Retaining wall materials and drainage behind the wall

A retaining wall has one job: hold back ground that wants to move. Most builders think about the wall itself, its material, its finish and how straight it sits along the boundary. The failures that come back to the builder are rarely about the block or the post. They are about what happened on the other side of the wall, where the ground filled with water, the drainage did not work, and the wall was pushed over by a load nobody had considered.

This article covers the wall, the backfill placed behind it and the drainage that carries water away from that backfill. It compares the three materials residential builders most often use, masonry, concrete and timber, and then works through the backfill, drainage and surcharge questions that decide whether any of them lasts. Drainage under a slab or beneath footings is a different problem with its own logic, and it is covered in our article on subsoil drainage around footings.

The commercial stake is the callback. A wall that moves in its first wet season costs far more to put right than it did to build, and the repair usually means taking the wall down, excavating the ground behind it and rebuilding it properly. The client sees a bulge or a crack. The builder pays for the excavation, the disposal and the time, and the reputation takes longer to recover than the money does. Builders who want structured CPD on sites like this can start with the PointCPD course catalogue.

Why the ground behind a wall is the real load

Soil pushes sideways against a wall, and the push grows with depth. Dry, well-drained soil is the easiest to hold back. Wet soil is heavier and pushes harder, and water that sits in the backfill does something worse: it builds hydrostatic pressure, which acts on the wall in every direction the water can reach. A wall designed for the soil alone can be overwhelmed by the water that gets behind it, even when the soil itself would have been fine.

Clay is the hardest ground to retain. It holds water for a long time, it swells as it gets wet, and it loses strength when it is saturated. A wall that is perfectly adequate in a dry season can be pushed out of plumb in a wet one, because the clay behind it has grown heavier and more active at the same time. Builders on clay sites should assume the ground will be wetter than they would like, because it will be.

Comparing masonry, concrete and timber for the wall itself

Reinforced masonry block is the familiar choice for a solid wall that will be seen from the house. Its strength comes from the steel placed in the cores and the grout that fills them, and both depend on being done properly in the field. A block wall needs a sound footing, correct bond and reinforcement at the points the design specifies, and it needs a way for water to leave the back of the wall. The block itself is forgiving of moisture. The mortar joints and the cores are where a poorly built wall gives way.

Cast concrete is strong in compression and works well where the wall has to hold a lot of ground or sits close to a structure. It needs formwork that will not move under the wet concrete, reinforcement placed as drawn, and time to cure before the ground is loaded against it. Concrete walls do crack, and the reinforcement is there to control where the cracks go. The drainage still matters, because a concrete wall with no relief behind it is simply a dam that has not yet failed. Our article on formwork stripping times for residential work covers the timing that governs when the wall can be loaded.

Timber sleeper walls are common on residential sites because they are quick and familiar, and they suit lower walls in good conditions. Their weakness is the ground. Timber in contact with wet soil for a long time is what rots, and the posts and bearers that carry the wall are the parts that matter most. Use timber treated for ground contact, keep the timber off wet ground where the detail allows it, and make sure the backfill behind the sleepers is free-draining so water does not sit against them. The WoodSolutions guidance on timber and moisture is a sensible reference for the detail.

Backfill: what goes behind the wall and why it matters

Backfill is the material you place against the back of the wall once it has cured or set. Its job is to transmit the soil load and to let water pass through. The material the excavation produced is almost never suitable. Clay spoil from a trench is the most common mistake on a residential site, because it is convenient and it is already on the ground. It holds water, it swells, and it adds the most pressure exactly when the wall is least able to resist it.

Use free-draining granular material placed in the zone behind the wall, and specify it in the works so the crew does not substitute the spoil on the day. Place the material in layers and compact each layer, but do not drive heavy compaction equipment close to a wall that has not been designed for it. A compactor working close to the back of the wall can push it over in the same way a load of soil can, and the damage may not show until the wall moves in the next rain.

Drainage behind the wall: giving the water a path out

A drained wall needs three things: a free-draining zone that lets water reach the base, a drainage pipe or relief at the base that collects it, and an outlet that carries it to a lawful point of discharge. Each one can fail on its own, and a wall with two of the three is still a wall with a problem. The zone is usually a band of clean granular material running the full height of the backfill. The pipe is perforated and laid to fall to the outlet. The outlet must discharge somewhere the rules allow, clear of the building, and not onto a neighbour’s land.

Filter fabric separates the drainage zone from the surrounding soil. Without it, fine particles migrate into the aggregate and the pipe over time, and the drainage slowly stops working without any visible sign. Use a fabric that is designed for the purpose, lay it so the joints overlap, and make sure the fabric is not torn during placement, because a torn fabric is a hole the soil will find.

Weep holes or drainage relief through a masonry or concrete wall let water escape from the backfill at intervals along the height. They need to be open, clear of mortar and sheeting, and placed where the drainage zone will reach them. A weep hole that is blocked by the first course of mortar, or that opens onto a paved surface that holds water in front of the wall, does not do what it was put there for. Check them before the backfill goes in, because they are very hard to clear after.

