Installing a fence on top of a concrete block wall is a popular way to add height, privacy, and style to a boundary in New Zealand. With the right preparation and fixings, it's a project a competent DIYer can tackle — but there are a few important structural and regulatory considerations to get right first.
Check Your Boundary and Consent Requirements
Before you pick up a drill, confirm where your legal boundary sits. Installing a fence on the wrong side of a boundary wall can cause disputes with neighbours. Check your property title and, if in doubt, have a surveyor peg the boundary.
In New Zealand, fences and walls are generally covered by Schedule 1 of the Building Act 2004, which lists work that may be exempt from a building consent. However, the total height of your structure — wall plus fence — matters. As a general guide, a fence that brings the combined height above a certain threshold, or that is in a special character or heritage zone, may require a building consent. Always confirm with your local council before starting work, as rules vary by district. You can find guidance at MBIE Building Performance.
Also consider the Fencing Act 1978, which governs cost-sharing with neighbours for boundary fences.
Assess the Existing Block Wall
Not all concrete block walls are built to carry the extra load and wind forces of a fence. Before proceeding:
- Check the wall's condition — look for cracking, spalling, or signs of movement. A compromised wall may need a builder or structural engineer to assess it.
- Confirm the wall is solid or grouted — hollow, ungrouted blocks may not hold anchor bolts reliably.
- Consider wind loads — New Zealand has significant wind zones (as defined in NZS 3604). A tall fence in a high-wind zone puts considerable lateral force on the wall and its footings.
If you have any doubts about the wall's structural integrity, consult a Licensed Building Practitioner (LBP) before proceeding. Find one at Licensed Building Practitioners.
Choose Your Fence Posts and Fixing Method
The most common approach is to fix steel post shoes or base plates to the top of the block wall, then slot or bolt fence posts into them. Here are your main options:
| Method | Best For | Notes | |---|---|---|| | Bolt-down post base plate | Solid/grouted block walls | Uses chemical anchors or expanding bolts into the block | | Core-fill post | New or being-rebuilt walls | Post set into a hollow core filled with concrete | | Surface-mount bracket | Lighter fences, decorative use | Bolted to the wall surface; less robust for tall fences | | Welded steel frame | Metal/aluminium fencing | Custom fabricated; very strong but requires a metalworker |
For most timber or aluminium privacy fences, a bolt-down post base plate fixed with chemical anchors (such as those from Sika) into solid block is the most reliable DIY-friendly method.
Step-by-Step Installation Guide
1. Plan Your Post Spacing
Standard post spacing is 1.8 m to 2.4 m centres for timber paling or lapped-and-capped fences. Mark post positions along the wall with a chalk line to keep them straight.
2. Drill Anchor Holes
Using a rotary hammer drill with a masonry bit, drill holes into the top of the block wall to the depth specified by your anchor bolt or chemical anchor system. Blow out dust with a bulb blower — this is critical for chemical anchors to bond correctly.
3. Install the Anchors
Follow the manufacturer's instructions precisely. For chemical anchors, inject the resin, insert the threaded rod, and allow the full cure time before loading (this can be several hours depending on temperature).
4. Fit the Post Base Plates
Slide the base plate over the threaded rod, check it is plumb and aligned, then tighten the nut. Use a spirit level on two faces of the plate before fully tightening.
5. Set the Posts
Slot your timber or aluminium posts into the base plates and secure with the supplied bolts. Check each post for plumb before tightening.
6. Fix the Rails and Infill
Attach horizontal rails between posts, then fix your chosen infill — timber palings, lapped weatherboards, aluminium slats, or trellis. Work from one end to the other, checking for level as you go.
7. Finish and Treat
For timber fences, apply a quality exterior stain or paint to all cut ends and surfaces. Products from Resene or Dulux are widely available at NZ hardware stores and are formulated for our UV and moisture conditions. Treat cut ends before fixing where possible.
Safety Notes
- Working at height: If the wall is more than 1 m high, take care when working on top of it. Use a stable platform or scaffold rather than leaning over.
- Dust: Drilling masonry generates silica dust. Wear a P2 respirator as required by WorkSafe NZ guidelines.
- Underground services: Before drilling into the wall base or nearby ground, check for buried cables or pipes.
When to Call a Professional
Consider hiring a qualified fencer or builder if:
- The combined wall-and-fence height exceeds 2 m, or a building consent is required
- The block wall shows structural damage or movement
- You are in a high or very high wind zone and need engineered post sizing
- The fence runs along a retaining wall (different structural rules apply)
- You want a custom steel or aluminium frame welded to the wall
A professional fencer will also know local council requirements and can ensure the job is done to a standard that protects your property value. You can find experienced local fencers on tradedirectory.co.nz.
