Prevention

Rodent proofing: keeping rats and mice out for good

Rodent proofing an Auckland house: the 12 mm and 6 mm gaps that matter, which mesh and fillers rats cannot chew, and why repellents and ultrasonic plugs fail.

The short answer

Rodent proofing means closing every gap a rat or mouse can use to get into a building, about 12 mm for a rat and 6 mm for a mouse. What holds is galvanised mesh, sheet metal, mortar and packed copper or stainless steel wool, not chicken wire or expanding foam on its own. Clear the animals first, then seal, then cut back whatever lets them climb to the roof. We survey and seal as part of our rodent control work.

What is rodent proofing?

Rodent proofing, also called exclusion, is the work of finding and closing the gaps rats and mice use to get into a building, so that the population outside cannot refill the space you have cleared. It is the part of rodent control that lasts: traps and bait remove the animals inside, and proofing stops the next ones. A rat needs a gap of about 12 mm, and a mouse about 6 mm, the width of a pencil.

Those figures come from university extension guidance, which puts the rat gap at slightly more than half an inch and the mouse gap at slightly more than a quarter, and they matter because people consistently underestimate them. A gap you could not get a finger into will take a mouse, and gnawing enlarges a small gap quickly, so a crack that is nearly big enough soon is.

Where rats and mice get into an Auckland house

In most Auckland houses the gaps are in the same handful of places: subfloor vents with perished or missing mesh, pipe and cable penetrations through floors and walls, the gap under the garage door, gaps at the eaves and where the roof meets the wall, and drains with a cracked pipe or a dry gully trap. Ship rats come in high, while Norway rats and mice come in low.

The species decides where you look first. A ship rat in the ceiling arrived along a branch, a fence or a cable and got in at the roofline, so the climbing route is part of the gap. A Norway rat under the floor came through a vent, a drain or a burrow, and sealing the roof achieves nothing against it. Mice can use either, but in practice they are usually at ground level and close to the kitchen.

Inside, check under the sinks, behind the dishwasher, the oven and the washing machine, around the hot water cylinder and wherever a pipe, cable or duct passes through a floor or a wall; the US Centers for Disease Control and Prevention's checklist adds floor and dryer vents and the join between floor and wall. Outside, walk the foundations, the vents, the service entries and the roofline.

At a glance

The usual gaps, and what closes them

The usual gaps, and what closes them
Where	Who uses it	What closes it	What does not
Subfloor and wall vents	Norway rats, mice, ship rats	Galvanised wire mesh, 13 mm aperture for rats, 6 mm where mice are the problem	Fly screen, plastic mesh, chicken wire
Pipe and cable penetrations	Mice, both rats	Copper or stainless steel wool packed tight, finished with mortar or sealant	Expanding foam or sealant on its own
Under the garage or an outside door	Mice, Norway rats	A door seal with a metal edge or kick plate, leaving a gap under 6 mm	Rubber or brush strips alone
Eaves, soffits and the roof to wall junction	Ship rats	Sheet metal or mesh fixed over the gap; a roofer for flashings	Foam stuffed into the gap
Branches, cables and fences touching the roof	Ship rats	Cut back vegetation; the climbing route is the gap	Spikes or repellents on the roof
Drains and gully traps	Norway rats	Repair by a drainlayer; keep the gully trap's water seal	Bait down the drain
Compost bins	Norway rats, mice	Galvanised mesh under the base, holes no bigger than about 12 mm	An open-bottomed bin on bare soil

What materials are rat proof?

Metal and well-made masonry. Galvanised wire mesh of 16 to 19 gauge with a 13 mm aperture keeps rats out, a 6 mm mesh keeps mice out, and sheet metal, mortar and sound concrete hold against both. Plain steel wool works for a while but rusts, so copper or stainless steel wool packed tight is better. Plastic, rubber, vinyl, timber and foam are all chewable.

The detail that makes the difference is the edge. Rat and mouse incisors curve slightly inward, which, as University of Nebraska–Lincoln extension guidance explains, makes it hard for them to start gnawing on a flat, hard surface and easy once they have an edge or a rough spot to bite on. So every repair should finish flush, with no lip of mesh or loose mortar to get the teeth under.

For the record, the extension figures are 24 gauge or heavier for galvanised sheet metal, 19 gauge mesh with a 13 mm square aperture for rats, and 24 gauge mesh with a 6 mm aperture for mice. Lightweight chicken wire is not on the list, because rats pass straight through its openings.

Can a rat chew through concrete?

Not through sound, well-finished concrete, but rats do gnaw concrete block, crumbling mortar and the rough edge of poor concrete, along with aluminium sheeting, timber, plastic and rubber. Their incisors need an edge to bite on, which is why a crack or a broken corner gets enlarged while a smooth, flat face is left alone.

In practice that makes the concrete around a pipe that was knocked through and patched roughly, or the mortar of an old brick foundation that has gone soft, a weak point worth checking, while a properly poured slab is not. Where we find gnawing on masonry, the repair is fresh mortar finished smooth, with a metal plate or collar where a pipe goes through.

