French Drain Comparisons

Offering reliable landscaping services to homeowners in British Columbia since 2013

Picture of Natasha Maerz

Natasha Maerz

Owner/Office Manager

Backfilling With Soil (Clay) vs. Round Rock

If we haven’t met before allow us to introduce ourselves. We are Back 40 Landscaping and our expertise is in retaining walls, hardscaping and drainage. We also like to bring education to those looking for retaining wall and drainage solutions.

French drains are a great solution for a water-logged backyard and drainage problems. Many different materials and methods are used when installing a French drain system. With numerous different combinations of methods, and materials that can be installed, it can be tough to know which drainage system you should have installed.

So, what are french drains?

 French drains are typically built by digging a trench, putting filter fabric and drainage pipe in the trench, and backfilling (filling up the trench) with either clay soil or round rock. While there are many other parts of a French Drain install that are integral to how the water gets out of your system, like type of pipe used, we firmly believe that one of the most important parts to making your French Drain work its best is what is used to backfill the trench. Here’s why:

Backfilling the French Drain system, and how it affects the void space in your trench

French drain installation:

French Drains can be backfilled by either using the excavated clay soil from your trench, or using 1 ½’ round rock.

Using Clay soil:

You might be wondering what is so wrong with using clay soil to backfill the French drain trench?

Let’s think about why you need a French Drain installed in the first place. It is probably because you have drainage issues,  your yard is constantly wet, and you want to be able to enjoy it without having it be a soggy mess. The reason your yard is always wet is because you have clay soil in it, and clay soil does not allow water to go through it. If your yard was sandy, you wouldn’t be having a water problem, because water can percolate through sand no problem, and not allow surface water to pool.

If you use the same material that you already have in your yard as backfill, you will have the same drainage issues shortly after your system is installed. Yes, a  drainage pipe will be put in the ground which should carry your water problems away; however, water has to be able to get into the pipe that was placed. What ends up happening is the system works initially because the clay soil is uncompacted, and allows water to pass through it into the drainage pipe. Shortly afterwards though (2-24 months) this clay soil compacts just like the rest of your yard. Once it compacts again water will sit on the surface of your lawn ( surface water), and not penetrate the ground to get into the drainage pipe.

Using clay soil as backfill

Clay soil advantages:
  • No soil needs to be hauled out, saving on disposal costs.
  • No rock needs to be hauled in, saving on material costs.
  • Because the excavated soil is being used as backfill, no machines are needed (trench can be dug by hand, backfilled by hand).
  • This method is quick to install, as there are less steps to install the system.
Clay soil disadvantages:
  • When there is no void space in the trench the surface of the yard dries slowly, and the subsurface never dries out completely. As we said before, this is because water cannot percolate through the clay soil.
  • Using soil gives your system a short lifespan, as clay soil will compact and not be able to provide the drainage you are looking for.
  • System is Expensive for how long it lasts (2-24 months).
  • Grass roots will be waterlogged due to standing surface water. This will cause a yellowish tinge to your lawn as its oxygen supply is cut off.

Using 1 ½’ round rock:

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.

Using 1 ½’ round rock as backfill:

Round rock advantages:
  • Your lawn and ground water will dry out quickly because the water can freely move into the drainage pipe.
  • The void space in the trench acts as an underground reservoir, and collect water after heavy rainfall if the outlet drain is unable to keep up with it.
  • Because there are no fine materials in the system, it will not plug
Round rock disadvantages:
  • Dump truck loads of soil have to be hauled out of your yard and disposed of.
  • The same amount of soil that is hauled out, needs to be replaced with rock. Adding to the cost of materials and hauling costs.
  • Heavy equipment will need to be used to bring the rock to your yard and to backfill the trench with the rock.
  • If your yard has limited access for machines, wheelbarrows may need to be used. Increased labor may be needed to bring the excavated soil out, and the rock in.

Why are some systems built with no void space?

Using round rock to backfill your French Drain system is more costly than using clay soil because of having to buy the rock, needing more labor and more equipment. Some customers look for a quick fix that will work for a little while because others systems that include using round rock are not within their current budget.

What to consider?


Backfilling with clay soil is the cheapest option up front as there are no disposal costs.  However, if you take into consideration that in a couple of years your system will need to replaced, it can be more expensive in the long run.


Your property’s lifecycle: If you plan on only living in your home for a short period of time a system that is cheaper may be the best option for you.

 Lifespan of your system: If you want a system that will last centuries, you want to go with a system that uses 1 ½’ round rock. If you are okay with replacing the system in a couple of months-years a system with clay soil as a backfill will do its job.

Now that you have reviewed the advantages and disadvantages of void space vs. no void space in a French drain install, you hopefully have come to a conclusion on what will work best for your budget, and needs.


Interested in how much a French Drain costs?