Surcharge: the loads nobody puts on the drawing

Surcharge is any extra load placed on the ground behind the wall. It is what the wall has to resist on top of the soil itself, and it is easy to overlook because it is not part of the wall. Vehicles parked on a driveway behind a wall, a shed or garage built close to the top of it, a pool, a spoil heap stacked during the build and the footings of a neighbour’s structure are all surcharge. Each one adds pressure that the design may not have counted.

The design should allow for the surcharge the site actually has, both now and in the future. A wall that holds a quiet garden bed is a different proposition from one that holds a driveway the family will use every day, and a wall built on a boundary with a neighbour’s footing close behind it needs that footing to be considered in the design. Ask the engineer the question directly: what loads will sit behind this wall for its life, and which of them have been counted?

During construction, do not stack spoil behind a wall that has not been designed to carry it. Spoil heaps, stored materials and machinery parked close to a newly built wall are all surcharge, and they are often placed without anyone thinking of them as a load. A crew that puts a skip of rubble behind a half-built wall on a Friday has created a design problem that nobody on the drawing knew about.

Timber, moisture and the detailing that keeps it sound

Timber can be a sound choice for a lower wall, but the details decide whether it stays sound. The posts and bearers are the most exposed parts, because they sit in the ground and in the water that collects at the base of the wall. Use timber treated for ground contact, set the posts so the timber is kept off standing water where the detail allows, and avoid cutting into treated timber in a way that exposes untreated heartwood to the soil.

The drainage behind a timber wall matters just as much as it does behind a masonry one, and it is often neglected because the material looks cheap and temporary. A sleeper wall with a clay backfill and no relief will fail in the same way a block wall does, usually sooner, because the timber has less resistance to the soil and to the rot that wet ground brings. Treat the drainage as part of the wall, not an optional extra.

A worked example: a sleeper wall on a sloping block

A builder on the north coast was building a new house on a sloping block with clay soil. The design called for a timber sleeper wall to hold back a driveway at the side, and the wall was within the scope the builder had been given. The excavation produced a large volume of wet clay, and the crew, short of time and wanting the driveway formed before the rain, backfilled the wall with the spoil and did not install any drainage behind it.

The wall looked fine for the first few weeks. Then the first heavy wet season arrived. The clay behind the wall swelled and the sleepers began to lean. The driveway was on top of the wall, carrying the weight of the car that used it each day, and that surcharge had never been part of the conversation with the engineer. By the time the bow was visible, the wall had moved far enough that the posts were loose in the ground.

The fix was the one nobody wanted. The driveway was lifted, the wall was taken down, the clay was removed and replaced with free-draining backfill, and a drainage pipe with filter fabric and an outlet to a lawful discharge point was installed at the base. The wall was rebuilt to the engineer’s design, with the driveway loads counted. The builder told the client exactly what had happened and what it would cost, and the cost was many times what the original backfill would have taken to do properly.

Getting the design right and knowing when to call an engineer

Not every low garden wall needs an engineer, but many residential walls do, and the line is set by what the wall is doing rather than by its appearance. Once a wall is retaining significant ground, sits close to a boundary or a building, or carries a load from a structure above or beside it, the design belongs to an engineer. The council or the certifier may set requirements of their own, and those should be confirmed before the excavation starts.

Give the engineer the information that matters: the ground conditions as found, including whether the soil is clay, the water table if it is known, the surcharge that will sit behind the wall, and the drainage and outlet you intend to use. A design built on assumed ground is a design built on guesswork. If the ground turns out to be different on the day, stop and get the design reviewed rather than adjusting it on site.

Keep the design, the material delivery records and the inspection notes together for the job. If a wall moves later, these records show what was designed, what was built and who checked it. Where the work is covered by the Home Building Compensation Fund (HBCF), a structural failure within the cover period can become a claim, and the records are what show the wall was built to the design.

Checks to make before the first load of backfill

The last point to check is the one that is most often rushed. Before any soil goes against the wall, walk the whole length and confirm what is in place. The wall has cured or set to the strength its design requires. The drainage pipe falls to the outlet, the outlet is clear and discharges where it should, and the filter fabric is in place and undamaged. The weep holes are open. The backfill material is the free-draining type that was specified, not spoil.

Confirm, too, that nothing is sitting behind the wall that the design did not allow for. A stack of pavers, a skip, a pallet of mortar or a parked machine is surcharge that will be counted by the wall whether anyone intended it or not. Move it, or make sure the design accounts for it before it stays.

Once the backfill is in, the work is not over. Walk the wall after the next heavy rain and look for bulges, new cracks, water coming through the face and the outlet running clear. A wall that is checked after its first wet season is far more likely to be sound long after that, and the record of that check is the best evidence that the work was done properly.

Questions builders ask

• Drainage
• Materials
• Retaining Walls

Published 9 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.