Problems with french drain trenches

Offering reliable landscaping services to homeowners in British Columbia since 2013

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Natasha Maerz

Owner/Office Manager

When built correctly and with quality materials, French Drains are a fantastic way to deal with a homeowners water issues.

The first step to installing a French Drain ( after figuring out where the water will end up and how much pipe you will need) is to dig the trench. A sloped trench is dug with an excavator 16” deep and 16” wide.

The trench is sloped so once the system is installed it can help the pipe do its job by bringing water to the discharge station.

What happens if the French Drain System is not Deep, Or Wide Enough

When digging a trench for a French drain pipe system the deeper and wider that the trench is dug, the more surface area there will be within the trench.

This is important, as the surface area of the trench dries out the subsurface of the yard.

Other factors that help the subsurface stay dry are the rock that is used in the system. Big round rock allows a bigger void space in the trench, and this air movement creates an underground drying effect.

The big void space made by the rock also acts as an underground reservoir for the water entering the trench.

This is particularly helpful when there is heavy rainfall or large amounts of snow melting at once.

Backfilling The Drainage Pipe with Dirt

When backfilling a French drain, it may seem like a good option to use the dirt that was dug up so that you do not need to worry about where to dispose of it.

The problem with using the excavated soil is that native clay does not allow water to go through it. 

When you backfill with dirt, no fabric is placed in the trench, and when there is no fabric in the bottom of the trench and encapsulating the drain pipe system, soil will end up in the pipe.

This means that even if water eventually makes its way through the clay soil and into the system, the likelihood of the drain pipe clogging is high, and the water will still have nowhere to go.

Instead your trench should be backfilled with round rock.

Why use  Round Rock to Backfill A French Drain Trench

When backfilling with 1 ½” round rock, your system will have an ample amount of void space due to the size of the rock that is placed in the trench.

Void space in a trench is important, because the greater amount of void space that is in the trench, the quicker your yard will dry.

Void space allows both the surface and subsurface of your yard to dry out quickly by allowing air movement in the trench, creating an underground drying effect. The water that is on top of your yard is able to go through the round rock, and into the pipe.

Once that water gets into the pipe, the pipe can take it to its discharge station, and your yard will be dry.

Your French drain trench is what sets your entire French drain system up, so it is imperative to do this step properly ( properly excavation width and depth, and proper backfill) so that the rest of your French drain installation is successful.