Retaining Wall Drainage in Spokane: Gravel Backfill, Drain Pipe, Weep Holes and Where the Water Can Go (2026)

Short answer: most retaining walls in the Spokane area don't fail because the concrete or block was too weak. They fail because water got trapped behind them. A properly drained wall has a zone of clean, free-draining gravel directly behind it, a perforated drain pipe at the base, a way for that pipe or weep holes to get water out, and a ground surface above the wall that sheds water away from it. Where that water ends up matters too: local stormwater rules say you can't simply dump it onto your neighbor's lot.

This guide covers why drainage matters so much, what the code and industry standards say, and what a well-drained wall looks like in Spokane's freeze-thaw climate.

Why water is the real load on a retaining wall

A retaining wall is designed to hold back soil. Saturated soil pushes much harder than drained soil, because the water itself adds pressure (called hydrostatic pressure) on top of the earth pressure. Engineering references put it bluntly: when drainage fails, water pressure can roughly double the sideways force on a wall that was never designed for it.

The building code points the same way. Washington adopts the International Residential Code (IRC) statewide, and IRC Section R404.4 says walls that hold back more than 48 inches of unbalanced fill, or are over 24 inches tall and also carry loads beyond the soil, have to be designed to resist:

• Overturning
• Sliding
• Excessive foundation pressure
• Water uplift

Water is named right in the code. For walls designed under the International Building Code, Section 1807.2.3 requires a safety factor of 1.5 against sliding and overturning. A wall that's drained as designed keeps those margins. A wall with clogged or missing drainage quietly loses them.

The parts of a well-drained wall

1. Clean gravel directly behind the wall

The National Concrete Masonry Association (NCMA), which publishes the standard design manual for segmental (block) retaining walls, calls for at least 12 inches of clean, free-draining gravel behind the units, plus gravel filling the block cores. That gravel:

• Lets incidental water drop to the base instead of building up against the wall
• Keeps fine soil from washing out through the joints of the wall face
• Makes it easier to compact the backfill without pushing the blocks out of line

NCMA is clear that this gravel zone handles incidental water and isn't meant to be the main drainage path when a lot of water is coming toward the wall. If a slope, a downspout or a spring sends real volume toward the wall, the site drainage has to deal with that first.

For poured concrete walls, the drainage zone and outlets come from the wall's engineering or geotechnical report, but the idea is the same: free-draining material behind the wall, not native clay or silt packed against it.

2. A perforated drain pipe at the base

NCMA guidance calls for a perforated or slotted drain pipe at the bottom of the gravel zone:

• At least 3 inches in diameter for the main collection pipe behind the block (many builders use 4-inch)
• Sloped so gravity carries the water out of the wall zone
• Outlets spaced 20 to 50 feet apart along the wall face, either through the wall or to daylight at each end

A pipe that has nowhere to go is just a buried tube. Every wall needs a planned exit point that's lower than the pipe.

3. Weep holes or outlets through the wall

On poured concrete walls, weep holes or pipes through the stem let water out from behind the wall. They need to stay open. Weeps that are buried by landscaping, clogged with fines, or frozen shut stop working, and the water pressure comes right back.

4. Filter fabric between the gravel and the native soil

Where fine soil sits behind the drainage zone, a geotextile filter fabric keeps silt from migrating into the gravel over time. A gravel zone that has filled with fines no longer drains, even though nothing looks wrong from the front.

5. Surface water directed away from the top of the wall

The best drainage is the water that never gets behind the wall. That means:

• Grading the ground above the wall so it slopes away from the wall or to a swale
• Not pointing downspouts or sump discharge at the top of the wall
• Watching for driveways and patios above the wall that sheet water toward it

Why drainage matters more in Spokane

Spokane-area building codes rate the region's weathering as severe and set the official frost depth at 24 inches to the bottom of a footing, under the City of Spokane and Spokane County joint frost depth policy. For walls, that means:

• Wet soil freezes and expands. Saturated backfill that freezes behind a wall pushes on it every winter, which is one more reason to keep the soil behind the wall drained
• Frozen outlets back water up. Weep holes and pipe outlets in shade can ice over, so we put outlets where they can drain and keep them clear of snow piles where we can
• Spring melt is the test. Snowmelt on frozen ground runs off instead of soaking in, and a lot of it heads downhill toward the top of a wall

Where the water is allowed to go

Draining a wall means sending water somewhere, and in the Spokane area that's regulated. The Spokane Regional Stormwater Manual, used by Spokane County, the City of Spokane and Spokane Valley, says:

• Runoff from a developed site has to leave in the same manner and location as it did before development
• Flow may not be concentrated onto down-gradient properties where it used to sheet flow
• Drainage can't be diverted to points that didn't receive runoff before

In practice, that means you can't pipe your wall drain to a single outlet that dumps onto a neighbor's yard. We plan where each outlet goes, whether that's daylighting on your own lot, a gravel area, or another approach your jurisdiction accepts. If the wall comes with a permit, the drainage plan may be part of the review.

Signs a retaining wall has a drainage problem

• Leaning or bulging in the middle of the wall
• Water seeping out of joints or the base, or wet streaks after rain or melt
• Weep holes that never drain even after heavy rain
• Soil washing out through block joints or a sinkhole forming behind the wall
• White crusty deposits (efflorescence) on the face of concrete or block
• Ice forming at the base of the wall in winter

A wall showing any of these should be looked at before the next freeze-thaw season. Some can be fixed by clearing outlets or regrading above the wall. A wall that's already leaning usually needs to be rebuilt with drainage done right.

Drainage and permits

Drainage doesn't change whether you need a permit, but height and loads do. In unincorporated Spokane County and Spokane Valley, walls up to 4 feet from the bottom of the footing that don't support a surcharge are generally exempt. The City of Spokane is stricter. Our retaining wall permit guide covers each jurisdiction, and our guide to choosing the right retaining wall compares block, poured concrete and boulder walls.

How Raven builds drainage into every wall

We build poured concrete and block retaining walls across the Spokane area, and every one gets:

• Free-draining gravel behind the wall, not native soil packed against it
• A perforated drain pipe at the base with planned outlets
• Filter fabric where the native soil calls for it
• Footings below the 24-inch frost line where the design calls for a footing
• Grading above the wall so surface water goes somewhere other than behind it

See our retaining wall pages for Spokane, Spokane Valley, Liberty Lake and Mead.

Get a retaining wall quote

Call 509-350-2016 or request a free quote with the wall's length, rough height, what sits above it and your address. If you have an existing wall that's leaning or seeping, send a few photos and we'll tell you whether it can be fixed or needs to be rebuilt.

Sources

• ICC, Significant Changes: IRC Section R404.4 Retaining Walls (PDF)
• WAC 51-51-003, Washington's adoption of the International Residential Code
• County of Los Angeles Residential Code Manual, R404.4 Design of Retaining Walls (IBC 1807.2.3 safety factor and drainage) (PDF)
• NCMA Segmental Retaining Wall Installation Guide (PDF)
• CMHA TEC-004, Segmental Retaining Wall Design
• Spokane Regional Stormwater Manual (PDF)
• City of Spokane / Spokane County Joint Policy, Official Frost Depth (PDF)
• City of Spokane BUILD-08, Earth-Retaining Structures (PDF)