Retaining walls are one of the most technically demanding elements of residential construction, and one of the most frequently done incorrectly. A wall that holds back soil on a sloped property is under continuous lateral pressure from the retained earth — and that pressure increases significantly when the soil is saturated after rain. A wall that was built without adequate drainage, without the correct footing depth, or without engineering input where it was required is not just aesthetically problematic: it is a structural failure waiting to happen, and when retaining walls fail, they fail suddenly and completely.
Understanding the basics of what makes a retaining wall work will help you evaluate contractors, ask the right questions, and avoid one of the most common and expensive mistakes in home construction.
When an engineer is required
South African building regulations require that any retaining wall above 1.0m in height have engineering input. In practice, this means:
- Walls above 1.0m height must be designed by a registered professional engineer (PrEng) or a competent person as defined in the NHBRC guidelines
- The design must address soil conditions, surcharge loads (any structures, vehicles, or other loads above the wall), drainage, and structural capacity of the chosen material
- In many municipalities, building plans approval is required for retaining walls above 1.0m
Walls under 1.0m in nominal height that retain undisturbed, well-drained soil with no surcharge loads above can be constructed without engineering drawings in most cases — but the drainage principles described below still apply, and the contractor must understand them.
If you are looking at a significant retaining wall — above 1.5m, near a property boundary, near a structure, on expansive clay soils, or where the retained soil will bear additional loads — insist on engineering involvement. The fee for a structural engineer's assessment and drawing for a typical residential retaining wall is R3,000–R8,000. Against the cost of a wall failure, this is insignificant.
Drainage — the most critical element
The overwhelming majority of retaining wall failures in South Africa are caused by inadequate drainage. When water saturates the retained soil, the pressure on the wall increases dramatically — in some soil types, the pressure when saturated can be three to four times the pressure in dry conditions. A wall that was adequate for dry conditions can fail completely after heavy rain if the drainage is not correctly designed and built.
A properly drained retaining wall includes:
- Free-draining backfill: The material immediately behind the wall should be permeable — crushed stone or gravel, not clay or compacted soil. This allows water to drain vertically down to the weepholes or drainage pipe rather than building up pressure against the wall face.
- Geotextile filter fabric: A geotextile membrane between the free-draining gravel and the natural soil prevents fine particles from migrating into the drainage layer and blocking it over time.
- Weepholes: Openings through the wall face at regular intervals (typically every 1.5–2m horizontally) at the base of the wall allow water to escape. Weepholes that are too small, too widely spaced, or obstructed by mortar during construction defeat their purpose.
- Drainage pipe: For walls above 1.5m or in high water table areas, a perforated agricultural drain pipe at the base of the wall, connected to a discharge point, provides more reliable drainage than weepholes alone.
When you ask a contractor how they will handle drainage for your retaining wall, their answer tells you almost everything you need to know about their competence. A contractor who describes free-draining gravel backfill, geotextile, and weepholes in concrete or precast walls knows what they are doing. A contractor who talks only about the wall face and avoids the drainage question does not.
Material options
Concrete block (hollow or solid) is the most common residential retaining wall material in South Africa. Concrete blocks are available in many profiles and can be plastered or face-brick finished. They require a proper concrete strip footing below ground level, reinforced verticals filled with concrete in hollow block walls, and the drainage provisions described above. Well-built concrete block walls are durable and economical.
Face brick is used where a higher quality finish is required. Structurally similar to block, but with the visual quality of exposed face brick. More expensive than block construction but can match the aesthetic of existing structures on the property.
Precast concrete retaining systems (interlocking precast blocks like Terraforce, Beton Brett, or similar systems) are engineered interlocking block systems designed specifically for retaining walls. They are self-draining through the gaps between blocks (eliminating the need for separate weepholes), quick to install, and available in various finishes. They are particularly good for curved or terraced retaining walls. Higher cost per metre than block, but faster installation and engineered performance. The blocks must be installed on a level, compacted base according to the manufacturer's specification.
