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 19^th 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 (EZ-Drain) 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 Difference

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.

Visit www.TrenchDrain.biz for more trench drain information.

I think it’s time to tell you about a product that is a bit of a secret in the marketplace.  It is one that I’ve been selling for years but have not written about until now.  It is simple, economical, sturdy and versatile.  I find myself recommending it more now that I have installed it a few times and have experienced the ease and speed that it can be assembled.  I’m speaking of EconoDrain, a patented concrete trench drain former system manufactured by MultiDrain Systems, of Barium Springs, North Carolina.

Two types of Poured-in-Place Trench Drains

First a little background on poured-in-place trench drain systems needs to be covered.  Poured-in-place concrete trench drains come in two flavors:

1)      Frame and Grate Systems (or Traditional Trench Drain systems) – which requires that the contractor build the trench form, usually from wood.

2)      Trench Drain Former System – In addition to a frame and grate, a former system utilizes a disposable, pre-sloped form (or mold) that is used to make the drain channel.

The end result of both systems can be similar.   Both trench drain types are used to build a concrete channel with an embedded metal frame that supports an engineered grating.  There are some differences, however, which are discussed below.

Traditional Frame and Grate System – I think of traditional poured-in-place trench drain systems, or frame and grate systems, as one that is purchased from a foundry.  What you are actually buying is some quantity of cast iron grates and some cast metal rails.  The rails are used for making a frame to cradle the grates.  Traditionally, trench drain grates made by a foundry in the US have been 2 foot in length.  The width of the grates will vary based on the flow requirements of the channel.  However, a historically popular grate width seen in the U.S. has been 12”.  The rails used to hold the grates are often 4 foot in length.

So, for example, if you were going to install a 16 foot long trench drain using this system, you’d first purchase 8 grates and 8 rails from an iron foundry.

Two common foundries that manufacture this product are Neenah Foundry and East Jordan Iron Works.  These companies make large gray iron and ductile iron castings used in roadways and sewers.  Other plumbing fixture foundries, such as Josam, Watts and Zurn, manufacture frame and grate systems which are geared more for interior uses, such as maintenance facilities and warehouses.  Though there are differences in the design of each of these products, there is a great deal of application overlap.  And, in the end, each product requires a channel form to be constructed out of wood.

The act of building a wooden channel form can be a daunting task to the novice.  The wooden form is constructed and suspended in an excavated trench.  The metal rails of the drain are attached in some manner to the wooden form.  Because the rails are designed with an anchoring system, once concrete is poured around the form the rails become imbedded in concrete.  The wooden form which forms the trench is removed once the concrete is dry.  If a sloped trench bottom is wanted, the form can be constructed to produce the effect, or a mortar layer can be applied afterwards to slope the trench bottom.  It all sounds difficult.  But, it can be done in time with a skilled tradesman.

Trench Former System – A trench former system is similar to the traditional frame and grate product in-so-far that you are supplied a frame and grate.  However, with this type of system, you are also given a disposable, pre-sloped form.  The forms are specifically designed to attach to the frame, making the whole “wooden box construction episode” obsolete.  The forms and frames are assembled quickly and suspended in the excavated area by use of rebar.  As the frame and form are pre-engineered to a specific width and depth, less design work needs to be done in the field and installations are significantly quicker.  Forms are pre-sloped and can have rounded bottoms to give the resulting concrete channel improved flow characteristics.

The three most popular Trench Forming Systems on the market today are:

a)      EconoDrain (MultiDrain Systems) which uses an EPS mold to make round or flat bottom, pre-sloped channels that range in width from 4 inches to 24 inches.  The frame design allows the trench installation to be done with one concrete pouring event.

b)      Trench Former (ABT, Inc.) which also uses an EPS mold, offers channels that range in width between 6 and 24 inches.  Like EconoDrain, they have a patented framing system.  However, for proper installation of the Trench Former System, two concrete pours are required.

c)      FastForm (ACO) which uses a cardboard molding material to form the 12” and 24” wide channels.   Form assembly is required.

