Installing a timber fence on an existing concrete footing is very achievable as a DIY project, provided you use the right fixings and technique. The key is to avoid splitting the concrete by using purpose-made post anchors or shoes, drilling correctly, and keeping fixings well away from edges. Here's how to do it properly in a New Zealand context.
Understand Why Concrete Cracks
Concrete is strong under compression but weak under tension. When you drive a fixing into it — especially near an edge — the expanding force can cause radial cracking that spreads outward like a spider web. The most common causes are:
- Drilling or fixing too close to the slab edge or an existing crack
- Using the wrong type of anchor (e.g. a nail-in anchor in thin concrete)
- Over-tightening expanding bolts
- Using a post that traps moisture and causes freeze-thaw movement (less common in most NZ climates, but relevant in alpine areas)
Choose the Right Post Anchor System
The safest and most reliable method is to use a galvanised steel post shoe (post anchor). These are surface-mounted brackets that bolt to the concrete and hold the timber post above the slab, which also improves drainage and extends post life.
Common options available at Bunnings and Mitre 10 include:
- Adjustable post shoes – allow minor alignment tweaks after fixing
- Fixed post shoes – simpler and slightly stronger once set
- Standoff post bases – raise the post 50–75 mm off the concrete to prevent moisture wicking
For fencing, a standoff or raised post base is generally preferred because it keeps the end grain of the timber dry, significantly reducing rot.
Mark Out and Plan Your Fixing Positions
Before you drill anything:
- Snap a chalk line along the intended fence line.
- Mark each post position — typically 1.8 m to 2.4 m centres for standard paling or picket fences.
- Check that each anchor position is at least 75–100 mm from any slab edge, control joint, or existing crack. This is the most important rule for preventing cracking.
- If the footing is narrow (under 200 mm wide), consider whether surface-mounted anchors are even suitable — you may need to consult a builder or engineer.
Drill Correctly
Use a rotary hammer drill (SDS drill) with a masonry bit sized to match your anchor manufacturer's specification — typically 10–12 mm for M10 or M12 anchors.
Drilling tips:
- Keep the drill perpendicular (90°) to the surface. An angled hole causes uneven load and can crack the concrete.
- Drill to the correct depth — usually marked on the anchor or specified in the datasheet. Too shallow and the anchor won't grip; too deep wastes effort.
- Blow out the hole with compressed air or a bulb blower and brush it clean. Dust left in the hole dramatically reduces anchor holding strength.
- Avoid hammer-only mode near edges — use rotary-hammer mode and let the bit do the work gently.
Choose the Right Anchor Bolt
For post shoes on concrete footings, you have two main options:
| Anchor Type | Best For | Notes |
|---|---|---|
| Wedge anchor (expansion bolt) | Thick concrete (>150 mm) | Strong, permanent; don't over-torque |
| Chemical anchor (epoxy) | Thin slabs, near edges, cracked concrete | Slower to set but gentler on concrete |
| Screw anchor (Tapcon-style) | Light-duty or thinner slabs | Easier to install; lower load rating |
Sika and Hilti both offer chemical anchor systems widely used in NZ. For most residential fence post shoes, a quality wedge anchor or screw anchor is sufficient — check the post shoe manufacturer's fixing specification.
Do not over-tighten. Use a torque wrench if specified. Over-torquing an expansion anchor is one of the most common causes of cracking.
Set the Post Shoe and Timber Post
- Insert the anchor bolt into the cleaned hole.
- Place the post shoe over the bolt and tighten to the specified torque — finger-tight plus the recommended turns, or use a torque wrench.
- Check the shoe is plumb and aligned with your chalk line before fully tightening.
- Slot the timber post into the shoe. Use H3.2 or H4 treated timber for any post in contact with or close to concrete and ground level — this is required under the NZ Building Code for durability.
- Fix the post to the shoe using the supplied galvanised bolts or coach screws — don't substitute with untreated fasteners.
Allow for Movement
Timber expands and contracts with moisture and temperature. If you're running a long fence run, control joints in the fence design (a slight gap between panels, or a break every 6–8 metres) help prevent the fence from racking and putting stress back onto the concrete anchors.
Does This Work Require a Building Consent?
In most cases, a standard residential boundary fence under 2.5 m high is exempt from building consent under Schedule 1 of the Building Act, but this depends on your specific site, zone, and council rules. Always confirm with your local council before starting. Some areas (e.g. near boundaries, in special character zones, or on sloped sites) have additional requirements. Check MBIE Building Performance for general guidance.
When to Call a Professional
Consider hiring a qualified fencer if:
- The concrete footing is cracked, thin, or of unknown quality
- You need to install posts into a structural slab (e.g. a driveway or building foundation)
- The fence is over 2.5 m high or on a retaining structure
- You're unsure about the fixing specification or load requirements
A Licensed Building Practitioner (LBP) may be required for certain fencing work — check LBP for details. You can find experienced local fencers on tradedirectory.co.nz.
