How do I make a fence survive high winds?

Why posts fail rather than panels, the depth and concrete detail that keep them standing, and the panel choices that reduce wind load in the first place.

Difficulty: intermediate
Time: 5 hr
Read: 4 min
Safety: warning

Short answer

Set posts about a third of their above-ground height into the ground, in a hole roughly 300mm across, on hardcore, with the concrete domed above ground level to shed water. Then reduce the load: slatted or hit-and-miss panels let wind through and survive gales that flatten close-boarded ones.

A 1.8m fence panel is a sail roughly three square metres in area, and in a gale it can be carrying a substantial load. The failure is almost always at the post — either it snaps at ground level where it has rotted, or the whole post and concrete lever out of the ground.

Safety

Check for buried services before digging post holes — electricity, gas and water runs are often shallow along boundaries. Postcrete and cement are caustic: wear gloves and eye protection. Never work on a fence in high wind; a panel caught by a gust is heavy and unpredictable. If a fence adjoins a highway or is over 2 metres high, planning rules apply.

What you'll need

Posts
Postcrete or concrete
Hardcore
Post hole digger or spade
Spirit level
Gravel boards
Metal post spurs (optional)

What makes a fence survive high wind

Step by step

Work out why the last one failed.Snapped at ground level means the post rotted where it entered the soil. Levered out with the concrete attached means the hole was too shallow or too narrow.
Set posts deep enough.About a third of the height standing above ground. A 1.8m fence therefore needs roughly 600mm in the ground, which means a 2.4m post.
Dig a wide enough hole.Around 300mm across, so there is a decent collar of concrete around the post. A narrow hole gives a thin sleeve that snaps out under load.
Put hardcore in the bottom.100mm or so of compacted hardcore under the post so the end grain is not standing in water, and so the post can drain rather than sit in a concrete cup.
Dome the concrete above ground level.Finish the concrete proud of the soil and sloped away from the post on all sides so water runs off. Concrete finished level or dished holds water against the post and rots it.
Treat cut ends and the ground zone.Cutting a tanalised post exposes untreated timber. Treat every cut, and give the section that will sit in the ground extra treatment.
Fit gravel boards.A gravel board keeps the panel itself out of contact with wet soil, and is a cheap part to replace when it eventually rots. It also reduces the panel height slightly, which reduces load.
Choose a permeable panel where wind is a problem.Hit-and-miss, slatted or louvred panels let air through and take far less load than close-boarded or lap panels. On an exposed boundary this is the biggest single improvement.
Use concrete posts or metal spurs on the worst runs.Concrete posts do not rot and are effectively permanent. Metal repair spurs bolted to a sound post and concreted in give a similar result on an existing fence.
Brace long runs and corners.End posts and corner posts take the most load. Set them deeper and consider a strut or a heavier post section at those points.
Check the fence annually.Push each post firmly. Movement now is a fallen fence in the next gale, and it is far easier to deal with in advance.
Plant a hedge in front of it if you can.A permeable hedge takes the force out of the wind before it reaches the fence, and shelters far more of the garden than the fence does.

Tips

A third of the above-ground height in the ground. That is the rule that matters most.
Dome the concrete so water runs off the post rather than into it.
A slatted panel in an exposed garden survives gales that flatten a close-boarded one.

Common mistakes

Concreting the post into a sealed cup with no drainage below — Water runs down the post and collects in the bottom of the concrete, and the post rots from the base up. Hardcore below the post lets it drain.
Finishing the concrete level with or below the soil — Water pools around the post at exactly the point where it is most vulnerable. Dome the concrete above ground level and slope it away.

If it doesn't work

Post snapped at ground level

Cause: Rot at the soil interface — Fix: Replace the post, or bolt a metal repair spur to the sound upper section and concrete that in.

Whole post and concrete levered out

Cause: Hole too shallow or too narrow — Fix: Reset deeper, in a wider hole, on hardcore, with a domed collar.

Panels blown out of the slots

Cause: Wind load on a solid panel — Fix: Fix panels to the posts with brackets or clips, and consider replacing with slatted panels.

Whole fence leaning

Cause: Progressive post movement on an exposed run — Fix: Reset the worst posts properly and consider concrete posts, then plant a permeable hedge to reduce the load.

Questions people ask

How deep should a fence post go?

About a third of the height that stands above ground. For a 1.8m fence that is roughly 600mm, in a hole about 300mm across, on a bed of hardcore.

Are slatted fences really better in wind?

Yes, substantially. Letting roughly half the wind through means the panel takes far less force, and it also shelters more of the garden behind it than a solid fence does.

Should I use concrete posts?

On an exposed boundary or where posts have rotted repeatedly, they are worth the extra cost and effort — they do not rot and last decades. They are heavier to handle and less attractive.