Recommendations For Lot Grading and Drainage
Poor yard surface drainage can lead to cracks and movement in house slabs, severe foundation problems, slope instabilities, and retaining wall failures. Conversely, a little preventive maintenance and drainage improvement work can mitigate the potential for soil related distress at far less expense than structural solutions for failures. Residents of Southern California tend to downplay the importance of good drainage after several months of sunshine. Unfortunately this is too often a costly mistake.
Many, if not most of the homes in San Diego County are constructed on clay soils that are expansive. This means that when the soil becomes wet – it expands. This expansion can be up to 150 percent of the dry volume and exert pressures up to 500 pound per square foot. This expansive force is the cause of the majority of the cracked slab problems in this county and can break a perimeter foundation. By improving the grading and drainage and employing the recommendations that follow, it is possible to minimize fluctuations in soil moisture content beneath the foundation and floor slab and this is usually all that is needed to prevent structural problems caused by moisture changes in the soil. The same process causes movement in patio or driveway slabs.
I particularly want to emphasize the critical importance of good drainage with homes that have a raised foundation with interior piers. Wet clays become unstable and fluid and individual piers that support the center of older homes with raised foundations are particularly vulnerable to movement when the soil becomes wet. The small base or footprint of the piers makes them likely to sink in the mud. This is true of exterior deck footings as well.
Ideally homes should be located on the “high” ground with slope contours directing surface flow away from the structure. Unfortunately, many homes in the real world are on poorly graded lots where the soil level is too close to flat and too much water is allowed to saturate into the soil close to the foundation. This problem is greatly compounded when large amounts of water come off the roof and dump close to the house.
Most water problems can be fixed by exterior grade changes, rain gutters and surface drainage. You want the water to move away from the house with the greatest of ease. Generally, the ground should slope down away from the house at a rate of one half inch per foot for the first five feet. The more slope the better. Well compacted soils which force most of the water to run across the surface are preferred and mulch, bark or rock that slows the flow should not be used. Impervious surfaces like concrete sidewalks and driveways can slope less, with almost any positive slope being effective.
After the water is moved away from the foundation, it must be channeled to the street along a trough or swale that should slope ¼ inch per foot all the way to the street gutter. That will require two feet of fall in 100 feet. Where this cannot be achieved, catch basins with yard drains may be used. Water can be directed toward the basins and carried through a minimum 3 inch solid wall drain lines to the street. (4 inch drains are a better choice) The flexible black corrugated drainage lines should never be used because they are so easily crushed or cut. It is worthwhile to invest in ABS or PVC drains that are thicker walled and much less vulnerable to damage than the cheaper materials. Considering the effort involved installing the drains, it is well worth the small additional cost.
Although, most of the homes I inspect fall short of these standards without apparent signs of distress, it is foolhardy to dismiss their importance. No one has come up with a reliable way of predicting which houses will have water or grading related problems in advance, and repairs to foundations or slabs can be very expensive.