Gabion walls (wire mesh baskets filled with stone) are increasingly used for larger retaining walls and riverbank stabilisation. They are permeable by nature (no drainage issues), aesthetically distinctive, and absorb energy from earth movement. They require geotextile behind them to prevent soil loss through the stone fill. Gabion walls need a contractor experienced in gabion construction — incorrectly tensioned wire baskets bulge and deform over time.
Timber (railway sleepers or treated pine sleepers) is used for garden retaining walls under 1.0m where aesthetics are the priority. Timber retaining walls have a limited service life (typically fifteen to twenty years for treated pine, longer for hardwood sleepers) and require the same drainage principles as masonry walls. They are not appropriate for walls subject to vehicular surcharge loads.
Mass concrete is used for walls requiring very high structural capacity — typically walls retaining significant height with surcharge loads, or walls near structures. Always requires engineering design.
Footing requirements
A retaining wall without an adequate footing will overturn, slide, or settle unevenly. The footing must be below the depth of seasonal ground movement (typically 400–600mm in most South African conditions, deeper in frost-prone areas), wide enough to resist overturning forces, and cast in stable ground. In expansive clay soils (common in many Gauteng and Limpopo areas), footings may need to be deeper or wider to account for seasonal soil movement.
Ask any contractor what their footing specification is. A proper answer will include depth below ground level, width, and reinforcement (for walls above 1.0m, reinforced concrete footings are standard practice).
Questions to ask a contractor
For a wall above 1.0m, will you provide engineering drawings? This is not optional. If the contractor says drawings are not required for walls of that height, they are either unfamiliar with the regulations or are advising you incorrectly.
How will you handle drainage behind the wall? As above — this question separates informed contractors from uninformed ones.
What is your footing specification? Depth, width, reinforcement, and whether the footing will be inspected before backfilling.
What backfill material will you use, and where will you source it? Free-draining gravel or crusher run behind the wall, compacted fill elsewhere. Clay or expansive subsoil used as backfill is a common mistake.
Do you have examples of retaining walls you have built that are more than five years old? Long-term performance is the real test. A wall that looks good at completion but develops movement or drainage problems in year two or three reveals poor construction. Ask for references specifically from clients whose walls have been standing for several years.
What to budget
Concrete block retaining wall (supply and build, per linear metre at 1.0m height): R800–R1,500/m
Concrete block retaining wall at 1.5m height: R1,200–R2,200/m
Terraforce or equivalent precast interlocking system (per linear metre at 1.0m height): R1,200–R2,000/m
Face brick retaining wall (per linear metre at 1.0m height): R1,500–R2,500/m
Gabion wall (per linear metre at 1.0m height): R1,800–R3,000/m depending on stone type and availability
Structural engineer fee for wall design: R3,000–R8,000
Note that these prices assume correct drainage construction. Any quote that appears to be significantly cheaper than these ranges should be interrogated — drainage materials (geotextile, gravel, drainage pipe) have a real cost, and a quote that omits them is saving money on what is functionally the most important part of the construction.
Red flags
No mention of drainage. Any contractor who quotes a retaining wall without discussing drainage is either uninformed or planning to omit it.
Dismissing the need for an engineer on walls above 1.0m. This is a legal and structural requirement, not a bureaucratic inconvenience.
Using excavated soil as backfill. Reusing clay or expansive subsoil as backfill directly behind a retaining wall is one of the most common causes of premature wall failure. Ask specifically what material will go behind the wall.
Shallow footings on visual grounds. A footing that is barely below the soil surface is not adequate for a structural retaining wall. If you can observe footings before backfilling, check the depth.
No building plans for walls requiring approval. In most municipalities, retaining walls above 1.0m require building plans submission and approval. A contractor who skips this step leaves you with a structure that is non-compliant with local bylaws and may create problems when you sell the property.
Final note
A retaining wall is a structural element, not just a garden feature. The principles that make it work — correct drainage, adequate footings, appropriate materials, and engineering input where required — are non-negotiable for anything that holds back significant soil. Spend the time to find a contractor who understands this, and do not allow cost pressure to compromise the drainage details. A wall built correctly will last for decades without problems. One built incorrectly will be a recurring expense and a potential hazard.