The French drain is a simple system of diverting rainwater or springwater away from homes to prevent water damage. It consists of a trench angled away from the structure toward a low point where water can safely collect. In a French drain system, the builder employs a perforated pipe, sand, landscape cloth and varying sizes of stone or gravel to filter water to the street or sewer systems.
French Drain Construction
Depending on the length and complexity of the system, homeowners can construct the French drain themselves or hire a contractor that specialises in property drainage problems. To design the drain, the trench must incline downward no less than 1 foot for every 200 feet, according to the website Renovation Headquarters. Dig the trench 6 feet wide and 2 feet deep along this slope, removing rocks, debris and tree roots from the trench walls. Line the trench with stone or gravel to allow the water to filter through the trench easily. Perforated plastic pipe, 4 to 6 inches in diameter, is laid on top of the stone, and more gravel is laid on top to cover it. Topsoil is sometimes laid on top of the rock and planted with grass seed, but this is for appearance and not necessary for the efficient operation of the drain.
Many different sizes of gravel or crushed stone are used for French drain systems, depending on the consistency of the soil. Sometimes, a variety of sizes is used to increase the drainage ability in different types of soils. The most common types of stone aggregate used in French drains are the 3/8-inch pea gravel, 7/8-inch drain rock and larger sizes of river rock. According to the website MD Vaden, the finer the stone, the less likely the walls of the trench will collapse.
Pea gravel is a fine size of crushed rock, 3/8 to ¼ inch in diameter, often considered the standard for construction of simple French drain systems. It is about the size of a pea and made out of small, smooth stone, generally basalt rock. This size is small enough to allow water to filter through the stone without soil particles clogging the system. For French drains, the washed variety prevents dust and impurities from impeding water flow.
Larger sizes of river rock are often used to improve the appearance of the French drain on the surface of the property. This can work effectively for drainage, but you may need to add sand to prevent clay soils from compacting between the crevices of the larger river rock. River rock comes in a wide variety of sizes, but use no larger than 1 1/2 to 3 inches for lining French drains to allow for adequate flow capability.