How do I stop rats getting into my compost bin?

What actually attracts rats to a heap, the physical changes that exclude them, and why the usual folk deterrents do nothing.

Difficulty: beginner
Time: 1 hr 30 min
Read: 4 min
Safety: warning

Short answer

Rats come for food and stay for the dry shelter. Stop adding cooked food, meat, fish, dairy, bread and pet waste, keep the heap damp and turn it regularly, and stand the bin on fine galvanised wire mesh so they cannot tunnel up into it. A heap that is wet, disturbed and holds no cooked food is not worth their trouble.

A compost bin does not create rats. It offers them two things they want: an undisturbed, dry, warm cavity, and, if you are adding the wrong things, food. Remove both and they move on, because rats live in most urban and suburban gardens whether there is a compost bin or not.

Safety

Rat urine can carry leptospirosis, which is a serious illness. Wear gloves whenever you handle a heap rats have used, cover any cuts, never eat or smoke while working, and wash your hands thoroughly afterwards. Damp the material down before turning it so you do not breathe dust. Keep tetanus cover current. If you use bait, remember it is lethal to hedgehogs, cats, dogs and birds of prey that eat poisoned rats — professional control is the safer route.

What you'll need

Fine galvanised wire mesh, roughly 6mm
Spade
Fork
Paving slab for the base (optional)

Step by step

Stop adding anything cooked.Cooked food, meat, fish, dairy, bread, oil and pet waste. These are the reason a heap becomes a destination rather than somewhere they pass through. Raw vegetable peelings are far less attractive.
Bury kitchen waste rather than tipping it on top.Anything visible on the surface is an advertisement. Pull the material aside, add the waste, and cover it.
Line the base with fine wire mesh.Lift the bin, lay 6mm galvanised mesh over the ground and up the sides by 100mm or so, and drop the bin back on. Rats tunnel up from below; this is the change that works.
Keep it damp.Rats want a dry nest. A heap kept at wrung-out-sponge dampness and covered from heavy rain is unattractive to them and composts better at the same time.
Turn it regularly.Disturbance every few weeks is enough to stop anything nesting in it. A heap left untouched for a year is a very comfortable place to live.
Check for runs and holes.Smooth tunnels about 50mm across at the base of the bin, greasy smear marks on the sides, and droppings the size of a large grain of rice. That confirms it rather than the occasional sighting.
Close the gaps in the bin itself.Cracked plastic daleks, gaps under lids and open-sided wooden bays all give easy entry. Mesh the gaps or replace the bin.
Move to a closed system if the site is bad.In a garden backing onto a watercourse, a takeaway or a railway line, a sealed rotating tumbler or a bokashi bin plus a wormery may simply be the right answer.
Ignore the folk deterrents.Human hair, chilli powder, mothballs, ultrasonic devices and used cat litter do not deter rats in any dependable way, and the last of those introduces a genuine health risk.
Get help if there is an established population.Repeated sightings in daylight, burrows and runs mean a colony rather than a visitor. Your council or a pest controller can deal with it safely; amateur baiting poisons wildlife and pets.

Tips

6mm galvanised mesh under the bin is the single most effective thing, and it costs very little.
A rat seen once in a hard winter is not an infestation. Burrows, runs and daytime sightings are.
If you must compost cooked waste, use a sealed system designed for it rather than adapting an open heap.

Common mistakes

Adding cooked food to an open or dalek bin — It turns a compost heap into a food source, and no amount of deterrent competes with a reliable meal.
Putting down poison bait in the garden — Rats that eat bait are then eaten by hedgehogs, owls, cats and dogs, and secondary poisoning is common. It is also easy to place bait where children or pets reach it.
Abandoning the bin and leaving it undisturbed — An untouched heap is ideal nesting habitat. Turning it is both the compost fix and the rat fix.

If it doesn't work

Tunnels at the base of the bin

Cause: Access from below — Fix: Lift the bin and set it on 6mm galvanised mesh.

Nest of shredded material inside the heap

Cause: Dry, undisturbed, warm cavity — Fix: Turn it thoroughly and keep it damp; turn every few weeks thereafter.

Rats returning despite mesh

Cause: Cooked food still going in, or a food source nearby — Fix: Stop all cooked waste and check bird feeders, pet food and neighbouring bins.

Daytime sightings and multiple burrows

Cause: Established population, not a visitor — Fix: Contact the council or a pest control professional rather than baiting yourself.

Questions people ask

Does having a compost bin cause rats?

No. Rats are present in most gardens already. A bin that holds cooked food and is never disturbed gives them a reason to concentrate there; one that holds only raw plant waste and gets turned does not.

Will rats in the heap contaminate the compost?

Treat it as contaminated: wear gloves, do not use it on salad or anything eaten raw, and put it on ornamental borders or under fruit bushes instead. The heat of a properly hot heap deals with most of it, but a cold heap does not.

Is a wormery or bokashi bin rat-proof?

Both are sealed and both are far less attractive. Bokashi in particular is designed for cooked waste, though the fermented output still needs burying or adding to a heap afterwards.