Retaining Wall Drain Pipe Installation: Pipe, Stone, Outlets

Retaining wall drain pipe installation follows a simple idea: give water behind the wall an easy way out, so it never builds up against the back of the wall. A properly drained wall has three parts working together. A zone of clean crushed stone directly behind the wall lets water drop down instead of soaking the soil. A perforated drain pipe at the base of that stone collects the water. And an outlet carries it out the ends of the wall or through weep holes to somewhere it can drain away.

Skip any one of the three and the wall is at risk. In Massachusetts, where wet soil freezes and expands against walls every winter, poor drainage is one of the main reasons retaining walls lean, bulge or crack.

This guide covers how retaining wall drainage works, which pipe and backfill to use, what height of wall needs it, and how to add drainage to an existing wall. It comes from Gerrior Masonry & Landscape Construction Corp. of Woburn, MA, which builds block and fieldstone retaining walls across greater Boston.

Why a retaining wall needs drainage

Soil behind a wall pushes on it all the time. When that soil fills with water, it gets heavier and pushes harder, and the water itself adds pressure. Engineers call this hydrostatic pressure. A wall designed to hold back dry soil can be overloaded by the same soil when it is soaked.

New England adds two more problems:

Freeze-thaw. Water expands when it freezes. Saturated soil behind a wall freezes, pushes, thaws and settles, winter after winter. Each cycle can move the wall a little.
Clay and ledge. Many yards north of Boston have clay soil or ledge close to the surface. Both hold water near the wall instead of letting it soak away, so the drainage has to do more of the work.

At what height does a retaining wall need drainage?

Any wall that holds back soil benefits from drainage, and a short wall that stays wet is still exposed to frost. In practice:

Low garden walls that hold back only a little soil often get by with clean stone behind them and open joints, without a pipe.
Walls that hold back a real slope, or have a lawn, driveway or patio above them, should have a stone drainage zone and a pipe.
Taller walls follow the block manufacturer's installation guide or an engineer's design, which spell out the drainage stone, pipe and any reinforcement.

Height also affects permits. The International Residential Code, which the Massachusetts building code for homes is based on, exempts retaining walls up to 4 feet tall (measured from the bottom of the footing) from a building permit unless they hold back an extra load such as a driveway, a building or a slope above, according to the ICC code text. Towns can add their own rules, so check with your building department before building a tall wall or one near a property line.

The parts of a retaining wall drainage system

Drainage stone behind the wall

A column of clean crushed stone runs up the back of the wall from the base to near the top. Water moving through the soil reaches this stone and drops straight down to the pipe instead of pressing on the wall. The width of that zone is set by the block maker's instructions or the wall's design.

Filter fabric

Landscape filter fabric goes between the native soil and the drainage stone. It lets water through but keeps fine soil from washing into the stone and clogging it, which matters a lot in silty or clay soil.

Perforated drain pipe at the base

The perforated pipe sits at the bottom of the stone, behind the lowest course of the wall, and runs the length of the wall with a steady fall toward the outlets.

Outlets and weep holes

Water in the pipe has to leave. The pipe either runs out the ends of the wall to daylight, ties into a solid pipe that carries the water to a lower spot or a dry well, or exits through outlets along the face of the wall. Mortared stone and concrete walls get weep holes near the base so water behind them can get out. An outlet that ends in a low spot full of water, or gets buried in mulch, does nothing.

A cap on top

The top of the drainage stone is covered with a layer of less permeable soil, and the ground above the wall is shaped so surface water runs along or away from the wall instead of pouring straight down behind it. Downspouts should never empty behind a retaining wall.

Curved fieldstone retaining wall along a driveway with a band of crushed stone at its base and a planted bed above — A fieldstone retaining wall by Gerrior with a strip of crushed stone along its base.

Choosing the drain pipe

Perforated or solid?

Perforated pipe collects water, so it goes behind the wall in the drainage stone. Solid pipe carries water without letting it in or out, so it is used for the run from the wall to the outlet. Many walls use both.

Holes up or holes down?

