Drains Drain, house exterior

Published on September 11th, 2016 | by Henri

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Installing a French drain

Q.   How do you correctly install a French drain?

A.  The proper installation of a French drain depends upon where it is to be installed. The most common use is to capture and reroute water from higher ground behind a building to prevent it from reaching the building and causing leakage.

A “True” French Drain

A ditch is dug at the base of the hill, a few feet from the building’s foundation and as deep as necessary to capture the subsurface water (common in areas with rock ledges). The land contour is adjusted to end up as a swale to capture the surface water (rain and melting snow) running down the hill, and to lead it around the house. The uphill side of the trench is reshaped and made steeper, and the area between the house and the trench is sloped away from the foundation to the trench.

The sides of the trench should be lined with geotextile fabric to keep soil particles from clogging it.

A couple of inches of egg-sized stones are spread at the bottom of the trench and a perforated pipe laid over the stones.

In a true French drain, the trench is completely filled with the same type of stones and left open to capture surface water. If there are young children around, it is advisable to top the stones with much heavier stones (referred to as “surge” in some parts of the country) in order to keep the children from playing with the smaller stones, which could end up breaking windows. (It has happened.)

Variation: a Curtain Drain

By contrast, a curtain drain is similar to a French drain except that the trench is backfilled with stones to about a foot or so below the final grade, geotextile fabric is laid over the stones and  two to three inches of coarse sand are placed over the fabric to keep its pores from getting clogged with soil particles.

The trench is then backfilled with native soil, with a concave surface to form a swale, and grass is planted.

If your question is referring to an interior drainage system, the term French drain is somewhat  of a misnomer and often misapplied.

Although it is always best to stop basement and crawl space leakage from the outside, there are cases where this may be impossible or too difficult to do. Such cases are when the leakage is caused by a rising water table or an expanding spring during periods of extensive rainfall and melting snow, and when there are expensive plantings, which cannot, or should not, be disturbed close to the foundation.

Interior Drainage Systems

Sump pump

Sump pump

In my 60 years experience in the residential construction industry, dealing with hundreds of basement and crawl space leakages, I have found it necessary to install interior drainage only a few times.

In such cases, 12 to 15 inches of the concrete floor of a basement is cut and removed from around its perimeter. Any stone substrate and soil are dug out to the base of the footings.

A deeper hole is dug where a sump is best situated to collect water and evacuate it to the outside where it can flow away from the building. The top of the sump should be slightly below the top of the existing concrete slab.

A couple inches of stones are laid on the bottom of the trench, a perforated pipe is laid over the stones and the trench is backfilled with stones to within three to four inches of the top of the slab.

The pipe is connected to the sump, and stones are placed around the sump.

It is a good practice to place 1/2-inch wide strips of plywood (wrapped in plastic to make later removal easy) against the foundation walls.

New concrete is poured over the stones and finished with a very slight slant toward the plywood strips and the sump. The plywood strips are removed, leaving an open space in case of leakage through the walls.

A good-quality submersible sump pump is installed in the sump, connected to an electrical outlet and piped to the outside, generally through a band joist. The horizontal part of the pipe should be slightly slanted to drain completely in order to avoid freezing. The pipe should discharge onto a splash block and the grade should slope.

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