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Retaining Wall Drainage: How to Build It Right in Utah

Worker placing drain pipe behind retaining wall

A retaining wall stays standing when water has somewhere to go. The proven setup is a layer of free-draining gravel directly behind the wall, a perforated pipe low in that gravel, filter fabric to keep soil out, and an outlet that carries the water away. Surface water above and below the wall has to be managed too. Any wall that holds back soil needs this, whatever its height, and in Utah some cities require a drainage plan before they issue a permit.

Below is how each part works, what the published standards specify, the mistakes that cause failures, and how to spot trouble early.

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TL;DR

  • Water pressure behind the wall is the main reason walls fail, so drainage is a structural part of the wall, not an add-on.
  • The standard system is clean drainage stone at least 12 inches deep behind the wall, a 4-inch perforated pipe at the base, non-woven fabric and a pipe outlet.
  • Short walls need drainage too. Height changes the engineering, not the need for water to escape.
  • Utah cities set their own rules. Lehi, for example, asks for a drainage plan on walls of 4 feet or more.
  • Leaning, bulging, cracks along the top and soil staining or wet spots are signs to have the wall looked at.

Table of Contents

Why water is what pushes a wall over

Soil on its own is heavy. Soil full of water is heavier, and the water adds its own sideways push. Colorado State University’s PlantTalk guidance notes that most retaining walls fail because of pressure from water or moisture building up behind them, which is why a wall should let that water drain down and away.

BRANZ, a building research body, explains the mechanics: water buildup increases the soil load and lateral pressure while reducing bearing capacity. In other words, drainage controls the loads the wall was designed for. Snohomish County’s retaining wall guidance says the same in plain terms: walls can fail when water builds up in the soil behind them, so a drainage system is a necessary component of the design.

The Concrete Masonry & Hardscape Association (CMHA) adds two more reasons to drain a segmental block wall: water can erode soil and carry fine particles out through the wall face. The right design depends on the groundwater conditions at your site.

Utah homes often sit on hillsides, with irrigated lawns above and driveways or patios below. That puts a lot of water in the ground behind many walls, from snowmelt, summer storms and sprinklers.

The parts of a drainage system that works

A wall that drains well is a layered assembly. Each layer has a job.

Drainage stone behind the wall

Immediately behind the blocks goes clean, angular, free-draining stone, not the native soil. CMHA says to place at least 12 inches of free-draining aggregate behind a segmental wall. The Utah state Typical Gravity Wall Detail specifies drainstone (AASHTO No. 57 or equivalent) extending at least 12 inches behind the blocks, and for blocks with hollow cores it calls for filling the vertical core slots with the same stone.

“Clean” matters. Gravel mixed with dirt or road base holds water instead of passing it. Washed rock keeps its open voids, which is what lets water fall to the bottom of the wall.

Perforated pipe at the base

The stone only moves water to the bottom. A pipe carries it out. The Snohomish County guidance calls for a perforated PVC pipe at least 4 inches in diameter in the lowest layer of drain rock, sloped so water flows by gravity to an outlet. The Utah gravity wall detail also shows a 4-inch perforated pipe wrapped in a sock. Allan Block’s best practices recommend a 4-inch perforated pipe and, if you use rigid perforated pipe, setting it with the holes down.

CMHA sets a floor of 3 inches for pipe diameter and says the elevation and size should be chosen for the site. In practice, 4 inches is the number you will see most often.

Filter fabric

Non-woven geotextile separates the drain stone from the soil behind it. Snohomish County specifies it over exposed soil to keep the drain rock from clogging, and the Utah detail calls for non-woven fabric behind the blocks as well. Without it, fine soil slowly migrates into the gravel and the system silts up.

An outlet that actually leads somewhere

A pipe that ends in a dead end just fills up. Allan Block requires drain pipes to exit to daylight or connect to an underground drainage system. CMHA’s inspection guide recommends keeping the pipe sloped for gravity flow and daylighting it at a maximum of 50 feet, or tying it into a storm system. On a sloped lot, that usually means running the pipe to the end of the wall or to a lower point where water can leave without damaging a neighbor, a walkway or your own foundation.

Quick reference

Component What the published guidance specifies Source
Drain stone At least 12 in. behind the wall; AASHTO No. 57 or equivalent in the Utah detail CMHA, Utah detail
Drain pipe Perforated, 4 in. typical (3 in. minimum per CMHA), sloped to an outlet Snohomish County, CMHA
Fabric Non-woven geotextile between stone and soil Snohomish County
Leveling pad 6 in. minimum crushed rock in the Utah gravity wall detail Utah detail
Outlet Daylight, or connect to a storm system; daylight at 50 ft. max per CMHA CMHA inspection guide

These are reference points from public guidance. Your wall’s real design depends on its height, soil and loads.

Where the pipe goes: bottom, not top

Homeowners often hear conflicting advice, such as putting drainage at the top of a long wall in clay soil. The published specifications all place the collection pipe low, in the bottom layer of drain rock. Snohomish County puts it in the lowest layer, and the Utah detail ties its pipe into a drain system at the base. The reason is simple: water moves down through the gravel, so the lowest point is where it collects and can be carried out.

What belongs at the top is surface water control. Allan Block says surface water must drain away from the top and bottom of the wall, and that a swale above the wall should keep water from running over the crest. So the answer is both: a base drain for water that gets into the soil, and grading or swales to keep surface runoff from getting there in the first place.

Surface runoff and base drain paths at wall

Does a short wall need drainage?

Yes. BRANZ states that all retaining walls must have drainage behind them. A 2-foot or 3-foot wall holds back less soil, so the forces are smaller, but the water can still push, freeze and wash soil out. What changes with height is the engineering: CMHA notes that conditions such as groundwater within or above the wall height, tiered walls, steep slopes at the crest or toe, extra load above the wall, and soft, high-plasticity clay or fill soils need extra attention to stability.

