“It never rains in Southern California”! In seven days starting in mid-December 2010, the Los Angeles area received over seven inches of persistent rainfall, averaging over one inch per day. On a typical 7,500 square foot city lot, that is almost 4,400 cubic feet of water, or roughly 33,000 gallons. Where does that water go? Unless a property is well-drained, too much of that rain-water ends up under the house undermining foundations, seeping through slabs, and flooding crawl-spaces. This in turn contributes to settling of floors and foundations, buckling of wood floors, musty odors, mold and mildew.
When rain hits the ground, it either runs off or soaks in. Soil can only absorb water at a limited rate. In very sandy soil water moves straight down at a high rate. In anything other than very sandy soil, water moves laterally as well as vertically through the soil. Water in saturated soil next to the house is moving towards the dry soil under the house. When the water hits the footings of the house, it percolates down, under the footing, and then comes back up under the house to the same level as the water outside the house.
Uncontrolled roof-water is the most common source of water under the house. Gutters are usually the easiest way to capture and control roof-water. Gutters redirect roof-water into downspouts. But if the downspouts empty the water next to the house, the concentration of water is magnified at the place of discharge and can actually make the threat more severe. Rain-water from gutters needs to be taken away from the house to a safe discharge. This can be done with a paved surface that does not allow the water to flow back towards the house, or with an underground pipe that takes the water to the street or other safe discharge location.
Water that lands on the yard next to the house needs to flow away from the house. If the yard is flat or sloping towards the house, the water will pond and saturate the soil, ending up under the house. To avoid this, the ground should slope away from the house at a rate of ½ inch per foot or more and go into a drainage system or onto yard with slope of 1/4 inch to the foot beyond the first 10 feet. Any water collected in a drainage system needs to be taken to an approved storm-water system, most commonly the street.
Water that soaks into the ground in the yard and neighborhood above the house can work its way downhill towards the house, eventually working its way under the house. When we have prolonged, continuous rains, such as the seven days in December of 2010, soils become saturated and lateral underground movement increases. This is when some water intrusion problems become more visible. The longer the period of continuous or frequent cycles of rain persists, the more likely subterranean water problems are likely to become apparent. These types of problems can sometimes be alleviated by improving the uphill surface drainage on the property, but often a French, or subterranean, drain system with perforated pipe and gravel is the most effective treatment.
Though it does not rain frequently in Southern California, when it does, the rains can create severe problems, especially where provisions for drainage are minimal or poor. In those cases, the answer to the question, “Where does all the water go?” is often, “In or under the house!”
