Waterproofing Block and Retaining Walls
By Lemoon's Painting
A retaining wall is best waterproofed on the soil side, where water arrives, together with gravel and a drain that relieve the pressure behind it. A coating on the face you see cannot hold back water pushing through the block; it can only protect that face from weather and let the wall breathe. Painting a wet retaining wall with a dense film leads to blistering and peeling.
On the hillside lots of Bonita, Spring Valley and La Mesa, a great many back yards are held up by block retaining walls. And a great many of those walls share the same look: a dark damp band along the bottom, a white crust in the joints, and paint that bubbles and flakes no matter how often it is redone.
The wall is not misbehaving. It is holding back wet soil, which is its job. The question is what can be done about the water, and from which side.
Which side of a retaining wall should be waterproofed?
The soil side. Water arrives from the earth behind the wall, so the right place to stop it is on the face that touches that earth, together with gravel backfill and a drain that relieve the pressure. A coating on the visible face can protect that face from weather, but it cannot hold back water being pushed through the block from behind.
In the trade these are called the positive side, where the water comes from, and the negative side, where it is trying to get out.
How a well-built retaining wall handles water
A retaining wall built to manage water usually has several parts working together:
- A waterproofing coating or membrane on the soil face of the block.
- Gravel backfill directly behind the wall, so water drops down instead of soaking into the block.
- Filter fabric keeping fine soil out of the gravel.
- A perforated drain pipe at the base that carries the water away.
- Weep holes through the wall near the bottom, as a relief path.
Many older walls were built without some of these, and nothing on the visible face makes up for the ones that are missing. Some walls in hillside neighborhoods were also extended or added to over the years, so the back of the wall may not be what you would expect.
The two sides compared
| Soil side (positive) | Visible face (negative) | |
|---|---|---|
| What it does | Stops water before it enters the block | Protects the face from weather; lets moisture escape |
| When it can be done | When the wall is built, rebuilt or dug out behind | At any time |
| What it involves | Excavation, drainage, membrane or coating | Cleaning, repair, priming, a breathable or cementitious finish |
| Who does the digging and drainage | Usually a landscape, masonry or general contractor | Not needed |
| Limits | Major work; disturbs the yard | Cannot stop water pushed through from behind |
When the soil side is open
The best moment to waterproof the soil side is when the soil is already out: a wall being rebuilt, a new wall going in, or drainage being replaced behind an existing one. The excavation and drainage belong to other trades. Coating the exposed back face of the block while it is open is waterproofing work, and it is worth coordinating so that face is cleaned, prepared and coated before the gravel and the soil go back. It will not be reachable again for a very long time.
What a coating on the face can do
For most homeowners, the soil side is not open and is not going to be. That does not make the visible face a lost cause. Done properly, the work there can:
- Protect the face from rain and irrigation spray.
- Use a breathable masonry finish that lets moisture out instead of trapping it under a film.
- On damp walls, use a cementitious coating made to tolerate some moisture from behind, which usually holds up better there than ordinary paint.
- Restore the look of the wall in a way that lasts much longer than repainting over the old failures.
What it should never do is seal the face with a dense, non-breathing paint over a wall that is wet from behind. That is the cycle many walls are stuck in: paint, bubble, peel, repaint.
Weep holes, caps and the ground
Three details make a large difference.
Weep holes must stay open. They are not a defect, and a little staining below them is normal. Painting or caulking them shut traps the water they exist to release.
The cap on top of a freestanding garden or property wall is where rain enters. A cracked or porous cap feeds water straight down into the cores of the block. Sealing or repairing it is often worth more than anything done to the faces.
The base. Soil, mulch or planters piled against the bottom of a wall keep it wet. Where the white crust is the main complaint, our article on efflorescence explains how the salts are cleaned off and why the water source comes first.
Signs that are not about paint
A wall that leans, bulges outward, shows wide stepped cracks that are growing, or has pulled away from a slab or a fence next to it is showing structural movement. That calls for an engineer or a qualified contractor before anything else. A coating would only hide the evidence.
What Ebenezer looks at
At the estimate, he looks at where the water is coming from, whether the weep holes are open, what state the cap and the joints are in, and how the existing coating has failed. He will tell you honestly what the visible face can and cannot achieve, and when the real fix sits on the other side of the wall.
More on how the work is planned is on the waterproofing service page, and the hillside lots and mature landscaping of Bonita are described on its page. If your wall is peeling or blooming white, ask for a free estimate and get an answer about your wall, not a promise that paint can do something it cannot.
Common questions
Can you waterproof my retaining wall from the front?
Only partly. A coating on the visible face can protect it from rain and refresh how it looks, and some cementitious coatings resist moisture from behind better than paint. But no coating on the front reliably stops water that the soil is pushing through the block. That has to be handled on the soil side and with drainage.
Why does the paint keep peeling at the bottom of my block wall?
Usually because water is moving through the wall from the soil side or wicking up from the ground, and it pushes the paint off as it tries to escape. A dense paint film makes it worse. The source of the water has to be reduced first, and the finish chosen so the wall can breathe.
Should the holes along the bottom of my wall be sealed?
No. Those are weep holes, built in so water collecting behind the wall can drain out instead of building up pressure. Sealing or painting them shut traps that water. They should be kept clear, and a little staining below them is normal.
When does a retaining wall need an engineer rather than a painter?
When it leans, bulges, has wide or stepped cracks that are growing, or has moved away from a fence or a slab next to it. Those are structural signs. Ebenezer will tell you plainly if what he sees belongs to another trade before any coating is discussed.