Common practice is to lay perforated pipe with the holes facing down. Water rising in the stone enters the pipe from below and drains away along the bottom of the pipe instead of sitting in it.

What size?

Four-inch pipe is the most common size behind residential retaining walls. Three-inch pipe turns up on short garden walls. The block manufacturer's guide or the wall design is the final word.

Corrugated or rigid?

Flexible corrugated pipe is easy to lay along curves but can sag and hold water in low spots. Rigid PVC holds a steady slope and is easier to clean out with a hose or snake. On a long or important wall, rigid pipe is worth it.

Sock or no sock?

A fabric sock around the pipe keeps out fine soil, but in silty or clay soil the sock itself can clog. Wrapping the whole stone zone in filter fabric and leaving the pipe bare inside clean stone avoids that problem.

Best backfill material for a retaining wall

In the drainage zone: clean, washed, angular crushed stone. Clean means no stone dust or sand mixed in, because fines fill the gaps the water needs.
Under the wall: a compacted base of crushed gravel, deep and wide enough for the wall's height and the frost.
Behind the drainage zone: the native soil, compacted in layers.
Never in the drainage zone: clay, topsoil, stone dust or sand.

For a wider look at wall materials, see retaining wall materials compared. For the build itself, see how a retaining wall is built on a slope.

How to add drainage to an existing retaining wall

Adding drainage to a wall that was built without it depends on the wall's condition:

The wall is sound, the problem is surface water. Start above the wall: regrade so water runs away from the top, add a shallow swale, and move downspouts. This alone fixes many wet walls.
A mortared stone or concrete wall with no weep holes. Weep holes can sometimes be drilled through near the base so trapped water can get out. It helps, but only if there is stone behind the wall for water to drain through.
A dry-laid or block wall that is still straight. Soil can be dug out behind it in sections and replaced with filter fabric, drainage stone and a pipe. Any digging starts with a Dig Safe notice; Massachusetts requires at least 72 hours, not counting weekends and holidays, according to Dig Safe.
A wall that is already leaning or bulging. Drainage won't straighten it. That wall needs to be taken down and rebuilt with proper drainage.

Signs a retaining wall is not draining

The wall leans forward or bulges in the middle.
Water seeps out the face or base after rain, or the ground in front stays soggy.
White, chalky deposits (efflorescence) on mortared stone or concrete, which shows water moving through the wall.
Cracks in mortar joints or a concrete wall.
Soil washing out through the joints, or the ground behind the wall sinking.
Blocks or stones shifting after a winter.

Mortared fieldstone retaining wall beside a sidewalk holding back a planted slope, with small round weep outlets near its base — A mortared fieldstone wall by Gerrior. The small round weep outlets near the base let water out from behind it.

When to call a mason

A low garden wall is a reasonable weekend project. Call a professional when the wall holds back a real slope or a driveway, when it is tall enough to need a permit, when the soil is clay or ledge, or when an existing wall is already moving. Our retaining walls page explains how we build block and fieldstone walls, and you can see finished work in our landscape and retaining wall gallery. For what drives the price, see the retaining wall cost guide.

Common questions

Do all retaining walls need a drain pipe?

Not all. Low garden walls often drain fine through clean stone and open joints. Walls holding back a real slope, or with a lawn or driveway above, should have one.

Where should the drain pipe go?

At the base of the drainage stone, behind the lowest course of the wall, sloped toward outlets at the ends or through the face.

Can I use sand behind a retaining wall?

Not in the drainage zone. Sand and stone dust pack down and hold water. Use clean, washed crushed stone.

Why is my retaining wall leaning?

Usually water and frost pressure behind it, a base that was too shallow or soft, or a wall too tall for its design. Leaning walls need rebuilding, not just drainage.

Should the pipe have a sock?

In clean stone wrapped in filter fabric, a bare pipe usually works better, because a sock can clog in fine soil.

Gerrior Masonry & Landscape Construction Corp. is a family-owned company in Woburn with over 30 years building retaining walls and drainage. We are fully insured and our work is guaranteed. Call (781) 933-4157 or request a free estimate for a new wall or one that needs help.