Despite their name, French drains aren’t actually from France. In fact, they’re named for a Massachusetts judge and farmer, Henry French, who first wrote about them in 1859. The idea was simple: Get water away from the places it tends to collect by carving out an easy path for it. However, unlike a ditch, this drainage path is totally underground.
What is a French Drain?
A French drain is a trench that houses a perforated pipe buried beneath layers of gravel. When water pools on the ground above, gravity pulls it through the gravel and into the pipe, where it drains out the end.
How a French Drain Works
Water will always follow the path of least resistance. While a steep hill is the ideal solution for draining water, flatter areas pose drainage problems. French drains circumvent these issues by creating a path that transfers the water to a spot where it no longer poses a problem, such as a storm drain or rainwater collection system. The French drain works by encouraging groundwater to seep into the drain with layers of gravel. Water travels through gravel much more efficiently than soil, so once water soaks into the soil, it quickly enters the pipe. A permeable fabric prevents the gravel or other foreign objects from entering the pipe, leaving it free of debris and ensuring maximum drainage potential.
Requirements for a French Drain
The most crucial requirement for a French drain is the slope. If you’re unable to install the French drain with at least a 1% grade drop (a 1-inch drop over 10 feet), the drain will not function as intended. Installing a French drain without enough slope will result in standing water in the drain and improper drainage. Additionally, a French drain must be able to take the water to a location where it no longer poses a threat. This can be a storm drain, retention pond, or simply a safe spot away from your home.
When to Consider Installing a French Drain
Think a French drain might be the right solution for your water woes? Here are a few common problems that can be solved with a French drain.
1. Foundation Damage Due to Poor Drainage
If your home experiences flooding or foundational damage due to groundwater, rain, or snowmelt, a French drain may take care of the problem for good. A properly installed exterior French drain will completely circle your home’s foundation, exiting at the lowest point to drain the water away from the home. To do this, the entire drain must be installed at an angle that promotes drainage. This type of French drain is often called a weeping tile system and is intended to relieve hydrostatic pressure on the walls of the foundation. If water has become an interior issue, it might be more beneficial to install an interior weeping tile. This entails installing a moisture barrier called a drainage board along the outer walls to deflect water into a French drain that circles the entire interior circumference of the foundation. The drain then redirects the water out of the home via an exterior drain or sump pump.
2. Hydrostatic Pressure on Retaining Walls
Like foundation walls, retaining walls can be compromised by hydrostatic pressure. A French drain installed on the upper side of a retaining wall and routed to drain away from the wall will pull water out of the ground and remove hydrostatic pressure.
3. Flooding or Excess Surface Water
Does your lawn or garden hold an excessive amount of surface water after wet weather? Installing a French drain might be what it takes to reclaim your lawn. While French drains are designed to remove groundwater along the pipe via perforations, you can install drains in the ground anywhere water tends to collect. This way, the drains immediately remove any water before it has a chance to enter the ground, as well as any water that’s already in the ground.
How Much Does a French Drain Cost?
French drain installations are very labor intensive, which can add up to high costs if you choose not to do the labor yourself. Even if you do opt to do the job yourself, keep in mind that the materials are not cheap. A 10-foot section of perforated corrugated pipe with aggregate and mesh fabric will cost around $80. If you choose to rent digging equipment to save your back, you can expect to pay around $300/day for a 1-ton mini excavator. Many homeowners opt to hire professionals to ensure the installed French drain operates as intended. Additionally, a professional will have the tools and know-how required to tie the drain into a municipal storm drain, should it be necessary. For professional exterior French drain installation, you should plan on paying around $5,000, though this number will vary depending on your drainage requirements. Interior weeping tile installations are much pricier, falling between $10,000 and $20,000.
