Rats or mice? How to tell what's in your roof

Droppings - the reliable one

This is the check that actually settles it, and you can do it with a torch and a look in the roof space or along a skirting.

• Mouse: 3-6mm, pointed ends, look like dark grains of rice, scattered widely
• Rat: 12-18mm, much thicker, often concentrated along a run rather than scattered
• Possum: considerably larger again, often in clusters, and usually with a strong smell

Noise, and what it tells you

Mice are light and fast - a scratchy, skittering sound, often quite continuous. Rats are heavy enough that you can usually hear individual footfalls, and you'll sometimes hear one moving along a beam or joist.

If something wakes you up, it's more likely a possum. They're heavy enough to thump, and they make a low guttural growl or hiss that nothing else in a New Zealand roof makes.

Why it changes what you do

Mice need more, smaller entry points closed - a mouse gets through a gap about the width of a pen. Rats need bigger gaps but travel further and are warier of new objects, which is why rat trapping often fails in the first week.

Possums are a different job again, and they're the one most likely to leave you needing to replace insulation.

Other signs worth checking

• Gnaw marks - rats leave larger, coarser marks; mice leave fine paired grooves
• Greasy rub marks along a regular route, which suggests rats
• Disturbed or flattened insulation, which suggests something heavier
• A musky smell in an enclosed space