# Installing French Drains



## Daniel Holzman (Mar 10, 2009)

A French drain is simply a stone filled trench, named after a Mr. French who developed the technique for draining land back in the 1800's. Personally, I use the term perimeter drain as it is much more descriptive. The purpose of a perimeter drain is to collect groundwater and cause it to flow either away from the property by gravity, if that is possible, or into a sump pit where it is pumped away (your case).

An exterior perimeter drain is generally preferable to an interior drain. Interior basement drains are commonly installed because the house has a drainage problem, and it is too costly or difficult to install an exterior drain. However, you may be able to install an exterior drain yourself, with possibly a Bobcat with a small excavator on it.

The typical design is to dig down to approximately one foot below the footing, install a 4 inch diameter perforated PVC pipe in a gravel bed. The pipe is generally wrapped in filter cloth to minimize the intrusion of silt. The 4 inch pipe is placed around the entire house, and connects to the sump pits via a solid 4 inch PVC pipe at convenient locations. The perforated pipe is typically laid flat, with the holes facing down. The perforated pipe is backfilled to the surface with 3/4 inch stone or similar granular material. This creates a free draining zone around your house that collects surface water and allows it to flow rapidly downward to the pipes, where it is collected for draining into the sump pit.

Typically 4 inch pipe is adequate to collect groundwater, in rare cases you need a bigger pipe. The idea is that the groundwater under the house is kept approximately 1 foot below the basement level, so of course the sump pit has to be deeper than 1 foot below basement level, and the float must be set to turn on when the water reaches approximately a foot below the basement slab. You can cut it closer, say 6 inches below the slab level, which will save a little money on pumping costs.

As I say, you may be able to install such a system yourself. The main costs are the excavation, pipe and filter cloth, and granular backfill. You already have the sump pits installed, and the pumps, so there is no additional cost there.

The two systems you mentioned I am not familiar with, so I can't comment on whether you could put them in. An interior installation would require cutting through the concrete and installing essentially the same system inside the basement, which may also be doable, the difference being that you would need to either break the concrete with a jackhammer or sawcut it.


----------

