Winning the yearly winter rat race

It is the time of year when you hear a scurrying in the roof space and the anxiety levels start to climb.

Insulated, predator-free, cosy roof spaces are a haven for rats and mice in winter. And if you have a garden with a few fruit trees, you're giving them a nutritious diet as well as providing them with a warm, dry home to raise their young.

"It's lovely for them, if you think about where you'd want to live, particularly now we're getting down to zero at night, you've got all that insulation, there's no predators there for them," Gerwyn Jones from Pest Management Association of NZ Incorporated says.

While they are enjoying their rent-free roof space home, rodents can do some serious damage when they are up there, Jones says.

"The problem with older houses is you've got wires up there, in some of the even older ones, you can have water pipes up there."

Mark Sutcliffe, owner-operator of BMT Plumbing in Martinborough, has been operating in South Wairarapa for 27 years. He has a collection of rodent damage yarns that will strike fear into any human house dweller.

Sutcliffe recently got a call out one Sunday from a homeowner who had just returned from a three-week holiday.

"I arrived, and when I jumped out the wagon, no bull, water was coming out of the front door."

A rat had chewed through pipes in the ceiling and there had been three weeks of water pouring down, he says. A bathroom, two bedrooms and laundry were wrecked.

"All the Gib on the ceiling had dropped onto the floor, it was that wet.

"They had air conditioning vents hanging down, all the lights were hanging down, so it was fully open, you can look up and see the roof.

"Every door you couldn't close, because they're wood and they've swelled up."

That whole part of the house had to be stripped out, dried out with industrial fans and replaced, Sutcliffe said. It was the worst damage he had ever seen.

"I mean, this was major, if he didn't have insurance, the dollars… four rooms that had to be re-gibbed, re-painted, floors done, ceiling done, doors taken out and new doors put in because they had swollen. It was a shocker."

Rats can cause trouble in other parts of a property, too. Wellington city resident Stephen was mystified when his partner Lisa called to say their new car, a 2021 RAV 4 hybrid, was misbehaving.

"She had got in the car and it was just awful to drive. And there was a message coming up on the screen in the car saying 'engine trouble: contact service agent'."

They took the car to the service centre as it had been misfiring, he said.

"I assumed it would be fixed under warranty, and they say, 'Well if it's something inherently wrong with the car, but it could be something else like rats eating through the wires' and I just sort of laughed."

It turned out that was exactly what had happened. The rats had chewed through wires and had stuffed the fuel injection, he said.

"It had been in our garage, and we kept the dog food in the garage. And so obviously the smell of dog food had attracted them."

Stephen had to front up the money to get the car fixed and said he is now much more diligent about sweeping up dropped dog food.

"We just had to cough up - and replace the dog food with rat poison."

Think this could not happen to you? Check the small print of your insurance policy. The Insurance Council of New Zealand says rodent and vermin damage is "typically excluded under most regular household and motor policies". Its consumer guide has more detail on common exclusions.

How to keep rats out

Want to protect your household from a rodent-related deluge? First, keep your section tidy, Jones said.

"Make sure you haven't got trees coming up against your building. Because rats, they're very great climbers and jumpers and what they'll do, is they'll come up onto a tree or branch and they'll jump from that branch then onto your roof."

Keep your compost bin as far away from the house as possible, he said. And tidy up fallen fruit. Black rats, in particular, loved a good feed on fallen citrus, he said.

An external check for possible points of entry was the next job to tick off. Gaps in weather boards should be blocked off.

"With some older houses like mine, you can put your hand up and feel, oh yeah, they can get into the wall that way and next thing they'll shimmy themselves up."

Check too for signs of activity, he said.

"If you've got a hole under the building or you've got the weather boards and there's gaps in between and you're thinking will a rat get through there?

"If you see cobwebs there, you know that a rat hasn't gone through it, but if you don't see any cobwebs around a hole or burrow, you're thinking something's been going through there."

Jones suggested trapping as a good first option, but some basic principles of ratty behaviour must be understood.

The placement of the trap is very important, he said.

"Most people when they put a rat trap down or mousetrap they put it in the corner with the tongue you put your bait on facing outwards - completely wrong."

Traps are best set hard against a wall, he says.

"Maybe halfway along a beam rather than in the corners.

"Half the time when you catch rats it is not when they go in for the bait, it's when they're running along the wall, as they basically know where the corners are and they run into the trap and you catch them that way."

Rats are neophobic, so some rat-like cunning is required to catch them, he says.

"If I had a rat in the roof I would look to trap it. And the trouble you get with traps in a roof is with all the insulation there, it's hard to put a trap that are rats going to come across.

"So, what I would do is have a piece of ply board and then screw some traps on to the piece of baseboard so I can place that where I want to inside the roof space."

If you just put a trap up there and set it, you would likely spook the rat, he said.

"They'll know something's up there that wasn't there yesterday, and they're very shy.

"I set the traps to off, so they're not live, they're not in the kill position and then I'll put something like dried mango, which they love, that's the Achilles heel for black rats.

"Put dried mango just on the trap and around and just leave it for about two or three days in off position.

"And then maybe on the fourth day or third day, I will then set the trap and then put the dried mango in the trap.

"So, they come along thinking, 'that's that place I went to have some food the other day, that's ok'. Bang, you get them that way."

Always use trap stations outside to protect pets and birds, he said. And always put poison in stations.

"Make sure the trap is secure because when that kill bar comes down, if the trap is not secure you'll lose kinetic energy from the trap, if it's screwed into something or locked into place you get more whoomph."

Mice could do a great deal of damage too, Jones said.

"Mice, like rats, they must continue to gnaw to keep their teeth down, keep incisors down nice and sharp and they will do that on electrical cabling. They will do it on waterpipes, they will go through the installation."

Mice were also a more serious source of contamination than rats, he said.

"They urinate constantly. From a contamination point of view, rats are relatively clean. When they do their droppings it's normally in a set place, mice will just continue to defecate all the time."

Don't like the idea of trapping or baiting? Tempted to invest in an ultrasonic device? Do not bother, Jones said.

"Ultrasonics don't work, you're wasting your time."

Other DIY deterrents, such as peppermint oil, were also ineffective, he said.

"What happens is they'll get used to the smell, they'll just overcome that, like anything, any smell they'll get used to that."

The biggest myth of all was that rats searched for water after consuming poison bait, he said.

"Which is an absolute load of cobblers. People say, 'Oh they've eaten that bait, they go looking for water and that's why they're chewing the pipes' - no they don't.

"They're chewing pipes because they're trying to keep their teeth down. The bait itself will basically cause their blood to thin and their organs start to shut down."