French Drain Calculator

French Drain Design and Function

French drains collect and redirect groundwater using a gravel-filled trench with a perforated pipe at the bottom. Water flows through soil into the gravel, then into the pipe, which carries it away to a discharge point. This simple system solves wet basements, soggy yards, and foundation moisture problems by intercepting water before it reaches problem areas.

The gravel serves multiple functions beyond supporting the pipe. It creates a high-permeability zone that draws water from surrounding soil like a sponge draws up liquid. The voids between stones provide temporary storage during heavy rainfall, preventing saturation and allowing gradual drainage. This buffer capacity prevents overflow during storm events that would overwhelm a pipe alone.

Landscape fabric wrapping the gravel prevents soil infiltration that eventually clogs the system. Water passes through fabric easily, but soil particles don't. Over decades, even the best fabric slowly clogs, but proper installation extends French drain life from years to decades. The fabric also simplifies backfilling by containing the gravel so topsoil doesn't mix in and reduce effectiveness.

Installation Best Practices

Successful French drain installation starts with identifying the problem area and outlet location. Water must flow downhill continuously from the collection area to the discharge point. Walk the route with a hand level or laser to verify slope before digging. Any low spots create standing water that defeats the purpose and can freeze in winter to block the pipe.

Trench width typically ranges from 6-12 inches for residential work. Wider trenches collect more water but require more gravel. The pipe sits near the bottom with 2-3 inches of gravel underneath and 6-12 inches above. Some installers place pipe on one side of the trench rather than center to maximize the gravel collection area. Either method works if fabric wraps the entire gravel mass.

Perforated pipe should have holes facing down, counterintuitive as it seems. Water enters the gravel from below as the water table rises, so holes on the bottom collect most effectively. Holes facing up fill with sediment from above more quickly. Use 4-inch corrugated or PVC drain pipe with a filter sock if fabric alone isn't sufficient protection. Connect multiple pipe sections with couplers and seal joints to prevent soil infiltration that causes clogs.

Common French Drain Applications

Foundation drainage prevents wet basements by intercepting groundwater before it reaches walls. Install French drains along the exterior footing or in the interior perimeter trench before pouring the slab. Exterior drains work best but require excavation around the foundation. Interior drains are easier to retrofit but don't protect the foundation itself, only prevent interior water intrusion.

Yard drainage solves low areas that stay wet after rain. Install French drains across the slope above the wet area to intercept surface and near-surface water before it reaches the problem zone. Outlet to a lower area, storm drain, or dry well. For large wet areas, install drains in a herringbone pattern with branches feeding a central main drain line.

Retaining wall drainage prevents water buildup behind walls that causes failure. Install a French drain directly behind the wall base before backfilling. Some systems incorporate drain pipe into the wall construction itself, exiting through weep holes at regular intervals. Without drainage, hydrostatic pressure can exceed wall strength and cause bulging, cracking, or complete collapse during saturated soil conditions.