How do French drains and trench drains differ?
Many people – including contractors! – aren’t aware of the difference between trench and French drain. There is a significant difference between the two, and we’ll explore it with some general information below about the types of drains. But first, and generally speaking, French drains are used to remove ground water while trench drains are used to quickly remove surface water.
The History of French Drain
For starters, there’s nothing Françoise about French drains. They originated in Massachusetts (USA) during the 1800s by way of farmer Henry French, who later wrote a lengthy book on farm drainage. (French, Henry F. (1859). Farm drainage: the principles, processes, and effects of draining land with stones, wood, plows, and open ditches, and especially with tiles. New York: Orange Judd & Company.)
Originally, the drains were hand-dug trenches which were re-filled with a thick layer of gravel in the base and standard soil extending to the surface. The gravel base offered a sturdy, yet porous, conduit for water to be collected and drained from the surrounding water-drenched subsoil. Henry French later began lying stacked, though slightly spaced, roof tiles at the center of the gravel conduit to help facilitate water transportation (an early form of clay piping). These tiles, after a time, became perforated before evolving to clay pipe. As perforated piping became more widely used, the size of the gravel used to fill the drain had to be “engineered”. Coarse gravel was used surrounding the perforated clay tile which gave a high permeability to the drain. Finer sized gravel was used as a protective layer between the coarse gravel and the soil which helped filter fine dirt particulate from entering and eventually clogging the drainage system.
Modern French drains have evolved from their 19th century counterparts. While still hand dug in residential applications, they are more often excavated with machinery. Though some drains are still strictly gravel-filled trenches, the majority of French drains now use perforated (smooth walled or corrugated) pipe at the core of the gravel bed. The basic premise is still the same; provide a highly permeable, rugged structure to evacuate water from saturated subsoil.
Making a Conventional French Drain
A French drain is easy to make but a bit time consuming. First you’ll need to dig your trench (for a 3”- 4” diameter drainage pipe, a 9” wide trench works best). You will need to grade the trench during the dig; the best way to ensure a continuous slope is to measure as you go. Optimally, the gradient should be 1’ drop per 100 ft – or, a 1% slope.
After the trench is dug, line it with a filter cloth and pour in a layer of coarse gravel before setting the pipe. Half – to – 1” diameter rocks are typical, but it is best for them to be over 1”. They should have minimal particulates that would clog the piping. Some people put a special pipe sock (made from filtering material) over the pipe to keep out the finest of particulates.
To finish the French drain, cover the pipe with gravel, then fold and seal the filter cloth securely and backfill the trench with soil. Use coarse, sandy soil for better drainage.
New French Drain Products
The market has developed several innovative round French drain products that take away much of the work involved in installation. The structure of these products include universal components: a corrugated polyethylene pipe with perforations surrounded by polystyrene aggregate, all contained in fine mesh netting. The result is simple, a pre-packaged French drain that is much less time consuming than a conventional trench and can be cut off at any length.
NDS offers residential (EZDrain) and non-residential (EZflow) French drain systems.
While the design of both products is inherently the same, the EZflow is offered with 3”, 4” and 6” corrugated piping to accommodate varying groundwater saturation levels while the EZ-Drain only offers 4” piping for smaller, residential projects.
The deepest you can bury this type of French drain system is 10 feet, which poses nothing to worry about in most applications as it is more than enough for the system to sustain traffic. Actually, at 12 in. deep a system will withstand 16 thousand pound loads and single pass construction. Anything less risks damaging the product if vehicles drive over it, though the minimum depth to install a French drain system is 6 inches.
Another French drain system is Multi-Flow by Varicore.
Though Multi-Flow tries to separate itself from its humble roots, it is undeniably derived from French drain. It is a subsurface drainage product composed of corrugated piping wrapped in geotextile fabric (reportedly, it will not clog over time like French drains do). The product does have a few substantial differences, however, that at least make it superior to conventional French drains.
Multi-Flow touts itself as a vertical system, and it is. The system is only 1.25” wide but can come in 6” (seen below), 12” and 18” tall panels. The structure is a series of thin corrugated pipes stacked atop one another, creating more surface area than other products and therefore draining water more efficiently.
Multi-Flow can be installed in a 4” wide trench, which requires less excavation and backfill. The system recommends the use of coarse sand as backfill rather than native soil because the silt and clay particles would eventually clog the geotextile filter. During installation, fill the trench with clean, coarse sand.
Multi-Flow offers more flexibility than other systems, including couplers, tee connectors that join three or more segments of drain, end caps and side outlets. The product can bend in a 6” radius (enough for a 90 degree turn) but offers an optional 90 degree corner pipe, too.
Something to keep in mind is that, while Multi-Flow offers many basic PVC connections as part of its system, many of the connections will also be available at the local hardware store and can also be used on NDS’s EZ products.
What About Trench Drains?
The history of trench drains isn’t so clear cut. We’ve talked about this subject in one of our first blogs, “Consider History.” There, we made parallels between modern trench drain and such drainage systems as the aqueducts of Rome before its fall and the open sewers of Paris during the Middle Ages.
Over the course of the Roman Empire, there were 11 aqueducts that supplied various cities with drinking water. Over 200 miles of waterway used gravity to guide water into cisterns for distribution in cities like Rome. The system bored through mountains and was at times elevated on arched bridges so the aqueducts could follow gravity directly to the city. The above-ground troughs, which ran for only about 30 miles of the total system, are the most renowned pieces to the aqueducts and a model for modern trench drain.
Early in Paris’ history, drinking water was taken from the river Seine, and the waste water was thrown into the streets. Because the streets were unpaved, the city transformed into a swamp of foul-smelling mud. In the 1200s, the streets were cobbled and designed to have an open trench running down the center which would guide sewage back to the river Seine.
Disregarding the fact that the system drained sewage right back into the water supply, the open sewers helped spread the Black Death, which devastated the city. Thankfully, they were replaced by the famed vaulted sewers in Napoleon’s era, and shortly thereafter, by the sewer system in use today.
Today’s trench drains come in widths up to 2 ft. and depths up to 4 ft. They take after their predecessors but are built for construction purposes – not for sewers and drinking water. Typically, these drains consist of a preformed channel body and grating, which are installed in an excavated trench and then set in concrete.
Today’s trench drains can be cast from concrete, plastic, fiberglass, stainless steel and cast iron (though the last two are not as common). They use grates both to keep debris out and as a safety precaution, and many systems offer catch basins to collect sediment and small particles so that the drain never clogs.
The functional difference between the two is that French drains capture and evacuate ground water while trench drains are specifically designed to collect surface water before it can saturate the ground table. I suggest that you evaluate your situation, and if you want to catch the water before it can sog up your yard – or if your ground has high amounts of clay, which isn’t that permeable and can contaminate looser soils – you might just want to go with a trench drain.