Spray foam, chicken wire and other fixes that do not hold

There isn't a spray foam that keeps mice out on its own, because mice and rats chew through cured foam easily. Foam has one honest use, holding packed steel or copper wool in place so that the metal does the work. Chicken wire fails for the opposite reason: its holes are big enough for a rat to pass straight through.

Foam has a second weakness outdoors, which is that sunlight breaks it down, and the same goes for most of the quick fixes people reach for: fly screen or plastic mesh over a vent, a rubber or brush strip under a door on its own, a wedge of timber in a gap. Each of them is gnawable, and a rodent that wants to come through it will.

Do rodent repellents work? Peppermint, ultrasonic plugs and smells

Not for long enough to matter. Rats and mice may shy away from a new smell or sound at first, but they get used to it, and no repellent has been found to solve a rat problem for more than a very short time. Plug-in ultrasonic units are directional, do not reach behind furniture or into cavities, and rodents soon stop reacting to them.

That covers peppermint oil and the other scent products as well as the plug-in boxes sold as electronic rodent control. The point is not to sneer at anyone who has tried them, only to save the money. What rats hate, in practice, is a place with no food, no cover and no way in, and the natural deterrents that work are the dull ones: lidded bins, pet food lifted at night, firewood stacked off the ground and away from the walls, compost on a mesh base, and fallen fruit picked up.

Keeping them away permanently: the outside of the house

Permanently means two things kept up together: no food or shelter close to the house, and every gap closed and checked once a year. That is lidded bins, pet food and chicken feed in sealed containers, compost on a mesh base, vegetation and firewood away from the walls, and branches cut back from the roofline. A section on a reserve, stream or bush edge will always see new arrivals, so a maintained trap line belongs in the plan.

Health New Zealand's advice runs along the same lines: rubbish in metal or thick plastic containers with lids, no rubbish bags left out overnight where you can avoid it, no extra pet food left out, weeds and long grass cleared, and gaps and cracks in the house secured. For a compost bin, Predator Free NZ suggests wrapping the open base in galvanised mesh with holes no bigger than 1.25 cm, since mice and young rats get through anything larger.

If the rats are already inside, start with the signs that tell you where they are, because proofing comes after clearing, not before.

Clear first, then seal

Seal a building while rats or mice are living in it and you shut them in, where they either gnaw a new way out, often through something you would rather they left alone, or die in a cavity. Trap until the catches and the fresh droppings stop, then close the gaps, then keep one or two traps set where the runs were as an early warning.

For a mouse problem, the order of work is set out in how to get rid of mice.

When proofing is a job for us

Call us in when you cannot find the gap, when it is at roof height or needs a ladder on a two-storey house, when it is in the drains, when you are on a reserve or stream edge, when you have cleared rats from the same space twice, or when a landlord or body corporate needs a written list. Proofing is most of what stops a rodent problem coming back, so it is worth doing once, and properly.

We survey the building inside and out, give you a written list of every gap we found with its location and what will close it, and seal what is within reach as part of the job: vent mesh, pipe and cable penetrations, and door gaps. Roof flashings are a job for a roofer and broken drains a job for a drainlayer, and the list says so rather than pretending otherwise. You can book a rodent survey and treatment, check what rodent proofing costs in Auckland, or request a visit.

Questions

Rodent proofing questions

How much does rodent proofing cost?

Proofing a standard Auckland house, covering the survey and the sealing of vents, pipe and cable penetrations and door gaps, usually costs $280 to $560 incl. GST, quoted after we have surveyed the house. Roof and drain repairs by other trades are separate, and so is clearing the rats or mice inside first, which is priced as a rodent treatment.

Can I rodent-proof a house myself?

Most of it, yes. Fitting galvanised mesh over vents, packing pipe and cable gaps with copper or stainless steel wool and mortar, fitting a door seal with a metal edge and pruning branches back from the roof are all ordinary DIY. The hard parts are finding every gap and working at height, and roof flashings and drains belong to a roofer or a drainlayer.

Do I need to seal the roof if the rats are under the floor?

Usually not first. Norway rats work at ground level and below, so the subfloor vents, the ground-level gaps, the garage door and the drains come first, and roof sealing achieves very little against them. The roof matters for ship rats, the climbers, and if you are hearing rats overhead as well, both ends of the house need attention.

How long does rodent proofing last?

As long as the materials do and nothing new is cut into the building. Galvanised mesh and mortar last for years, while plain steel wool rusts out and needs replacing. Every renovation, new cable, heat pump or pipe makes a new gap, so check the outside of the house once a year, before autumn, when mice start coming in.

Is it worth proofing a house next to a reserve or stream?

Yes, more than anywhere. The supply outside never stops, so treatment without proofing just repeats itself every autumn. Pair the proofing with a maintained trap line along the boundary, which takes the pressure off the building, and keep a trap or two set inside where the old runs were as an early warning.

Sources

If you need someone

Having the house surveyed and sealed

Rodents

Rodent Control

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Identification