Retaining walls add value to your home and property only if properly built. An essential and often overlooked factor for a sturdy, long-lasting fence is effective retaining wall drainage. Imagine water silently pooling behind your retaining wall, creating immense pressure that can slowly push the wall forward, cause cracking or even lead to a complete collapse.

“Ever see a wall leaning forward, being pushed over from the top? That’s a sign of poor drainage behind the wall,” says Vince Christofora, professional engineer and owner of Woodstock Hardware in Woodstock, NY.

There are ways to help walls that are starting to show these signs. If you’re installing a new fence, getting the retaining wall drainage right from the start saves you time, money and headaches. “Installation of a retaining wall’s drainage features is not the place to cut corners or scrimp,” Christofora says.

This guide will explain why retaining wall drainage is essential and provide step-by-step instructions for adding drainage to new or existing retaining walls.

Why Retaining Wall Drainage Is Essential

When you don’t install adequate retaining wall drainage, moisture builds up in the earth behind it. As that soil soaks up water, it gains considerable weight and pushes with tremendous force—what engineers call hydrostatic pressure—directly against your wall’s structure. Getting drainage right is essential to preventing this pressure from potentially leading to structural failure. This can cause the wall to bulge, crack, lean or even topple completely. It can also lead to soil erosion behind and beneath the wall, even if there is a gravel and pipe drainage system in place, compromising its stability and potentially causing issues like frost heave in colder climates.

How water pressure affects retaining walls

Water is heavy, so when soil becomes saturated, its weight increases dramatically. This increased weight and the non-compressible nature of water push against the retaining wall. Clay soils that drain poorly are particularly problematic as they can hold much water and expand significantly, further increasing pressure. Proper retaining wall water drainage systems are designed to alleviate this hydrostatic pressure by providing a path for water to escape, which is why they are essential for DIY wall drainage solutions.

Step-by-Step Guide to Installing Retaining Wall Drainage

Several components work together to create an effective retaining wall drainage system.

Consider your conditions

Site conditions are your key determinant. “Dry areas and sandy, well-drained soils need less drainage. Wet areas with poorly drained and clay-type soils need more drainage,” Christofora says.

He recommends consulting with a structural engineer, if possible, especially if you’re dealing with poorly drained clay soil. If that’s the case, at the very least, aim toward drainage overkill.

Don’t skip the footing drain

Nearly every retaining wall should have a footing drain at its foundation. These have a pipe leading out, AKA daylighted, so water can flow out. “Given the cost of materials and the effort required to build any retaining wall, the increased cost of the footing drain is small compared to assuring the wall will perform over the long run,” Christofora says in approval of the gravel and pipe drainage system.

Pick the right pipe

There are two main choices: perforated pipe and solid pipe. Use the perforated kind under or behind your wall, for maximum subsurface drainage. Use solid to quickly carry water from your wall to the drainage site.

Don’t skimp on gravel amount

Add an angular aggregate—clean, crushed stone or gravel between 1/2- and 3/4-in.—as a base under the retaining wall and as backfill behind it. Convention says to add at least 12 inches of gravel backfill, but Christofora’s motto: The more the better. “Depending on the size and scope of the project, the minimal increase in cost of installing 24 inches of gravel behind the wall will provide increased drainage and maximize the structural integrity,” he says.

For existing walls, you may want to excavate to see if the gravel is at that 12-inch minimum. If needed, you could dig down far enough to see if there is the necessary footing drain and gravel under the wall as well. Just be careful not to undermine the wall.

Compact the soil

Although it might seem counterintuitive, use a compactor on the soil behind your completed retaining wall, after adding your 12-plus inches of drainage gravel. Water-saturated soil can cause tremendous hydrostatic pressure to be exerted on your retaining wall. Compacting the soil makes it less permeable, sparing your wall of potential (and potentially damaging) water weight. If hand-tamping, lay a 1- to 2-in.-thick layer of soil, tamp, then repeat until finished. If using a gas-powered tamper, you can lay up to six inches of soil between tamps.

