainage is to keep water from surface sources away from the foundation by sloping the ground surface and using gutters and downspouts for roof drainage. Subsurface drainage systems intercept, collect, and carry away any water in the ground surrounding the basement. Components of a subsurface system can include porous backfill, drainage mat materials or insulated drainage boards, and perforated drainpipes in a protected gravel bed along the footing or beneath the slab that drain to a sump or to daylight. Local conditions will determine which of these subsurface drainage system components, if any, are recommended for a particular site.
Figure 2-3F shows a dual drain system, which is the most robust option. Figure 2-4 shows a single-drain configuration. In both cases, provision is made to drain water originating at the surface that drains down along the foundation, as well as water that may accumulate under the slab. Figure 2-3F shows the best practice system for foundation perimeter drainage. It consists of two independent loops of perforated foundation drain, one inside the footing and one outside. These drain independently, either to daylight or to an internal sump. Figure 2-4 shows another option that is appropriate when site drainage conditions are good. It also allows drainage of the sub-slab gravel layer, through ducts that run through the bottom of the footing. These ducts should be placed as close to the bottom of the footing as possible, to avoid water pooling on the inside of the footing. Its single loop of foundation drain is on the outside of the footing, and drains to daylight or to an internal sump. It should be noted that the duct connection to the exterior of the footing can reduce the effectiveness of subslab depressurization radon mitigation systems by reducing the ability of the system to maintain sufficiently low pressures beneath the slab.
The final line of defense— waterproofing—is intended to keep out water that finds its way to the wall of the structure. First, it is important to distinguish between the need for dampproofing versus waterproofing. In most cases a dampproof coating covered by a 4-mil layer of polyethylene is recommended to reduce vapor and capillary draw transmission from the soil through the basement wall. A dampproof coating, however, is not effective in preventing water under hydrostatic pressure from entering through the wall. Waterproofing is recommended (1) on sites with anticipated water problems or poor drainage, (2) when finished basement space is planned, or (3) on any foundation built where intermittent hydrostatic pressure occurs against the basement wall due to rainfall, irrigation, or snow melt. Except on very dry sites, it is generally recommended to use waterproofing as a best practice. On sites where the basement floor could be below the water table, a crawl space or slab-on-grade foundation is recommended.