Retaining Wall Drainage: Stone, Pipe and Outlets That Keep a Wall Standing

Retaining wall drainage is the part of a wall nobody sees, and it’s the part that decides whether the wall stays standing. Waterlogged soil behind a wall is heavier, pushes harder, and in cold climates freezes and expands against the back of the block.

The fix is simple on paper: give water a fast path down and out before it can build pressure. Here’s how that works for block, concrete, timber and stone walls, and what to do if your wall is already holding water.

Why retaining walls need drainage

Soil alone pushes on a wall. Add water and three things happen: the soil gets heavier, water pressure builds behind the face, and in winter that water freezes and expands. A wall sized for dry, draining soil can be undersized for saturated soil.

Drainage is also how a wall keeps its soil. Without clean stone and fabric where they’re needed, water carries fines out through the joints, and voids open up behind the block. Poor drainage comes up again and again in why retaining walls fail.

The four parts of retaining wall drainage

1. Drainage stone. A column of clean, angular crushed stone directly behind the block, at least 12 in. wide per the Concrete Masonry & Hardscapes Association (CMHA). Family Handyman suggests 1/2- to 3/4-in. stone and notes that more is better on bigger walls. Clean means no fines: stone dust and crusher run pack tight and drain slowly.
2. Drain pipe. A 4 in. perforated pipe at the bottom of the stone, behind the base. Allan Block requires this toe drain on walls over 4 ft and on sites with poorly draining soil, and calls it out near driveways, below slopes and on tiered walls. If you use rigid perforated pipe, lay it holes down.
3. Outlets. The pipe needs somewhere to go. Allan Block vents drain pipes through the wall face or to lower ground at least every 50 ft, on at least a 1% slope, and marks and protects every outlet so it doesn’t get buried or plugged. A rodent screen on the end helps.
4. Surface water control. Keep water from reaching the stone in the first place. Allan Block calls for swales above walls so water doesn’t flow over the top, and for no ponding above the wall or at its toe. Route downspouts away, and finish the top few inches of backfill with compacted soil rather than open stone so the drain zone isn’t collecting the lawn’s runoff.

Drainage by wall type

Wall type	How it drains	Key detail
Segmental block	Clean stone behind the block, plus a perforated pipe at the base on taller walls and wet sites	Pipe vented to daylight; keep the stone zone clean during backfill
Poured concrete, brick or mortared stone	Weep holes through the wall, usually with stone and a pipe behind	About 1-1/2 in. holes every 8 ft on smaller walls; 3–4 in. holes every 3–4 ft on walls 6 ft and taller (Family Handyman)
Timber	Water drains through the gaps between timbers, with stone behind	Fabric behind the stone keeps soil from washing out through the gaps
Boulder or dry-stacked stone	Stone backfill behind and between the boulders	Fabric where the stone meets fine soil

Do you need landscape fabric behind a retaining wall?

Often, where clean stone meets fine soil. A non-woven geotextile between the drainage stone and the native soil keeps silt and clay from washing into the stone and clogging it, and Family Handyman recommends fabric at every soil-to-gravel interface. Where the soil behind the wall is sandy or the backfill is clean granular fill, many wall designs skip it. In clay and silt, it’s cheap insurance.

Common retaining wall drainage mistakes

• A drain pipe with no outlet, or an outlet that’s been buried, crushed or mowed over
• Downspouts or a sump line emptying behind or above the wall
• Backfilling the drain zone with native clay or stone dust
• Rigid perforated pipe laid holes up
• No swale above the wall, so the yard’s runoff pours over the top or soaks in right behind the block
• Driving equipment over the drain pipe during backfill

How to add drainage to an existing retaining wall

Start with the cheap fixes and work down:

1. Redirect downspouts and sump discharge away from the wall.
2. Regrade the top so water runs away from the wall, or cut a shallow swale that carries it to the side.
3. Find and clear any existing outlets or weep holes.
4. On concrete, brick or mortared stone walls, core-drill weep holes, as Family Handyman describes.
5. On a block wall that’s already bulging, take down the affected section and rebuild it with drainage stone and a pipe. Block walls are dry-stacked, so the same block can usually go back up.

If the wall is over 4 ft tall or holds up a driveway, have a pro look at it before anyone digs behind it. For a ground-up walkthrough of base, drainage and geogrid on a new wall, see our retaining wall design guide.

FAQ

Does a retaining wall need a drain pipe?

Allan Block requires a toe drain on walls over 4 ft and on sites with poorly draining soil, and recommends it near driveways, below slopes and on tiered walls. Short walls in sandy soil can often drain through the stone alone.

What kind of gravel goes behind a retaining wall?

Clean, angular crushed stone, roughly 1/2 to 3/4 in., with no fines. Plan on at least 12 in. of it directly behind the block.

Where does the drain pipe go in a retaining wall?

At the bottom of the drainage stone, behind the base, sloped at least 1% to outlets that vent through the face or to lower ground at least every 50 ft.

Do you need landscape fabric behind a retaining wall?

It’s a good idea where clean stone meets clay or silt, so fines don’t clog the stone. With sandy soil or clean granular backfill, many designs leave it out.

How far apart should weep holes be in a retaining wall?

Family Handyman suggests about 1-1/2 in. holes every 8 ft on smaller walls, and 3–4 in. holes every 3–4 ft on walls 6 ft and taller.

Can you add drainage to an existing retaining wall?

Partly. You can redirect water, regrade above, open outlets and drill weep holes in solid walls. A block wall that’s already moving usually needs that section rebuilt with stone and pipe.

Sources: CMHA, Guide to Segmental Retaining Walls; Allan Block, drain pipes; Allan Block, water management; Family Handyman, adding retaining wall drainage (2025).