EconoDrain – The Most Versatile Trench Drain Former Product

Of the products discussed above, no one has a product that is as versatile, easy to install or cost effective as EconoDrain.  EconoDrain has a patented frame and channel forming system that eliminates the time and materials required in building a traditional formed-in-place trench drain.  The frames, which come in 8 foot lengths, have specially designed anchor stand shoulders that accepts #4 or #5 rebar.  This rebar is the supporting member that suspends the frame and EPS foam in the trench where the concrete channel is to be formed.

Attached to the underside of the frame is a lightweight, pre-sloped form.  Where traditional forms are made of wood, the Econodrain form is made of expanded polystyrene (EPS).  This form is designed with a locking collar which holds the form in the frame during installation.  The center wedge, designed with “ears”, is easily removed after the pouring and setting of the concrete, allowing the remainder of the form to collapse and be removed from the channel.  Thereafter, grates can be placed into the frame.  Locking devices are also available to help secure the grates in place.

The EconoDrain former system can be made with painted steel, galvanized steel, stainless steel or aluminum frames.  The most common grating options are ductile iron slotted grates and bar grating.  However, with the help of a company called Trench Drain Systems (www.trenchdrain.biz), custom trench drain can be designed to meet your needs.  Custom stainless steel grates, decorative cast iron grates and custom polymer concrete grating (Jonite Grates) have been used with Econodrain.  Recently, MultiDrain and Trench Drain Systems have developed a pre-sloped, radius channel drain using custom forms made by EconoDrain.  These channels are used to make curved drains that can be used in running tracks, pools, driveways and fountains.  They utilize custom cast iron radius grating that cover curvatures that range from 3 feet to 55 feet in radius.  No other product or manufacturer can offer this degree of versatility in a poured-in-place trench drain system that can be achieved with Econodrain.

EconoDrain Installation Example

I recently installed a concrete former system trench drain in a home owner’s driveway.  Usually, in residential driveway trench drains, I recommend a 4 inch wide polymer concrete trench system (such as Polycast 600).  In this situation, however, a “river” of water was flowing down the driveway with a force that was moving yard structures.  A wider, more industrial trench system was required to divert the water.  We decided on using an 8 inch wide, poured-in-place concrete trench drain with a 10 inch wide grate containing a high percentage of open space.

We decided to place the drain at the bottom of the drive along the area we have marked with the measuring tape (see right).  We decided to direct the water to a cobble stone lined creek to the left, just behind the flowers.  The asphalt driveway was cut using a walk behind concrete saw.  The asphalt topping and soil was removed to a depth that allowed us to have 6 inches of concrete all around our form.  The drain frame and form was set in the excavation using #4 rebar.  I never took photos of these steps because I was busy getting the drain set.  The concrete truck was scheduled to be there soon!!

The photo to the right shows the drain just after we had placed the concrete.  We made a box to form a square end to the outlet end of the drain.  At this location, we connected a piece of corrugated pipe which ran below the surface of the cobble stone creek.  We let the concrete set overnight and removed the form the next day.  To remove the form, we had to first cut the metal support bars that hold the metal rails to their dimension.  For this we used a small angle grinder.  The EPS form actually seats around these bars during installation helping to hold the form in the frame.

Once the support bars were freed, a center wedge section of the form was removed, allowing us to pinch and collapse the remainder of the mold away from the new concrete channel wall.  The mold separated easily from the concrete because we had used a mold release agent on the form prior to pouring concrete.  After the EPS forms were removed, we cleaned the excess concrete from the metal rails with a scraping device and then swept the remaining dirt out of the drain.  The only thing left was to install the grates and bolt them into place (See below).

For more information on the EconoDrain trench forming system and how a system can be designed for your application, contain Trench Drain Systems (TDS) by calling 610-638-1221 or by emailing your request to sales@trenchdrain.biz.