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

A slipping slope, crumbling bank or unstable terrace: many gardens need support – yet hardly anyone fancies formwork, a mess of mortar and a professional-scale building site.

This is where a method that has long been familiar in professional circles, but remains something of an insider tip for private gardens, comes into its own: a retaining wall built almost entirely from stacked concrete bags. There is no conventional brickwork, no formwork and no concrete saw; instead, it is a kit-like system with clear rules and one crucial mistake that DIYers must avoid.

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

A retaining wall has one straightforward purpose: it holds back soil and water while creating a defined change in ground level. Common locations include garden slopes, terrace edges and areas alongside a driveway. Even low walls carry considerable loads, especially when wet soil allows water pressure to build up.

The dry concrete bag version is known as a gravity wall. In other words, it does not rely on complex reinforcement to remain in place, but on its own mass. Once cured, the individual bags form a solid block of concrete that resists the pressure exerted by the earth.

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

For DIYers, low walls in a private garden are particularly suitable, for example to create:

• support for a small slope behind a terrace;
• a raised bed or elevated planting bed;
• a garden seating step;
• reinforcement for a bank beside a garden path.

If the wall needs to be substantially higher, the slope is very steep or neighbouring buildings are involved, the work should be left to a specialist contractor. This simple concrete bag technique is also insufficient on clay, peat or generally unstable ground.

Legal requirements and planning: a sketch can prevent costly mistakes

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

A basic paper plan is worthwhile:

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

Even this simple preparation helps prevent running out of concrete halfway through construction – or ending up with an understrength structure simply because “there were still a few bags left over”.

Preparing the ground: where most DIYers go wrong

By far the most common reason retaining walls lean or crack is inadequate ground preparation. Placing concrete bags directly on soil or soft topsoil is an invitation for future cracks.

Foundation depth and the frost line

The wall needs a load-bearing foundation below the frost line. In many areas, a depth of around 40 to 50 centimetres is adequate; in upland locations or places particularly prone to frost, up to one metre may be required. 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 carefully. A spirit level is essential: even a few millimetres of slope can add up over the wall’s length and create a clearly visible lean.

Geotextile and drainage to prevent water pressure

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

• Lay geotextile fabric in the foundation trench, then bring it up behind the wall later.
• Place a layer of drainage gravel or crushed stone over it.
• Optionally, install a perforated drainage pipe at the base of the proposed wall, with an outlet to the side.

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

Building the retaining wall: stack concrete bags like building blocks

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

Turn the bags correctly and lay the first course

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

Lay the first course tightly together on the prepared gravel base, leaving as few gaps as possible. It must be perfectly level. Minor adjustments can be made with a rubber mallet and gentle tamping.

Stagger joints and add reinforcement

Lay the next course with staggered joints: each bag should be offset by approximately half a bag length from the course below. This overlaps the joints and makes the structure behave like masonry. The staggered layout significantly 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, this connects the wall and its base into one solid mass.

“The cardinal mistake made by many amateur builders is to omit reinforcing bars. The wall may appear stable at first, 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 crucial stage begins: watering. Thoroughly soak each bag with a garden hose until its paper outer layer is evenly wet and settles slightly. The water then spreads through the bag, 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 against it. Only after several days – around seven days is often recommended – will the concrete have reached sufficient strength for careful backfilling to begin.

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

Backfilling, drainage and appearance: keeping the wall stable over time

After the concrete has cured sufficiently, the final major task is filling the area behind the wall. Keep drainage gravel directly against the wall; farther back, use existing soil or a well-compacted crushed stone and sand mix.

Importantly, do not put 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 part of the wall may be useful to allow water to escape in a controlled way. Combined with a drainage pipe, these greatly reduce water pressure on the wall.

There are several options for anyone who prioritises appearance:

• render the wall and match its colour to the house or terrace;
• apply natural stone slips;
• grow climbing plants such as ivy or climbing hydrangeas;
• position timber cladding or a gabion-style feature in front of it.

The one cause of failure that can ruin everything

With this method, one issue is absolutely vital: water management around the wall must be correct. Insufficient drainage, missing geotextile or simply backfilling with heavy, wet soil are the most frequent 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.”

In addition, many people underestimate the wall’s own weight. Once a concrete bag wall has cured, it is virtually impossible to correct. A crooked first course, poorly compacted ground or missing reinforcement can barely be concealed later.

Practical advice and useful additions for DIYers

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

• Check local building rules and boundary clearances.
• Assess the soil type: is the slope mainly sandy, clay-based or rocky?
• Choose a realistic maximum wall height – when in doubt, build it 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 begin with a small section measuring two to three metres. This provides a safe opportunity to practise handling the bags, compacting the gravel and watering the concrete before taking on an entire row along a slope.

The method can also be combined with other construction techniques. For instance, a low concrete bag wall can serve as a solid foundation for dry-stone walling made from natural stone above it. It is also suitable as the rear wall of a raised garden bed, provided a well-draining growing medium is used inside.

Anyone who is unsure can at least seek assistance with the planning. A brief appointment with a structural engineer or experienced landscape contractor costs less than a wall collapsing later, and helps ensure the concrete bag method is used sensibly and safely.