Build a Retaining Wall with Concrete Bags: A Practical Garden Guide

A slipping slope, a crumbling embankment, an unsteady terrace: many gardens need support – but few people fancy formwork, a mortar mess and a professional-scale building site.

This is where a method long familiar in professional circles comes into its own, yet still sounds like an insider tip for domestic gardens: a retaining wall made almost entirely from stacked concrete bags. There is no conventional brickwork, no formwork and no concrete saw – instead, it is more like a modular system with clear rules and one critical mistake that DIYers must avoid.

What a retaining wall made from concrete bags can do – and what it cannot

A retaining wall has one straightforward purpose: it holds back soil and water while creating a defined difference in ground level. Common locations include garden slopes, terrace edges and areas beside a driveway. Even low walls carry considerable loads – especially when the soil is wet and water pressure builds up.

The dry concrete bag version is a type of gravity retaining wall. In other words, it does not rely on complex reinforcement but on its own weight. Once cured, the numerous bags form a solid concrete mass that resists pressure from the earth.

The concrete bag method is excellent for small garden projects – not for tall, safety-critical structures next to a house, garage or driveway.

For DIYers, it is particularly suitable for low walls in private gardens, for example:

• supporting a small slope behind a terrace;
• creating a raised bed or elevated planting bed;
• forming a garden seating step;
• stabilising an embankment alongside a garden path.

As soon as the wall needs to be significantly higher, the slope is very steep or neighbouring buildings are involved, the job should be left to a specialist contractor. The simple concrete bag method is also inadequate on clay, peat or generally unstable ground.

Regulations and planning: without a sketch, costs can mount up

Before the first bag even goes into the trolley at the builders’ merchant, check local planning and building rules. Many local authorities require a formal notification or permission above a certain height, or for walls directly on a property boundary. Starting work without checking can lead to trouble with both neighbours and the authorities.

A rough paper plan is worthwhile:

• Record the wall’s length and intended height.
• Estimate the thickness of each concrete bag layer.
• Use these figures to calculate the approximate number of bags needed.

Even this basic preparation prevents concrete running out halfway through the build – or a structure ending up too weak simply because “there were still a few bags left”.

Preparing the ground: where most DIYers go wrong

By far the most frequent reason for leaning or cracked retaining walls is poorly prepared ground. Putting concrete bags straight onto soil or soft topsoil is an invitation to future cracking.

Foundation depth and the frost line

The wall requires a load-bearing, frost-free foundation. In many areas, a depth of around 40 to 50 centimetres is sufficient; in upland locations or regions particularly prone to frost, up to one metre may be needed. Excavate an even, as-level-as-possible trench along this depth.

Fill the trench with a layer of crushed stone or coarse gravel and compact it thoroughly. A spirit level is essential: even a few millimetres of slope will add up over the wall’s length and create a clearly visible lean.

Geotextile and drainage against water pressure

Water is a retaining wall’s greatest enemy. It collects behind the structure, puts the soil under pressure and can push entire sections of wall outwards. A straightforward but effective drainage arrangement is therefore worthwhile:

• Lay geotextile fabric in the foundation trench, then bring it up behind the wall later.
• Add a layer of drainage gravel or crushed stone on top.
• If required, install a perforated drainage pipe at the base of the future wall, draining out to the side.

The geotextile stops fine soil particles from clogging the gravel zone. This allows water to drain away for many years without compromising the construction.

Building the retaining wall: stacking concrete bags like blocks

Once the foundation and drainage are complete, the part most DIYers enjoy begins: stacking the bags.

Turning the bags correctly and laying the first course

The bags contain dry concrete, which only cures when water is added. Before laying them, turn or flip each bag over completely so that the more cement-rich part of the mix will face the visible side of the wall. This improves both appearance and surface strength.

Lay the first course on the prepared crushed-stone base, with the bags tight together and as gap-free as possible. It must be perfectly level. Minor adjustments can be made using a rubber mallet and gentle tapping.

Staggering joints and adding reinforcement

Place the next course with staggered joints: each bag should be offset by around half a bag length from the row below. This crosses the joints, making the structure work like masonry. The staggered arrangement noticeably improves stability.

For additional security, insert vertical steel reinforcing bars. Push them down through the concrete bags from above so that they extend into the foundation area. Once cured, the wall and its base will then form a single solid structure.

The cardinal error made by many amateur builders: they leave out reinforcing bars. The wall may initially seem stable, but it loses a considerable amount of safety over time.

Add water: how the concrete bag wall cures

Once the planned height has been reached, the decisive stage begins: watering. Thoroughly soak every bag with a garden hose until the paper outer layer is evenly wet and settles slightly. The water then spreads through the inside, allowing the dry concrete to react and harden.

The first 24 to 48 hours are critical. During this period, nobody should pull on the wall, knock it or place heavy pressure on it. Only after several days – around seven days is often recommended – does the concrete reach sufficient strength to allow careful backfilling to begin.

The paper wrapping used for many concrete bags is biodegradable. Sun, rain and frost gradually break it down, leaving only the concrete “block” visible. Bags with a plastic inner lining are unsuitable for this method: they prevent moisture exchange and look unattractive in the long term.

Backfilling, drainage and appearance: keeping the wall permanently stable

Once the concrete has cured sufficiently, the final major task follows: filling the area behind the wall. Keep drainage gravel directly against the wall; farther back, natural soil or a well-compacted crushed stone and sand mix can be used.

Important: do not place water-retaining material directly against the wall. Pure clay soil or fine earth without a drainage layer absorbs water like a sponge and creates substantial pressure.

Depending on the site, small openings or weep holes in the lower area may be useful, allowing water to escape in a controlled way. Combined with the drainage pipe, these greatly reduce water pressure on the wall.

For those who prioritise appearance, several finishes are possible:

• render the wall and match its colour to the house or terrace;
• cover it with natural stone slips;
• add climbing plants such as ivy or climbing hydrangeas;
• place timber cladding or a gabion-style screen in front of it.

The one source of failure that can ruin everything

With this method, one factor is absolutely vital: water management around the wall must be right. Insufficient drainage, missing geotextile or simply backfilling with heavy, wet soil are the most common causes of later damage.

Anyone who cuts corners on drainage and ground preparation is building a time bomb into the slope – even if the wall initially seems rock-solid.

There is another issue: many people underestimate the wall’s weight. Once cured, a concrete bag wall is virtually impossible to correct. A crooked first course, carelessly compacted ground or missing reinforcement can scarcely be disguised later.

Practical advice for DIYers and useful additions

Garden owners planning this kind of project should use a short checklist:

• Check local building requirements and boundary clearances.
• Assess the soil type: is the slope mainly sandy, clay-rich or rocky?
• Choose a realistic maximum wall height – when in doubt, build lower.
• Calculate sufficient concrete bags and steel reinforcing bars in advance.
• Include drainage pipe, geotextile and gravel from the outset.

Anyone using this technique for the first time should ideally start with a small section measuring two to three metres. This makes it possible to practise handling the bags, compacting the crushed stone and watering safely before tackling an entire slope.

Combining it with other construction methods can also be useful: for instance, a low concrete bag wall can provide a solid foundation for dry natural-stone walling above it. The technique also works as the rear wall of a raised garden bed if well-draining growing medium is used inside.

If in doubt, it is worth getting support at least for the planning stage. A short appointment with a structural engineer or an experienced landscape contractor costs less than a wall that later collapses – and helps ensure the concrete bag method is used sensibly and safely.