Retaining Wall Drainage: Weep Holes, Gravel & Drain Pipe

The Reason Retaining Walls Fail

A retaining wall holds back soil. But the thing that actually destroys most failed walls isn't the weight of the dirt — it's the water in it. When rain saturates the soil behind a wall, that water has weight and it has pressure. The pressure pushes outward against the wall with surprising force, and in Oregon's long wet season the soil stays saturated for months. A wall built without drainage is a wall on borrowed time.

The fix isn't a stronger wall so much as a smarter one: drainage that relieves the water pressure before it can build. This guide covers the three components that do that. For the broader drainage picture, see our Oregon drainage guide.

Hydrostatic Pressure: The Force You're Fighting

When soil behind a wall fills with water, you get hydrostatic pressure — the outward push of standing water against the back of the wall. It grows with the depth and saturation of the soil, and it acts continuously. Over a wet Oregon winter, a saturated backfill can exert enough sustained pressure to bow, crack, lean, or topple a wall that would easily hold dry soil.

The entire goal of retaining wall drainage is to never let water accumulate back there. Keep the backfill draining freely and the pressure never builds. That's the principle behind all three components below.

Component 1: The Drain Rock Backfill

Directly behind the wall goes a zone of clean, free-draining gravel — drain rock — rather than native soil. This gravel layer gives water an easy vertical path to fall straight down to the drain pipe at the base, instead of saturating the soil against the wall.

The catch, as with any Oregon drain, is fine soil. Silt and clay will migrate into the drain rock and clog it, so filter fabric separates the native soil from the gravel zone, keeping the rock clean and draining for the life of the wall. Skip the fabric and the gravel slowly silts up, and the drainage you built quietly stops working.

Component 2: The Drain Pipe at the Base

At the bottom of the gravel zone, behind the wall's footing, runs a perforated drain pipe — the wall's footing drain. Water falling through the drain rock collects in this pipe and flows to an outfall, carried away from the wall entirely.

This is collection, so it's perforated pipe in the gravel; the run to the outfall switches to solid pipe so the water doesn't leak back into the ground. Our perforated vs solid drain pipe guide explains the distinction, and our French drain cost in Oregon guide covers similar pipe-and-gravel work. The pipe needs slope to a real outfall, just like every drain — daylighted downhill is ideal.

Component 3: Weep Holes

Weep holes are openings near the base of the wall face that let any water that does reach the wall escape to the front rather than building up behind it. In segmental block and masonry walls, they're a secondary relief — a pressure release valve that complements the gravel and pipe behind. You'll see water trickling from them during heavy rain, which is exactly what they're supposed to do: that's pressure leaving the system instead of pushing on the wall.

Weep holes alone aren't enough drainage for a significant wall — they're the visible part of a system that depends on the gravel and pipe behind it. But on smaller walls, and as backup on larger ones, they matter.

Putting It Together

A properly drained retaining wall, from back to front:

1. Native soil, separated by
2. Filter fabric, then
3. Clean drain rock against the wall, with
4. A perforated pipe at the base sloped to an outfall, and
5. Weep holes in the wall face as relief.

Build all five and water never gets the chance to load the wall. Leave out the drainage and the wall is relying on brute strength against a force that works on it every rainy day.

When the Water Comes From Uphill

On sloped Oregon lots, a retaining wall often sits below ground that sheds water down toward it. In that case, intercepting the water before it reaches the wall's backfill helps enormously — a curtain or interceptor drain across the uphill side catches groundwater moving downslope. Our curtain drain vs French drain guide covers that approach, which pairs well with proper wall drainage on hillside sites.