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
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.
Plant Bed with Drain
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
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.
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
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).
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