Keeping water drained away from home foundations is important for three reasons. First, if there is living space on the other side, the owner will want it to stay dry. Good drainage is the first step toward accomplishing that, then waterproofing. Second, soils supporting a foundation need to stay at a consistent moisture level to prevent settlement, heave, or differential movement. Last, but not least, drains are required by the building code. Here are a few things to keep in mind:
- Code requirements: The International Residential Code (IRC), in Section R405.1, requires drains “around all concrete or masonry foundations that retain earth and enclose habitable or usable spaces below grade.” The IRC goes on to provide details about what kind of drains are sufficient. However, very well-drained soils are an exception and mixtures such as sand and gravel do not require drainage.
- Moisture: There are two zones of subsurface moisture: the aeration zone (where both water and air exist) and the saturation zone. Generally, the saturation zone is everything below the water table, which is the level at which water rises to in a well. The saturation zone is seldom an issue in residential construction—soil moisture is the concern.
- Loss of soil moisture: Soil moisture beneath a foundation is lost in a triangular configuration, so the deepest dry area is just outside the edge of the foundation and the ground beneath the middle of the slab remains saturated. Differential drying or differential amounts of moisture in the soil can create problems, especially in expansive soils, which in many parts of the country are more the rule than the exception. In some areas, homeowners actually have to water their foundations to maintain soil moisture.
- Surface drainage: Controlling surface water is critical to controlling soil moisture beneath the foundation. The ground surface should slope away from the house at between ½ and 1 inch per foot for at least 6 feet—10 feet is better. Be careful of poorly compacted backfill, though, because that will soon mean that the surface will slope back toward the house.
- Gutters: Downspouts should discharge on sloping surfaces at least 10 feet from the foundation. Where that isn’t possible, downspouts should discharge into drained catch basins.
- Trees: According to the U.S. Department of Housing and Urban Development, trees should be planted no closer to the foundation than their eventual height. This prevents tree roots from filling perimeter drains and inhibits the tree from sucking all the water from the soil, which could lead to settlement.
- Subsurface drainage: Perimeter drains should be made from rigid drain tile or perforated pipe. Although flexible corrugated plastic pipe can be used, care must be taken to prevent it from being crushed during backfilling. One simple method is to use Form-A-Drain, which is a combined footing side form and drain pipe.
- Drain pipes: Drain pipes should be positioned alongside the footing—the best spot is near its base. Although tile doesn’t need to be sloping, low spots (which can fill with silt) must be avoided. With flexible tile, a good location is on top of the footing, which helps keep them from developing low spots.
- Drainage boards: In wetter areas, drainage boards installed on a concrete foundation wall will allow water to drain quickly to the perimeter drain and will prevent any buildup of hydrostatic pressure next to the wall.
- Finally, remember that it is much easier to do foundation drainage right in the first place than to repair it later.