French drains are primarily used to prevent ground and surface water from penetrating or damaging building foundations.

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These are especially effective where homes are built on mountain sides and underground water travels toward and up against the house or trapped in gardens become waterlogged.

French drains are also used behind retaining walls to relieve ground water pressure or on the outer underground walls of basements and cellars.

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A French or Agricultural drain has a 110mm slotted hollow pipe laid along the bottom of a trench to quickly vent water out of the area. This pipe is encased with washed drainage stone wrapped in biddum cloth to facilitate the easy drainage of water and prevent the clogging of the slots in the pipe. The encased pipe at the bottom of the trench is then backfilled with clean drainage soil so that water easily seeps down through the upper levels of gravel or rock.

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The collected water is then directed by means of a system of pipes to the natural flow of water out of the area, if the landscape allows for this.

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In areas where the collected water cannot naturally be allowed to drain out of the area, then it is collected into a collection sump into which all the water drains. A submersible electric pump is installed in the sump.

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The collected water is then pumped out of the area or recycled into a grey water system.