Use plenty of filter fabric

Filter fabric, AKA geotextile underlayment fabric, is a permeable fabric commonly used as a filter between soil and gravel surfaces. It keeps dirt and debris from clogging the gravel and preserves retaining wall water drainage. Christofora recommends using it at every place where soil and gravel meet, including under the wall’s gravel footing, between the footing drain’s gravel and soil and under the gravel added behind the retaining wall.

Install (or add) weep holes

Weep holes are small, evenly spaced holes along the bottom section of your wall. They protect the structural integrity by allowing underground water to seep through, preventing pressure build-up. Every retaining wall should have them. And yes, you can add them retroactively—drill them into an existing concrete, stone or brick wall using a core drill and drill bit, which you can often rent at your local hardware or DIY home improvement center.

Your wall’s scale will determine weep hole size and spacing. For large walls, around 6 ft. or taller, incorporate 3- to 4-in. dia. weep holes every 3 to 4 feet. If your wall is smaller than that, make your holes more like 1-1/2 in. dia., spaced at 8-ft. intervals. (Though Christofora does note that these figures can vary depending on wall height as well as soil type, topography and other site conditions. “The more holes, the better,” he says.)

One caveat from Christofora: “Adding weep holes may help save the retaining wall water drainage, but if the wall was backfilled with soil and there’s no gravel behind it, you may not get enough drainage through the weep holes.”

Ensure there’s enough slope

Even with lots of correctly installed drainage around your retaining wall, if the soil on the other side doesn’t slope away sufficiently, that water won’t go anywhere. Any amount of slope will do, provided it doesn’t reverse a short distance away.
Keep in mind where the water will end up. You don’t want the excess water draining onto someone else’s land or into a pond, lake or river.

Other options, if you’re in a pinch: “Build a French drain or a dry well, where water can flow into a gravel trench or pit and then be absorbed into the ground,” Christofora says. “They’re both fairly involved projects, but can be grassed over or turned into attractive landscape features.”

Common Retaining Wall Drainage Mistakes to Avoid

  • Using improper backfill material: Using soil or fine gravel directly behind the wall instead of clean coarse drainage aggregate will prevent proper water flow.
  • Skipping a drainage pipe installation: Relying solely on gravel may not be enough, especially for larger walls or poorly draining soils. A pipe actively collects and removes water.
  • Failing to use geotextile fabric: Allows soil to clog the gravel and pipe system quickly, leading to failure.
  • Incorrect pipe slope or no outlet: The pipe must slope to an outlet so water can escape. A flat or improperly sloped pipe will hold water.
  • Inadequate compaction of backfill soil (above drainage zone): While the drainage gravel itself isn’t heavily compacted, the soil placed above and behind the gravel zone should be compacted in lifts to prevent settling and undue pressure, which is why it is necessary to know how to install drainage behind a wall properly.
  • Blocked weep holes or pipe outlets: Ensure these remain clear of debris.

FAQ

Can improper drainage lead to retaining wall failure?

Yes, absolutely. When you skimp on your wall’s drainage system or install it incorrectly, you’re setting yourself up for one of the main reasons retaining walls give way. The water pressure behind poorly drained walls can eventually overcome even a well-built structure’s designed capacity to hold back soil, resulting in unnecessarily expensive repairs.

About the Expert

  • Vince Christofora, PE, the proprietor of Woodstock Hardware in New York, brings over two decades of structural engineering experience to his retail business. His specialized knowledge of soil mechanics and water management systems has made him a trusted advisor in the community, where he offers customized drainage solutions for both weekend DIYers and professional contractors. Christofora has developed innovative approaches to preventing hydrostatic pressure buildup behind retaining walls across the region’s challenging topography through his work.

Sources

Author
Robert Maxwell
Robert Maxwell is a seasoned carpenter, builder, cabinetmaker, homesteader, stone mason and mechanic. He's been sharing his knowledge through digital and print articles since 2010. Robert lives in a self-built cabin in the woods in Northern Ontario, Canada with his wife and daughter.
Read More
Robert Maxwell