BRANZ also lists what to consider when choosing a wall system: soil conditions, drainage, water table level and distance to an adjacent slope. Loads above the wall count too, including sloping backfill, buildings and vehicles.

For a short garden wall in free-draining soil, the drainage assembly can be simple. For a taller wall, a wall under a driveway, or one holding a lawn that gets watered daily, it deserves real design.

Surface water and sprinklers

Much of the water behind a wall does not come from the ground. The New York City Department of Buildings inspection guidance lists intense rainfall, perched groundwater, seepage, irrigation, utility leaks and erosion among the water-related causes of wall and slope problems. Utah yards add a few of their own: roof downspouts that discharge near a wall, sprinkler zones that spray the backfill, and paved areas that shed runoff toward the crest.

Some practical checks:

  • Point downspouts and drain lines away from the top of the wall.
  • Keep sprinkler heads from soaking the soil right behind the wall.
  • Grade the ground above so water runs away from the crest.
  • Make sure the ground at the toe slopes away too, so water does not pond against the base. Allan Block warns against letting water pond at the toe or above the wall.

If you are redoing a yard, a xeriscape design cuts irrigation near the wall and makes this much easier to manage.

Common drainage mistakes

Using native soil or dirty gravel as backfill. It holds water. Use clean drain stone at the required depth.

Skipping the fabric. Fines move into the stone over time and the drain loses capacity.

Relying on weep holes alone. Holes through the wall face can clog, and NYC’s wall guidance lists clogged weep holes as a warning sign. They are a relief point, not a replacement for a drain behind the wall.

A pipe with no outlet or no slope. Water only leaves by gravity if the pipe runs downhill to a real exit.

Perforations facing up. Allan Block says rigid perforated pipe goes with the holes down.

Ignoring water during construction. Allan Block says surface water must be directed away from a partly built wall at the end of each day, and must not pond at the toe.

Forgetting what’s above. Tiers, slopes and driveways above the wall add load and water. They change the design.

Common drainage mistakes

Utah codes and permits

Rules differ by city, so check yours before you build. Some examples from public Utah documents:

  • Lehi: Chapter 12-C, added August 26, 2025, applies to walls retaining 4 feet or more. Permits require a drainage plan showing slopes, grades, pipes and detention or retention facilities, and walls must provide drainage unless designed for hydrostatic pressure. It also lists approved drainage options and the conditions under which extra drainage material can be omitted.
  • Leeds: the town’s code requires a free-draining gravel layer and filter fabric behind the wall, with a drainpipe that daylights to a proper outlet. It allows a synthetic drainage composite behind segmental block walls only if material-specific shear testing establishes the friction between the backfill and the composite.

Utah’s own typical gravity wall detail, cited above, shows what a public agency expects behind a block wall. Your city or county building department tells you the threshold height for a permit and whether engineering is required. A licensed contractor who builds walls in your area will already know these.

Warning signs your wall has a drainage problem

Walls should be checked regularly. The NYC Department of Buildings lists these signs:

  • Water collecting behind the wall or at the top because of poor drainage
  • Water leaking through cracks or clogged weep holes
  • Leaning
  • Cracks or holes in the soil or pavement along the top
  • Soil or pavement swelling at the bottom
  • Severe cracks, bulging or displaced material

If you see these, have a qualified professional evaluate the wall. NYC’s page points to a licensed engineer or registered architect, and a wall that is leaning or bulging should not wait. Fixing a drain early can be a modest job. Waiting for movement often leads to rebuilding, and our guide to when to tear out and rebuild a retaining wall walks through that decision.

Our perspective on drainage

We build retaining walls in Utah from Cedar City to Park City, and we treat drainage as part of the wall, not a line item to trim.

The reason is the one the engineering guidance gives: water behind a wall is the main load you cannot see. We would rather spend time on stone depth, fabric, pipe slope and a real outlet than explain a leaning wall two winters later.

Because our own licensed crew does the work with no subcontractors, the person who sets the pipe grade is on the same team as the person who stacks the blocks. That keeps quality control in one place and the schedule predictable.

Whether you choose a concrete block wall or natural boulders, ask your contractor to show you where the water enters, where it collects and where it leaves. We walk every customer through that during the free estimate.

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Build it with a crew that handles drainage

Our concrete block and natural boulder retaining walls are built by our in-house licensed crew. Concrete block retaining walls are priced at $25 to $45 per square foot on our site. Where a wall needs new grading first, our excavation and grading team prepares the site. Estimates are free.

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Ready to plan yours? Request a free estimate and we will look at your slope, soil and water sources.

FAQ

What is the best drainage behind a retaining wall?

The combination used in public specifications is clean drainage stone at least 12 inches deep, non-woven fabric against the soil, and a perforated pipe of about 4 inches at the base. The pipe slopes to an outlet that daylights or connects to a storm system. Surface grading above the wall completes the system.

Does a 2 ft retaining wall need drainage?

Yes. BRANZ states that retaining walls must have drainage behind them. A short wall may need only a simple assembly, but water still needs a path out. Soil type and what sits above the wall can increase what is needed.

Can I just use weep holes?

Weep holes can help relieve water, but they can clog, and a clogged one is a warning sign in the NYC guidance. A gravel layer and base pipe are the primary system.

Do Utah cities require a drainage plan?

Some do. Lehi’s code requires a drainage plan for permits on walls of 4 feet or more, and Leeds requires gravel, fabric and an outlet behind walls. Check your city’s building department for its threshold and rules.

Sources

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