Yes, it is sometimes possible to build a retaining wall directly on top of existing concrete, but it is not as straightforward as it sounds. Whether it is safe and legal depends on the condition and thickness of the existing slab, the height of the proposed wall, drainage provisions, and your local council's requirements. Getting this wrong can lead to wall failure, property damage, or compliance issues — so it pays to understand the key factors before you start.
Is the Existing Concrete Suitable as a Foundation?
Not all concrete slabs or pads are designed to carry the lateral and vertical loads that a retaining wall generates. Before building on top of any existing concrete, you need to consider:
- Thickness and strength — A typical residential path or driveway slab (75–100 mm) is generally not engineered to act as a retaining wall footing. A structural footing is usually 200–300 mm deep or more, depending on wall height and soil conditions.
- Condition — Cracked, heaved, or deteriorating concrete may not provide a stable base. Any movement in the base will transfer directly to the wall above.
- Reinforcement — Is the existing concrete reinforced with steel mesh or rod? Unreinforced concrete is more prone to cracking under load.
- Soil beneath — Even sound concrete can fail if the ground underneath is soft, poorly compacted, or subject to movement (such as expansive clay soils, which are common in many parts of New Zealand).
If you are unsure about the slab's suitability, a structural engineer or experienced retaining wall builder can assess it. You can find qualified local professionals on tradedirectory.co.nz.
Building Consent — Do You Need One?
In New Zealand, retaining walls are regulated under the NZ Building Code and may require a building consent depending on their height and proximity to boundaries or structures.
As a general guide under Schedule 1 of the Building Act 2004:
- Retaining walls up to 1.5 m in height that do not support a surcharge (e.g., a driveway, building, or slope above) may be exempt from consent.
- Walls over 1.5 m, or any wall supporting a surcharge, will almost certainly require consent.
- Proximity to boundaries, drainage, or other structures can also trigger consent requirements.
Always confirm with your local council before starting work — Schedule 1 exemptions have conditions, and councils can interpret them differently. Check MBIE Building Performance for guidance on consent exemptions.
Drainage Is Critical
One of the most common causes of retaining wall failure in New Zealand is poor drainage. Water build-up behind a wall creates hydrostatic pressure that can push the wall over or cause it to slide. This is especially important when building on concrete, because:
- The existing concrete may block natural drainage paths.
- You will need to incorporate weep holes or a agricultural drain (ag drain) behind the wall to relieve water pressure.
- BRANZ provides guidance on best-practice drainage for retaining structures.
Never seal the base of a retaining wall against an existing concrete slab without a proper drainage solution in place.
Wall Types Commonly Used on Existing Concrete
| Wall Type | Suitable for Existing Concrete Base? | Notes | |---|---|---|| | Concrete block (mortared) | Sometimes | Requires adequate footing thickness and level surface | | Timber (treated pine) | Rarely | Posts typically need to be set into ground, not surface-mounted | | Segmental retaining blocks | Sometimes | Dry-stacked systems may tolerate a concrete base if level and stable | | Poured concrete | Rarely | New concrete bonds poorly to old without preparation and reinforcement | | Steel sheet pile | No | Must be driven into ground |
For segmental retaining block systems (such as those using Allan Block or similar products available at Bunnings or Mitre 10), manufacturers often specify a compacted gravel base rather than concrete. Always follow the manufacturer's installation guidelines.
Preparing the Existing Concrete Surface
If an engineer or builder confirms the existing concrete is suitable, preparation is still essential:
- Clean the surface thoroughly — remove dirt, moss, oil, and loose material.
- Check for level — the base must be level (or stepped on a slope) to ensure the wall is plumb and stable.
- Mechanical fixing — for mortared block or brick walls, you may need to drill and epoxy-set starter bars into the existing concrete to tie the new wall in structurally.
- Waterproofing and drainage — plan your drainage layer and any membrane before the wall goes up, not after.
Height Limits and Structural Engineering
For walls over 1.0 m in height, it is strongly recommended — and often required by council — to have the design reviewed by a chartered structural engineer. They will assess whether the existing concrete can handle the loads, specify any reinforcement needed, and provide producer statements that may be required for building consent.
For walls over 1.5 m, engineering input is typically mandatory. Licensed Building Practitioners (LBPs) may also be required to carry out or supervise the work where it constitutes restricted building work — confirm the applicable LBP licence classes with your council or MBIE.
When to Call a Professional
You should engage a qualified retaining wall builder or structural engineer if:
- The wall will be over 1.0 m high, or supports any surcharge load.
- The existing concrete shows cracking, movement, or deterioration.
- The site has poor drainage, steep slopes, or expansive soils.
- The wall is close to a boundary, building, or services.
- You are unsure whether building consent is required.
A professional will ensure the wall is safe, compliant with the NZ Building Code, and built to last. Find experienced retaining wall builders in your area at tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
