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Hardscape. Concrete or paver block sidewalks can also control percolation of runoff into the backfill (Figure 3) — I’ve measured reductions in runoff percolation of between 300% and 500%.

Concrete or Paver Sidewalk

Figure 3. A properly sloped concrete or paver sidewalk will reduce the amount of runoff that percolates through the backfill (top). Where perimeter plantings are used to landscape, improve drainage by burying a sheet of polyethylene below the plant bed, with openings cut out for roots (bottom). Tie shallow perforated drain tile to solid pipe to carry water to daylight or a drywell.

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Plant Bed with Drain

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Again, the hardscape should be wide enough to cover the entire backfilled area, and the surface should slope away from the foundation walls. A less expensive technique is to bury a sheet of polyethylene in a plant bed. The poly should cover the backfilled foundation trench and slope to a perforated drain tile laid parallel to the foundation. Use solid pipe to carry runoff to daylight or to a drywell. In landscaped areas, cut openings in the poly to accommodate plant and tree roots. Buried poly works well, so long as the backfill has been compacted. With a negative grade, however, the poly actually directs the water into the foundation wall. Plant and tree roots near the foundation can also compound problems with uncompacted fill, because their root systems absorb water and cause the soil to reconsolidate quickly. In a drought, tree roots can pull so much moisture out of the soil that the foundation may settle.

Perimeter Footing Drains

Foundation perimeter drains work in both directions. They not only carry rainwater percolating down through the backfill away from the foundation, they also relieve excessive hydrostatic pressure from rising groundwater. By helping the backfill dry out more quickly, properly installed perimeter drains reduce lateral soil pressure, which in turn means that foundation walls can be designed to use more porous materials and less steel. There’s a right way and a wrong way to install perimeter drainage. Unfortunately, many foundation contractors and home builders labor under a false sense of security, reasoning that if complaints about leaky basements don’t surface within the first year or two after a project is completed, their construction techniques must be working. The fact of the matter is that basement water problems that occur within the first twelve months are usually related to waterproofing defects. Drain tile problems typically take many years to develop. Thus, many contractors have buried time bombs that will eventually blow up in their faces.

Holes Down

Although porous cement-based tile is still in use today, most residential contractors would agree that perforated 4-inch-diameter plastic pipe produces tighter joints and is easier to work with. Not all would agree, however, on which direction to place the holes in the pipe when installing footing drains. The answer depends on the type of pipe. Flexible HDPE (high-density polyethylene) is slotted all the way around, and some rigid PVC has a pattern of holes around the entire circumference. With these types of drain tile, there is no “right” direction because there are openings on all sides. Plugged holes on the bottom are cleared by water entering through the sides and top. The most popular drain tile, however, is rigid PVC that has just two parallel rows of holes close together along its length. The classic approach is to lay this type of drain tile with the holes facing down, in the five-o’clock and seven-o’clock positions. This allows a rising water table to enter the pipe at its lowest point.

Filter fabric.

While hydrostatic pressure helps to flush silt from the pipe, all buried drain tile should be surrounded with coarse gravel or crushed stone, and wrapped with a filtering material. Without a filter, silt will contaminate the stone and eventually enter and plug the holes in the pipe (Figure 4).

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Figure 4. Without a filter to keep silt from contaminating the surrounding stone, drain tile can be rendered useless within just a few seasons (left). Pipe that is pre-wrapped or “socked” with filter material will prevent drain tile from becoming plugged (above). Various geotextiles are available in rolls, and pre-wrapped or “socked” pipe — pipe that is manufactured with a filter sleeve already in place — is also available.