Do French & Trench Drains Need a Trap?

If you look under your kitchen or bathroom sink, you’ll find a U- or an S-shaped pipe underneath the drain opening. This is known as a drain trap. This is necessary to create an air trap to prevent sewer gasses from travelling through the plumbing and into your home.

However, a question arises; do outdoor plumbing systems also require a trap, especially French and Trench drains? Well, that depends.

French drains in general don’t require traps. When it comes to trench drains, it depends on where the opening is. If you’re opening it in the sewage line (which is against code, by the way), you will need a trap. If the opening is at a river stream or lake, you don’t need the trap. But that’s not all.

There are several other factors that you need to consider as well that you need to consider before embarking on your DIY venture, which we’ll discuss here in detail.

Understanding the French Drainage Systems

A French drain system is an outdoor drainage system that has you dig up a trench, lay the necessary plumbing, and cover it with rocks or pebbles. This type of drainage system is also known as the weeping tile because of its design. Whilst french drains can be found indoors (such as in basements), they are preferred outdoors.

A French drain vs. Trench drain
A French drain vs. Trench drain

The pipe within is perforated, which means that the flowing water (or the draining water) will seep into the trench and head into the pipes. This opening of your French trench is wherever you need water taken away from. The system can have multiple openings throughout your yard, which are typically referred to as drain fields.

As the water goes into the drain, it is assisted by gravity to reach the lowest point in the system, i.e., the exit point of the drain.

People assume that it is the French that came up with this type of trench, but that is not the case. In the mid-10th century, a farmer and judge from Concord, Massachusetts introduced the drainage system and named it after himself.

Installing a French drain can be very tedious, which makes many wonder whether the drainage system is of any use or not. 

French drains most often find use in prevent ground and surface water, such as storms, from damaging building foundations and were introduces as alternatives to open ditches or storm sewers. Since most French drains don’t open up in sewers, you don’t necessarily have to put in a trap.

You can, because after all that’s the spirit of DIY, you can do whatever you want, but it would just be an extra step that leads nowhere.

French drains present an exceptional opportunity for landscaping.

Understanding the Trench Drainage Systems

A perforated pipe covered in gravel to help drainage
A perforated pipe covered in gravel to help drainage

Trench drains are also mostly found outdoors with a few higher-end washrooms using them indoors rarely. These drains are also known as channel drains and are an above-ground drainage system; despite the fact that you have to dig a trench for them and set them into the ground.

To make a trench drainage system;

  • Start by digging a trench. This will have to be much narrower than for a French drain since you won’t have to fill it with rocks or pebbles.
  • Lay your plumbing. Make sure the flow of water is, again, assisted by gravity. Keep it slightly angled.
  • Connect the drain to a storm sewer or an environmentally responsible waterway
  • Cover the opening with a grate. It will give the impression of a long gutter in the ground.

Don’t confuse the trench drainage system with a gutter, though, even though they look the same. These trenches don’t (and shouldn’t) open into the sewage if you’re draining storm water. Since there is no risk of sewer gasses flowing out of the drain, you don’t need a trap for trench drains, either. 

Where French drains are difficult to install, they are relatively low maintenance. Trench drains are much easier to install, but require more maintenance since their opening is on the surface. You may even have to clean the pipes regularly, or at least clean out the grates.

Trench drains may be made of plastics or concrete, while the grate on top can be made of plastic or steel. The drains are permanent, and some people might not appreciate their outlook.

Installing A Trap on A French Drain System – If You Really Must

A French drain can open anywhere downhill; from a dry well or a rain garden all the way to a river or a lake. The goal is to ensure that the wastewater sewage system doesn’t end up being overloaded due to the excess water from storms. By draining it into the sewer, you’d only end up wasting clean water. 

If you believe that at any point your French drain system is coming in contact with sewage or if you have reason to believe that the outlet is otherwise contaminated and releasing gases, you may have install a trap.

For that, a U-trap is the best way. Installing a p-trap in a French drain system will be difficult and may be expensive, though.

  • Dig a trench wider than your pipes.
  • Lay the plumbing and install your trap right underneath the field, i.e., where all the water collects before flowing.
  • Lay the rock and pebbles, and you’re done.

The problem with installing a trap on French drain systems is that since water is seeping through the pebbles, it may not have enough pressure to flow through the trap, thus rendering the drainage system ineffective. 

Installing A Trap on a Trench Drain System – If You Really Must

We can’t stress this enough; keep your storm drains away from sewage lines. But circumstances may not always be on your side and you may need to install a trap on your trench system (even if you’re just being cautious).

To do so;

  • Dig your trench on a slight angle and lay the pipe (plastic or concrete) throughout the gutter.
  • Where your gutter is ending, dig a little deeper to make room for the trap. You need to make sure you are installing the trap near the end, so that the water has enough pressure to go through. Furthermore, this way you’ll only have to install one trap.
  • P-traps work best here. Install it, throw some dirt on top of it, compact it and you’re done!

How Much Can a French Drain Cost?

It goes without saying that a French and Trench drain pipe’s cost will depend on how extensive the system is. A French drain system installation is much more labor-intensive than a trench drain, but since you’re doing it yourself, you won’t have to worry about labor costs. The only thing you need to worry about is the material.

For French drains, small systems can cost you around $300-$600 without labor, while extensive projects can go as high as $6,000 – especially on larger fields which have drainage issues.

A P-trap, on average, costs $200-$365, depending upon the quality, size, and store where you buy it from. The drain itself costs $12-$32 per foot, so measure the length of your drain and multiply it with the cost to get a rough estimate. Rocks and pebbles costs depend on the type you buy. For example, river rocks to cover a 20 feet driveway (bad idea, by the way) will cost you around $600.

How Much Can a Trench Drain Cost?

Trench drains are a relatively cheaper option for DIY-ers since the only cost involved is that of the plumbing, a P-trap, and grate. You don’t have to buy rocks for this.

Again, the P-trap will cost you around $200 to $365, while the pipe you lay can be plastic or concrete. Plastic pipes will cost you somewhere between $10 to $20 per linear foot, depending on the size and quality you choose. Concrete will be cheaper but will require more efforts on your part.

Then there is the price of grates. Again, depending on the material and size, their cost will vary. Plastic grates cost around $5 to $50 per foot, while steel grates can get slightly $15 to $25 per foot. Depending on how long your drain is, the cost will rise. When installing a trench drain around your driveway, the best size is 5-6 inches. It’s enough to drain water sufficiently and be cost-effective at the same time.

This cost breakdown might seem like a lot, but when buying, just remember that by doing it yourself, you are saving yourself thousands of dollars; enough to apply the same in your next big DIY project. Good luck!

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