Yes, you can use metal post stirrups on a concrete driveway to support fence posts — it's a popular approach for NZ homeowners who want to avoid breaking up an existing concrete slab. However, getting it right depends on choosing the correct stirrup type, using the right fixings, and understanding the load your fence will place on the concrete. Done properly, surface-mounted stirrups can provide a solid, long-lasting fence foundation.
What Are Post Stirrups and How Do They Work?
Post stirrups (also called post shoes, post anchors, or post bases) are galvanised or stainless steel brackets that hold a timber or steel fence post upright on a hard surface. Instead of digging a hole and setting a post in concrete, you bolt the stirrup to the existing slab and drop the post into the bracket.
There are two main styles:
- Bolt-down stirrups — fixed flat to the surface with masonry anchors or chemical anchors.
- Drive-in stirrups — spiked into soft ground. These are not suitable for concrete driveways.
For a concrete driveway, you need bolt-down stirrups specifically rated for hard surfaces.
Choosing the Right Stirrup
Not all stirrups are equal. Key things to look for:
- Galvanised or stainless steel — essential in NZ's coastal and high-rainfall areas to prevent rust. Hot-dipped galvanised (HDG) is the minimum; stainless steel is better near the coast.
- Correct post size — stirrups are sized to match the post (e.g. 90 mm × 90 mm, 100 mm × 100 mm). A loose fit reduces strength significantly.
- Load rating — check the manufacturer's datasheet for the stirrup's lateral (side) load capacity. A tall or solid fence in a windy location puts significant force on each post base.
- Adjustable vs fixed — adjustable stirrups allow minor alignment corrections after fixing, which is handy on an uneven driveway surface.
You'll find a range of bolt-down post stirrups at Bunnings, Mitre 10, and PlaceMakers.
Fixing Stirrups to Concrete — Getting It Right
This is the most critical step. A stirrup is only as strong as its fixing into the slab.
Concrete Anchor Options
| Anchor Type | Best For | Notes |
|---|---|---|
| Sleeve anchors | General use, solid concrete | Easy to install, good holding power |
| Wedge anchors | Heavier loads, thicker slabs | Very strong, not removable |
| Chemical anchors | Cracked or thinner concrete | Resin-bonded, excellent strength |
| Dynabolts | Light-duty, thinner slabs | Common DIY option, check load rating |
For most residential fencing, sleeve anchors or wedge anchors (often called dynabolts in NZ trade parlance) are the go-to choice. Chemical anchors from brands like Sika or Ardex are worth considering if your slab is thinner than 100 mm or shows any cracking.
Minimum Slab Thickness
As a general guide, your concrete slab should be at least 100 mm thick for standard bolt-down stirrups with sleeve or wedge anchors. Thinner slabs risk cracking around the fixing point, especially under lateral fence loads. If you're unsure of your slab thickness, a fencer or concretor can advise.
Installation Tips
- Mark out post positions carefully before drilling — mistakes in concrete are hard to fix.
- Use a hammer drill with a masonry bit sized to match your anchor.
- Blow out the hole with compressed air before inserting the anchor — dust reduces holding strength.
- Torque anchors to the manufacturer's specification.
- Allow chemical anchors to fully cure before loading the stirrup.
- Check stirrups are plumb and aligned before tightening fully.
Wind Loads and Fence Height
NZ can be a windy place, and fence design needs to account for this. BRANZ and the NZ Building Code both recognise wind as a significant structural consideration. A tall, solid fence (like a 1.8 m timber paling fence) acts like a sail and puts enormous lateral force on each post base.
If your fence is:
- Over 1.5 m tall, or
- Solid-panel construction, or
- In a high-wind zone (Wellington, exposed coastal sites, Canterbury nor'west areas)
…then you should get a fencer or structural engineer to confirm your stirrup and anchor selection is adequate. Don't just guess on a windy site.
Building Consent — Do You Need One?
Most residential boundary fences under a certain height may be exempt from building consent under Schedule 1 of the Building Act — the exact threshold and conditions depend on your situation, but this depends on your specific situation, local council rules, and whether the fence is on a boundary, retaining anything, or near a public space. Always check with your local council before starting. You can find general guidance at MBIE Building Performance.
Timber Post Care
One advantage of stirrups over in-ground posts is that the timber post sits above the concrete surface, improving airflow and reducing ground contact rot. Make sure you're using H3.2 or H4 treated timber for any posts that may be exposed to moisture or ground splash — this is standard NZ practice for fence posts.
When to Call a Professional
Fencing is generally DIY-friendly, but consider hiring a qualified fencer if:
- Your fence is tall, solid, or in a high-wind area and you're unsure about load calculations.
- The driveway slab is cracked, thin, or of unknown condition.
- You need the fence to meet specific boundary or resource consent requirements.
- You simply want the job done right the first time.
Fencing is not restricted trade work under NZ law (unlike electrical or plumbing), so any competent person can do it — but a professional fencer brings experience and the right tools. You can find qualified local fencers on tradedirectory.co.nz.
Safety note: When drilling into concrete, always wear eye protection and a dust mask. Be aware of any services (pipes, cables) that may run under the slab before drilling. If in doubt, use a cable/pipe detector first.
