Can I put spikes or barbed wire on top of my fence?

The occupiers' liability position in plain terms, the height and signage conventions for anti-climb products, and the lawful deterrents that work at least as well.

Difficulty: beginner
Time: 20 min
Read: 5 min
Safety: warning

Short answer

It is not automatically illegal, but you owe a duty of care even to trespassers, so anything likely to injure exposes you to a claim and potentially to prosecution. Keep anti-climb products above about 2.4 metres, on your own structure, with clear warning signs — and prefer a prickly hedge, a taller solid gate, lighting and gravel, which are effective and carry none of the risk.

This question comes up constantly and the honest answer is more nuanced than either extreme you will read online. Occupiers' liability legislation imposes a duty even towards people who are not there lawfully, and there is a long-standing prohibition on setting anything designed to inflict injury. That does not ban every deterrent, but it does put the burden on you to show that what you fitted was reasonable.

Safety

Anything you fit that is capable of injuring somebody can leave you liable for that injury, including to a trespasser, and in some circumstances it can amount to a criminal offence. Never fit broken glass, spring guns, blades or any trap. If you are considering deterrents because you feel unsafe, that is a conversation with the police rather than with a hardware shop.

Step by step

Start from the duty you actually owe.Occupiers' liability law imposes a duty of care to lawful visitors, and a narrower but real duty towards trespassers in respect of dangers you know about. Fitting something sharp is knowing about the danger.
Rule out anything designed to injure.Broken glass set in mortar, blades, spring-loaded devices, traps, or electrified wire on a domestic boundary. These are not grey areas — they carry real criminal and civil liability and should never be fitted.
Understand where anti-climb products are conventionally acceptable.Rotating anti-climb toppings, anti-climb paint and similar products are widely used above roughly 2.4 metres, on your own building or structure, out of reach of the highway and of anyone passing lawfully. Below that height, and anywhere a person could brush against it, the position is much weaker.
Sign it clearly.Warning signs at regular intervals and at eye level, visible from every approach, in daylight and at night. Anti-climb paint in particular is normally used with specific warning signage, and unsigned use is a poor position to be in.
Check whose fence it is.You cannot fix anything to a boundary structure you do not own. Adding spikes to a neighbour's fence is both trespass and, if someone is hurt, an argument nobody wins.
Keep it away from the highway and shared access.Anything overhanging a pavement, a shared alley or a right of way brings the highway authority into it and hugely increases the chance of injuring someone entirely innocent — a child, a delivery driver, a window cleaner.
Prefer the lawful deterrents, which genuinely work.A dense prickly hedge, a taller solid gate, gravel underfoot, a PIR light, trellis on top of a fence that will not take weight, and removing climbing aids. These deter effectively and carry no liability tail.
Tell your insurer if you fit anything.Public liability cover under a home policy is what would respond to an injury claim. Check that fitting anti-climb measures does not affect it.
Escalate a real threat properly.Repeated trespass, intimidation or targeting is a police matter, and the police also run free crime prevention advice through designated officers. That is a much better route than improvised deterrence.

Tips

If your reason for asking is repeated intrusion, ask your local police force about crime prevention advice — many forces will assess a property free of charge.
Rotating anti-climb toppings are designed to spin rather than cut, which is a much better legal position than spikes.
Whatever you fit, photograph the signage and the installation date. If there is ever a claim, showing you took reasonable care is the whole defence.

Common mistakes

Fitting spikes at head height along a shared alley — The people most likely to be injured are lawful passers-by — a delivery driver, a child, a neighbour. That is the scenario where liability is clearest and least defensible.
Using anti-climb paint with no signage — The paint stays slippery indefinitely and transfers to skin and clothing. Without prominent warning signs you have created a hidden hazard, which is the least defensible version of it.

If it doesn't work

Neighbour objects to spikes on a shared boundary

Cause: Fitted to a structure that is not yours, or overhanging their land — Fix: Remove them. Establish who owns the boundary before doing anything, and put your deterrence on your own side.

Council or highways asks you to remove something

Cause: Projection over a public footway — Fix: Comply promptly. Anything overhanging the highway is their jurisdiction and it is not an argument worth having.

People keep climbing the fence anyway

Cause: A climbing aid nearby, or the fence is simply too low — Fix: Remove the bins, log store or furniture beside it and add trellis for height. That combination solves it far more often than anything sharp.

Alternatives

Defensive planting: Pyracantha, berberis, hawthorn, holly and blackthorn form barriers people do not push through. Lawful, effective, and valuable for nesting birds and insects. It takes two or three seasons to establish.
Trellis on top of a fence: Adds height, will not take a person's weight, and is entirely uncontroversial. It is the standard answer to a fence that is slightly too low.
Gravel and lighting: Noise underfoot and a PIR light on the approach change behaviour without touching anybody. Cheap and immediate.

Questions people ask

Is barbed wire illegal on a garden fence?

Not automatically, but on a domestic boundary it is a poor idea: it can injure lawful passers-by, and where it adjoins a public highway a local authority can require its removal as a nuisance. It also looks like the sort of measure that invites attention rather than deflecting it.

What about anti-climb paint on a drainpipe?

It is commonly used above 2.4 metres on a building you own, with warning signs. Below that height, or where anyone might touch it in normal use, it becomes a hazard you created.

Can I be sued by someone who was breaking in?

In principle yes, which surprises people. The duty towards trespassers is narrower than towards visitors but it exists, and deliberately setting something to injure is the fact pattern most likely to succeed against you.