DRAINAGE

Problems

Soggy carpets, buckled floors, damp basements, flooded yards, mold and mildew allergies and frazzled nerves accompany rain storms for many property owners.

Southern California is an area of very little rainfall, but it does rain. Because of the generally arid climate many homes have been built with little or no consideration for drainage problems. 

Although the Los Angeles area is in a semi-arid region, there have always been springs and streams, especially in the canyons and hills. The early native residents depended upon the natural springs and streams for their water supply. These springs and streams dry up in the summer and during prolonged drought they almost disappear. Few are active year around. Most of these water flows disappeared during construction booms which resulted from the desirability of the surrounding property. Much of that water is now running down storm drain systems. During grading operations – establishing building pads, roads, etc. – underground courses have been disrupted and redirected, perhaps unknowingly, when construction occurred during a dry spell. A grading operation may have exposed strata that will conduct water. The strata may emerge on some other property, and then so will the water. 

The best time to solve drainage problems is before or during construction. But drainage problems can be alleviated or eliminated after construction. The first step is to find possible sources of the water. A roof collects a lot of water and is the source of many basement water problems. The yard grading, the neighbor's yard or the whole neighborhood might be the source. 

Next, the water has to be collected. This starts with roof gutters and down-spouts. Water should not be allowed to come off anywhere it wants to. It should be brought down to where it can be handled. Gutters need to be inspected and cleaned periodically; they have a way of being plugged when they're needed the most. Catch-basins located at low, swampy spots are good collectors. Paved gutters are sometimes helpful, but only for surface water. The trickiest collection problem is sub-surface water. This can be collected by means of a "French" drain, weep drain, or subterranean drain, a drain that intercepts the water below the ground.

Finally, the water collected must be conducted away to a safe point of discharge. This usually requires underground piping and occasionally sump pits and pumps to remove the water.

Intuitive solutions are often unproductive. Many people build a sidewalk next to the structure or a little concrete curb; such devices will not stop water that is going under a building. The volume of water to be handled is much greater than most people realize. A storm that drops one inch of rain on a typical fifty by one hundred fifty foot lot has deposited over 4000 gallons of water. For most buildings that is more than the water that could be used by turning on every water outlet on the property and leaving it run for three hours, or about half of the water from a typical residential swimming pool. All of that water goes somewhere. It can go over, under, around or through obstacles. Building walls, waterproofing interiors, digging trenches, or just gritting one's teeth after the storm are poor solutions. 

Solutions

Roof and surface drains

Treatment for water intrusion often begins with adding rain gutters to the roof of the building. Water that falls next to the house and seeps into the ground forms a bubble of moisture under the building. When it passes under a concrete curb or footing, it rises to the same level on both sides. Gutters need to be large enough and positioned so as to intercept and carry off the water from the roof. If the gutter water is then discharged next to the house, it often only makes the problem worse by concentrating large amounts in one location. Downspouts from gutters must to be connected to a drainage system that conducts this water away from the house. 

Similarly, yard-water that puddles around the building can also make its way back under the building. The Uniform Building Code sets standards for yard grading to prevent seepage back under the building. The standard is for the exterior grade to be a minimum of 6 inches below the top of the footing, and to slope away from the house at a minimum of 2% (1/4 inch per foot) to an approved drainage device. Catch-basins (a box with a grate on top set at ground level and connected to drain pipes) can be installed in low spots around the building to carry off water. These typically have openings 8 inches square or larger. The typical drain inlets installed in decks and lawns that are 3 or 4 inches in diameter are often ineffective. Two or three leaves washed over them can severely restrict or eliminate their functionality. 

Once the water is collected it needs to be discharged safely. This usually means in the street gutter, but may also mean a nearby gully or canyon. Care must be taken not to create a biological hazard by the discharge of collected water. 

French (subterranean, foundation) drains

Water that is migrating under the surface is best intercepted and controlled by use of a French drain. People refer to many schemes as French drains. The term French drain means a trench, preferably with the bottom below the level of the sub-floor, or better yet below the bottom of the building footings, and:

  • the bottom smoothly graded and declining at the rate of one percent,
  • 4 inch perforated pipe laid on the bottom of the trench,
  • backfilled with gravel either all the way to the surface, or to within 8 inches of the surface
  • discharging to an appropriate drainage course or structure.

Sometimes the trench is wrapped with filter fabric to reduce root invasion. Wrapping with filter fabric has the disadvantage that it tends to fill up with silt which can stop the drain from functioning.

These French drains typically run all the way across one or two sides of a house, and occasionally three or four sides. The critical consideration is depth. Almost any water intrusion problem can be solved with a deep enough French drain. However, a French drain is expensive to construct, and the deeper it is the costs go up more than proportionately.

French drains are susceptible to silt intrusion and root invasion, since they are not sealed. Tall shrubs and trees should not be near the drain when possible to prolong drain life. Shallow drains (12 to 18 inches) have been known to clog up within 2 years when installed next to ivy. Other drains have been in service for fifteen or twenty years and appear to be functioning well. 

Sump pits and pumps

It is always preferable to conduct water by gravity to a point of safe discharge. Sometimes that is not practical, and sometimes it is not possible. In these cases, a sump pit and pump are required. A sump pit is any device or structure that serves as an underground pit or container to receive water. The pit needs to extend 18 inches below the drain inlet to allow for pumping operations. The pump is automatically actuated any time the water level is high enough to activate a capacitance switch. It is then pumped to the street or other suitable drainage course. 

Drainage Consultations

Drainage problems require on-site diagnosis and evaluation in order to propose treatments. If there have been geological reports, soil engineer reports, property inspector reports, termite reports, owner's observations, etc., it is helpful to have copies available at the site at the time of the on-site examination. It is preferable for the owner, or in the case of a building in escrow, the buyer be present at the time of the evaluation. Elusive water problems can best be described and evaluated more effectively person-to-person on the site than in written reports or proposals.

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