The most reliable way to stop squirrels, foxes and cats climbing your fence is to fit physical deterrent spikes along the top rail — but only if those spikes have irregular, staggered heights. Single-height flat spikes stop birds from landing but let climbing animals walk straight across them without discomfort. Staggered spikes at different heights disrupt the foot-placement that climbers rely on, making the fence top an unreliable surface they quickly learn to avoid.
Why ordinary bird spikes don't stop climbing animals
Most bird spikes on the market are designed around one problem: stopping pigeons from landing. They do that by presenting a flat row of upward-pointing prongs at roughly the same height. A pigeon can't land because there's no flat perch.
A squirrel, fox or cat is a completely different challenge. These animals don't land — they walk. They approach the fence top at speed and place their feet between or around the spikes. If every spike is the same height, there's a predictable rhythm to them, and a confident climber simply steps over each one in sequence. You've seen it happen: you fit cheap spikes, feel smug for a week, then watch a squirrel trot straight across them without breaking stride.
The fix is a mixed-height spike design. When spikes alternate between, say, 12 mm, 22 mm and 36 mm in no predictable pattern, there's no repeatable foot placement. The animal can't read the surface, loses confidence, and turns back. That's the principle behind ThornGuard's three-height design — 12 mm, 22 mm and 36 mm — and it's what separates a climbing deterrent from a perching deterrent.
Which animals are you actually dealing with — and does it change the approach?
Not much, in practice. Squirrels, foxes and cats all use fence tops as transit routes, and all three respond to the same physical deterrent. The differences worth knowing:
Squirrels are the lightest and most agile. They'll attempt a fence top repeatedly if there's a food reward on the other side — a bird feeder, fruit tree or vegetable plot. Covering every access point matters more with squirrels than with any other animal, because they will find the one gap you left.
Foxes are heavier and usually more cautious. They use fence tops as shortcuts between gardens at night. A fox that finds a fence top uncomfortable will typically reroute rather than push through — making deterrent spikes effective even when coverage isn't perfect. That said, a fox can also clear a 6-foot fence from a standing jump if it's motivated enough, so spikes on the top need to be combined with removing the motivation (securing bins, not leaving food out).
Cats — whether your own or a neighbour's — are persistent and nimble. They'll test a fence top cautiously before committing, which actually makes them more deterrable than squirrels: if the surface feels wrong, they'll back off before getting far. The challenge is that cats jump cleanly onto the fence top from some distance, so the spikes need to cover the full width of the top board, not just sit along one edge.
How to fit deterrent spikes so climbing animals can't get around them
Fitting spikes correctly is as important as choosing the right spikes. The most common mistake is running spikes along only the flat top of the fence while leaving the post tops, gate tops and any rail edges uncovered. Climbing animals will find those gaps.
Step 1: Measure the full run. Include the fence panels, every post top, any gate rails and any horizontal rails that run along the outside face of the fence. Animals don't skip sections — your coverage shouldn't either.
Step 2: Choose a spike strip that wraps. A flat strip only covers the top face. A strip that folds along its length can wrap over the top and down both sides of a fence rail, covering three faces with one piece. This matters most for posts and rails, where an animal approaching from the side would otherwise find an uncovered surface to grip.
Step 3: Cut to fit. Decent spike strips can be cut across each section with scissors or a utility knife, so you can work around post tops and awkward junctions without leaving gaps. Some strips also split lengthways, letting you cover a wider run from a single pack.
Step 4: Fix securely. On timber fences, screws are the most permanent fix — pre-drill if the wood is dry to avoid splitting. Cable ties work well on rails and post tops where drilling isn't practical. On surfaces you can't drill — rendered walls, uPVC fascias, gates — waterproof adhesive pads rated for outdoor use are the alternative.
Step 5: Check for bypass routes. Stand back and look at what an animal approaching from each direction would actually encounter. Overhanging branches, a shed roof that butts against the fence, a compost bin pushed against the panels — all of these let an animal bypass the fence top entirely. Clear them before or alongside fitting the spikes.
What about the fence posts specifically?
Fence posts are often overlooked and are a genuine weak point. A squirrel or cat approaching from outside will often run straight up the post face rather than climbing the panel. If the post top is bare wood, they'll simply use it as a stepping stone over whatever you've fitted to the panels.
Post tops need their own coverage. A spike strip that folds all four ways — over the top, down both sides, and along the front face — deals with this neatly. Alternatively, cut a short section of strip and fix it flat to the post top as a separate piece. It doesn't need to be elegant; it needs to be there.
Do deterrent spikes harm animals?
No — and this is worth being direct about. Deterrent spikes, when correctly specified, cause discomfort rather than injury. The animal feels an unreliable, uncomfortable surface and chooses not to continue. They don't get trapped, cut or stuck. The spikes that have a safety record are made from polypropylene with rounded or blunted tips — not sharpened metal.
ThornGuard spikes are made from UV-stabilised virgin polypropylene with this in mind. The goal is to make the fence top somewhere an animal decides isn't worth using, not to injure it. This also means the spikes are legal under UK law: you're not setting a trap or injuring wildlife, you're modifying a surface.
One thing UK law is clear on: if you discover a bird has built an active nest on or near your fence, leave it alone until the nesting season is over. Spikes can be fitted around it, but an active nest with eggs or chicks cannot be disturbed. If you're unsure what counts as active, the RSPB guidance is the right reference point.
How long will the spikes last outdoors?
This is where material quality actually matters. Cheap spikes made from recycled or unmodified polypropylene go brittle in UV light — one winter of freeze-thaw cycles and they're snapping under light pressure. We've heard from plenty of people who fitted budget spikes in spring, had them disintegrate by the following January, and had to start over.
UV-stabilised virgin polypropylene resists this. The polymer chains are more consistent and the UV stabilisers prevent the photodegradation that makes cheaper plastics brittle. It's not a dramatic difference in appearance — but it's significant in lifespan.
Common questions
Will spikes stop a fox that's determined to get over the fence?
Deterrent spikes make the fence top uncomfortable and unpredictable, which is usually enough to redirect a fox to an easier route. A fox that is highly motivated — by food waste, a denning site or cubs — may persist, so removing the attractant (securing bins, not leaving pet food outside) matters alongside fitting spikes. Spikes alone won't fix a food-incentive problem.
Do I need to cover every fence panel, or just the sections where I see animals crossing?
Covering only the sections you've observed is a reasonable start, but climbing animals will explore along a fence line if their usual crossing point is blocked. Squirrels in particular will systematically test for gaps. If you have the means to cover the full run, it gives more reliable results than patchy coverage.
Can I fit spikes to a fence I share with a neighbour?
You can fit spikes to your side of a shared boundary fence without needing permission, as long as the spikes face inward onto your property or sit on the top surface. Spikes that project over the boundary or that your neighbour could reasonably consider a nuisance are a different matter — a quick conversation usually resolves any concern before it becomes one.
Originally published at ThornGuard
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