The fence had been up four months when a southerly gale rolling in off Cook Strait put six panels flat across a driveway in Miramar. The posts themselves were sound; they’d simply been set 450 millimetres into the clay with a bag of concrete apiece, which is about right for a sheltered Kelburn backyard and nowhere near enough for a corner section catching the full run of wind off the strait.
Why Miramar breaks fences that would survive elsewhere
Miramar sits on the exposed southern edge of the harbour, with very little standing between most sections and the wind coming straight off Cook Strait. Wellington as a whole records gusts over 60 kilometres an hour on well over a hundred days a year, and the peninsula suburbs feel more of that than the sheltered inner-city valleys do. Add heavy clay soil that swells when wet and shrinks when it dries, and a fence post that would happily stand for decades in a drier, calmer suburb can work itself loose within a couple of winters here. The two problems compound each other: wind loads the panel, and clay movement loosens the post’s grip in the ground at exactly the wrong moment.
Post depth and footing size for clay
As a rough guide, a standard 1.8 metre timber fence in an exposed Miramar location wants posts set at least 600 to 750 millimetres deep, with 900 millimetres a reasonable target on a fully open corner section or anywhere close to the coastline. Footing diameter matters as much as depth — a narrow post hole gives the concrete very little to resist overturning, while a footing 300 to 350 millimetres across gives the wind load somewhere to be absorbed rather than simply levering the post out of the ground. Heavier round or square posts, rather than the thinner stock sometimes used on sheltered sections, also help resist the twisting load that gusty wind puts through a fence line.
Concrete, backfill and drainage around the post
Clay holds water, and a post hole that becomes a sump for surface runoff will keep the base of the post wet long after the surrounding ground has dried out, encouraging rot in timber posts and corrosion around galvanised steel ones. A layer of coarse metal or shingle at the bottom of the hole, below the concrete, gives that water somewhere to go rather than sitting against the post base. The concrete itself should be domed slightly above ground level and sloped away from the post, so rain sheds off rather than pooling at the exact point where timber meets concrete.
Panel design and wind load
A solid paling fence catches far more wind than one built with gaps, louvres, or a lattice top, and that difference matters more in Miramar than almost anywhere else in the city. Where the section is fully exposed, it’s often worth trading some privacy for a slatted or gapped design rather than paying for oversized footings under a solid panel that’s still likely to flex and loosen fixings over time. Fence height above roughly two metres, or any fence built on a retaining structure, generally warrants a specific engineering check on post spacing and footing size rather than a rule of thumb.
Getting post depth and footing size matched to the exposure a fence will actually see is worth checking properly before rails go up, particularly on an open corner section catching wind from more than one direction. It’s a conversation worth having directly with a Wellington fencing contractor who has priced jobs on this kind of ground before, rather than assuming a standard spec will hold.
Frequently asked questions
How deep should fence posts be set in Wellington clay? For a standard 1.8 metre fence, 600 to 750 millimetres is a reasonable minimum on an exposed section, increasing toward 900 millimetres on a fully open corner or coastal-facing boundary.
Why do fence posts loosen in clay soil over time? Clay expands when saturated and contracts as it dries, and that repeated movement gradually works a shallow-set post loose in its footing, especially where drainage around the base is poor.
Does a solid fence need bigger footings than a slatted one? Generally yes, because a solid panel presents far more surface area to the wind, which increases the overturning load the footing has to resist.