Retaining walls aren't just a pretty landscape feature—they also have an important job to do, holding soil, creating flat spaces, and preventing erosion. Behind the facade, though, is an unsung hero to their longevity: good drainage. Without good drainage, though, water will collect behind the wall, creating hydrostatic pressure that, over time, can lead to structural compromise and failure.
Imagine having a beautiful garden with a retaining wall that's the best on the block, and you watch it lean or crack during a heavy rain. That is what is occurring backstage with improper drainage. Rainwater that is not properly diverted collects behind the wall and exerts pressure on the wall that the wall was never designed to endure. If you are a do-it-yourselfer or are about to hire a professional, understanding how to address this water properly can save you time, dollars, and frustrations down the road.
In this post, we will be explaining why drainage matters with retaining walls, what mistakes homeowners typically make, and the types of drainage you can use to keep your wall strong for years to come.
Overview
A retaining wall, by its very nature, is meant to hold back soil. Water, however, is an unwelcome guest—it finds a way into every nook. When there is water behind a wall, it creates pressure on the wall. This not only weakens the strength of the wall in the long term but can also contribute to soil erosion, reduce the stability of the backfill material, and in the most severe case, result in catastrophic failure.
The correct systems work by allowing the water to drain. They may consist of gravel, perforated pipes, or even weep holes allowing the water to drain prior to the stage where it may become potentially dangerous. Additionally, a correct system will also protect your property from other issues including the development of mould and mildew, or even infestation by insects in stagnant water.
Common Drainage Issues and Solutions
No matter how good the intentions, there are several pitfalls that can turn an otherwise sound retaining wall into a time bomb waiting to explode. Some common drainage mistakes, and their solutions, are:
1. Inadequate Backfill Material
Poor backfill or no gravel will trap the water. When there is no suitable free-draining material present, there is no route by which the water may drain.
Solution: Install clean angular gravel, best graded to provide maximum void space. Behind the wall, an extra 12 inches (30.48 cm) is usually required to allow unhindered passage of water.
2. Lack or Insufficient Weep Holes
Insufficient provision of weep holes—or weep holes that are too small—will lead to water stagnation. Weep holes are tiny openings through which water will flow.
Solution: Drill weep holes across the bottom part of the wall, spaced regularly. They should be large (typically 2–4 inches in diameter) and adequately separated so that the water will be able to drain out.
3. Sloppily Sloped Drain Piping
Pipes that are sloped incorrectly may become clogged or lack proper capacity to empty the water quickly.
Solution: Install pipes used to collect water (e.g., French drains) so they slope gradually, but continuously (about 1–2%), to have the water flow constantly away from the wall.
4. Clogging by Finer Particles
Small particles eventually find their way into the system, causing pipes to get clogged and reducing efficiency.
Solution: Install a geotextile fabric between the aggregate and the soil. The filter excludes fine particles but still permits the passage of water.
Retaining Wall Drainage Types
You have a sense now of what may go awry. Next, we will cover the myriad systems used to keep walls from buckling. There are pros to each system based on your design and local environment.
French drains
A French drain is a gravel trench and perforated pipe that collects and diverts the water from the wall. French drains are one of the most sought-after solutions used where there is high rainfall or a high water table, and work because they catch the water from the full wall.
Weep holes
Simple and effective, weep holes are minute holes built into the wall. They allow the water to freely drain out, keeping hydrostatic pressure behind the wall to a minimum. They work best when used together with a reliable gravel backfill.
Drain pipes
They are typically installed towards the bottom part of the wall to catch and redirect the water. Gravel-packed perforated pipes, when installed with the correct slope, are an excellent conduit for draining.
GeoComposite Drainage Panels
Increasingly popular, geocomposite panels are made from a filter fabric and a core. They are particularly valuable in modern designs where aesthetic appeal and spatial concerns are a problem, offering a thin yet effective drainage system.
Conclusion
Good drainage is the key to every successful retaining wall. Knowing the pitfalls and choosing the right drainage system—a French drain, well-placed weep holes, or the new geocomposite panels—will have you assured that your retaining wall is not only beautiful but will stand the test of time year after year.
As with everything, a little extra planning and money spent on quality materials up front will pay a world of trouble later. Good luck with your building, and here's to maintaining those walls solid and water where it should be—away from your lovely landscape!
