French Drains: When You Need Them
If you have a soggy yard or a wet basement, the solution may be an exterior or interior French drain, a channel that collects water and diverts it safely away.
Water always flows downhill, and by the easiest route possible. That’s the basic concept behind a French drain, a slightly sloped trench filled with round gravel and perforated pipe that’s used to divert underground water away from your house. The name doesn’t come from the country. It’s from Henry French, a judge and farmer in Concord, Massachusetts, who promoted the idea in an 1859 book about farm drainage. French made his drains with clay tiles, but installers today usually use 4-inch-diameter plastic pipes. If you live on a slope and have a persistently wet basement or soggy lawn, a French drain could be the solution.
How a French drain works
French drains work by providing an easy channel for water to flow. Surface and subsurface water runs through the spaces between the round gravel and into the perforated pipe at the bottom of the trench. (Installers place the pipe with the holes facing down so the perforations don’t get clogged.) Water then travels freely through the pipe, which empties a safe distance from the house. The trench bottom should be sloped about one inch for every 10 feet in the direction you want water to flow. Depending on the situation, the water can be diverted to a low-lying area of the property, a drainage ditch, a dry well, or the street.
Use a shallow curtain drain to divert surface water
A shallow French drain, called a curtain drain, is a useful when you have a problem with surface water—a lawn that’s always soggy, for instance, or a driveway that keeps washing out because of water rushing across it. A French drain extending horizontally across your property, uphill of the area you want to dry out, skirts water to either side.
This type of drain doesn’t have to be very deep. To divert water around a house, a common size is 2 feet deep and 1.5 feet across. If the drain passes through an area with trees or shrubs, consider switching to solid pipe there, to reduce the risk of roots growing into the piping and clogging it.
Cost: $10 to $16 per linear foot.
Add a deep French drain to keep water out of the basement
If your yard is flat, or if a curtain drain across the top of the lot doesn’t keep water out of the basement, you may need a deep French drain. This type, often called a footing drain, runs around the perimeter of the house at the footing level. It’s easy to install during house construction, but much more difficult and expensive to add later. You might want to consider this option if you have a finished basement that you don’t want to disturb, but weigh that against the landscaping, decks, and walkways that will have to be ripped into to access the foundation.
Since French drains depend on gravity, on a flat property you may need to pipe the collected water to a basin in the basement, where a sump pump can lift it and send it to the storm drain system.
Cost: $12,000 for a 1,500-square-foot basement 6 feet deep; $4,000 for a crawl space with footings 2 feet deep.
Add an interior French drain to remove water where it enters
If you can’t keep water out of your basement, you can build an interior French drain to intercept the water where it comes in. Crews cut a channel around the perimeter of the basement floor, chip out the concrete, and install perforated pipe all the way around. Solid pipe then carries the water to a collection tank sunk into the floor, and a sump pump sends it out to the yard or a storm drain. The channel is patched with a thin layer of concrete, except for a small gap at the edge to catch any water that dribbles down the wall.
Cost: About $3,000
Build a French drain into a retaining wall
If you’re building a retaining wall on a hillside, incorporate a French drain behind the first course of stones or blocks. Otherwise water moving down the hill will build up behind the wall and undermine or even tip it. The pipe should rest on the same compacted gravel base or concrete footing that supports the wall. To protect the drain from clogging with silt, drape landscape cloth across the base or footing and up the slope before you add the pipe and drain gravel. As you near the top of the wall, fold the cloth over the top of the gravel, and top with several inches of soil.
Cost: The added cost to do this while building is very little—just the price of drain gravel ($25 a cubic yard) and pipe (50 cents to $1 a foot, depending on type).
Jeanne Huber is the author of 10 books about home improvement and writes a weekly column about home care for The Washington Post. She solved her first drainage mystery when her family’s frequent sneezing attacks led her to discover mildew coating the underside of their house’s roof. Turns out basement flooding (see Sign #6) was to blame.
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