Homeowners are asking what they should be doing to prepare for “the big one” a hypothetical storm-scenario being evaluated by the U.S. Geological Survey that could dump up to ten feet of water on California over 40 days. A popular saying is, “It never rains in Southern California!” But sometimes it does rain, and when it does, the consequences can be devastating.
Flooding of streets and hillside slippage are usually beyond the control of the individual homeowner, but there are steps homeowners can take themselves to reduce damage from rain storms in general, and which become even more significant in the event of “the big one.”.
Our experience is in treating ground-water intrusion problems, primarily for residential properties. In our work, we think of two types of storms of particular concern in Southern California: (1) Hard rain of 1 to 3 days; or (2) Persistent rain, whether hard or not, that persists over weeks at a time with little or no separation between storms. The first type of storm can simply overwhelm clogged or poorly designed drainage around a home and actually flood the home from the water getting too deep. This is a serious threat, but at least it is often visible to the homeowner. The second type of storm may flood the home, but in addition, it saturates the soil, continuously pushing more and more water into the soil. This type of storm saturates the soil to a greater depth, and water begins to move laterally underneath the ground. This is the type of water movement that is usually not visible, and because is it moving laterally underground may take days, weeks, or in some cases months before its effects are seen. This type of water movement can flood underfloor spaces, intrude into basements, saturate soil under slab-floors and footings, and create as much or more damage as a hard but brief rain. Typical home damage from ground-water includes mold and mildew issues under or in the house, actual flooding of floors and underfloor space, settling and/or cracking of foundations and concrete slabs, and warping of hardwood floors. In hillside areas, there is an additional hazard of land-slides.
For either of these two types of storms, there are fundamental principles that can be followed to minimize the hazard and damage to the homeowner. These are spelled out in the International Building Code. The overall idea is that water should not be allowed to pool or accumulate anywhere near the house, especially wood portions of the structure, but extending to all portions of the structure.
Roof Water
In our experience roof-water is the number one culprit to be addressed. The roof water falls represents a fairly large area that is dumped next to the foundation of the house in a relatively narrow area. This concentration can easily saturate the soil and lead to intrusion. Usually the easiest way to address this issue is to install gutters and downspouts, and take the water from the downspouts well away from the house, either over a concrete walk or driveway, or via underground pipe to the street.
The Grade Next to the House.
The building code calls for the grade next to the house to be at least 6 inches below and wood framing, which is usually at the top of the concrete foundation. Even though the wood framing may be covered with stucco, water can penetrate the stucco and cause mold, mildew, and wood-rot that can be very difficult and expensive to remedy.
Next the grade should slope away from the house at the rate of 6 inches over the first ten feet, and then 1/4 of an inch thereafter until reaching a drain device or the storm-drain system. This is a difficult requirement for many homes because they either were never graded this way when they were built, or sidewalks create dams that hold the water back against the house. Many homes have only a five foot side-yard, which means that some sort of physical drain system needs to be provided to get water away from the house. Water collected with such a system needs to be conducted safely to a natural drainage course or a storm-drain system to prevent undermining of the homeowners property and the property of the neighborhood.
On many existing homes it may be impractical to totally conform to these basic rules, so compromises need to be made. Such compromises should be consistent with the basic objective – capture as much water as is practical and get it safely away from the house. In some cases this may require the elimination of narrow planting beds adjacent to the house, or the installation of intensive drainage systems in such planting areas.
Subterranean Water
Subsurface water that builds up in the soil and moves towards the house, especially from a yard or neighborhood that slopes towards the house, may require a subterranean, or “French”, drain. This is basically a trench, usually deeper than the underfloor space or ideally deeper than the foundation, where the bottom is sloped in one direction, perforated pipe is installed along the bottom of the trench, and the trench is backfilled with gravel, with the excavated dirt typically being hauled away. The pipe discharges to an approved drainage system, as in the surface drain systems described above. In some cases, where there is not a suitable gravity route for the water, the water will need to be pumped to where it can then be drained off. The idea of this type of drain system is that water moving underground through saturated soil hits the more permeable gravel and percolates down to the bottom of the trench. There it begins to fill the trench, gets in the perorations of the pipe, and is conducted away from the property. When we get the type of storms where we have persistent, extended rain, the French drain becomes particularly useful for many properties.
Advance Preparation
Once the rains start, there is often little that can be done to prevent water intrusion until the rains stop. Therefore, it is important that homeowners prepare in advance for possible storms. Once appropriate systems are installed, they need to be regularly maintained. Gutters need to be kept clear of leaves and other debris, catch-basins need to be kept clean, sump pumps need to be tested, and blocked pipes need to be cleared. Typically preparations begin in October and November to prepare for the first rain. Sometimes it is important to repeat the maintenance after the first big rain to clear the system of accumulations from the rains. For vulnerable properties, additional maintenance may be required as the rainy season progresses.
While a super storm is not likely to happen, the buzz about it is a good reminder that we do sometimes get heavy rains here in the Los Angeles area and it's a good idea not to wait until it's too late to protect your property.
