A French drain is a common means of diverting water from one location on a property to another in order to divert excess rainwater or to dry up soil that is excessively waterlogged, such as land that is located near wetlands. Inspecting French drains is not within the scope of InterNACHI’s Residential Standards of Practice because they are typically located underground and not visible. However, inspectors may want to know more about this method of preventing flooding on a property because French drains may fail and subsequently cause visible signs of flooding at the exterior of a home’s property and potential moisture intrusion problems that affect the home itself.
What is a French Drain?
An exterior French drain is a trench 6 inches to a few feet in diameter that is dug along a declined slope on a property. It is then covered over with gravel to prevent excess mud and debris from entering the trench, and to prevent soil erosion on the inside. Perforated piping, called weeping tile, may be laid underneath the gravel to accelerate the movement of water through the trench. Gutter downspouts from a home’s roof may empty directly into a French drain system to alleviate flooding due to excess rainfall.
Advantages of French Drains
- French drains are an extremely effective way to channel water away from a home to prevent water damage to the foundation and the interior of below-grade rooms. For this purpose, French drains are generally installed underground around the perimeter of a home’s foundation.
- Lawns that would otherwise be too moist can be transformed and rendered useful for gardens or children’s play areas.
- French drains are relatively inexpensive to install, particularly if they do not require excavation of existing exterior structures, such as walkways and decks.
- Installing a French drain does not necessarily require special tools.
- Installing a French drain may be dangerous if there are water, power or communications lines buried in the vicinity. Many states, such as Massachusetts and Maine, require that municipal authorities be notified prior to digging.
- Digging may disturb natural water flows and can cause affected water to pool.
- Installing a French drain may require that existing structures, such as decks and walkways, be removed, especially when using a backhoe to dig the trench.
- Weeping tile is prone to clogging over time, and may require expensive excavation to correct.
- French drains that become clogged may overflow without warning and cause damage, such as flooding in yards and basements.
- Existing structures may need to be removed if a French drain must be excavated.
- Exterior French drains that are installed without a sump pump rely on gravity to direct water flow. An adequate slope for gravity drainage may be difficult to establish for properties that are located at the bottom of a steep slope.