Rats and mice. They squeeze through 9 mm holes, scuttle behind skirting boards and shin up drainpipes, and the damage they do is usually uninsurable. What's more, modern building construction may be making cosy homes for them. How can you keep them out?
There are 60 million rats in the UK – one for every person. Frighteningly, even this statistic could soon be out of date as experts warn that the rat population is poised to explode. And the bad news for tenants is that new forms of construction will do nothing to keep them out.

A report published last year by The Robens Centre for Public and Environmental Health at the University of Surrey concluded that the lack of a concerted effort by local authorities and water companies is allowing the rat population to expand unchecked. And now that autumn’s chill is upon us and summer’s ready food supply has dried up, pest control experts are bracing themselves for a hectic winter as rats and mice seek shelter from the cold inside snug, warm buildings.

Rats can undermine foundations, chew through electric cabling to cause fires and destroy telecommunications installations. To make matters worse, damage by vermin is excluded from most building insurance policies, making it doubly important that the problem is designed out from the beginning.

Vermin corridors

You never see “rat run” or “mouse motorway” written on construction drawings, but you may as well. Cavities built into walls, miles of communication cabling, suspended ceilings, raised floors, timber stud partitioning with its in-built voids all contribute to making buildings wonderfully hospitable environments for mice and rats to settle down in.

“The problem is growing,” says Dave Wingrove, a service manager for Rentokil Pest Control’s east London office, “particularly in older buildings that have been revamped and in newer buildings with extensive IT networks.” However, it is not all doom and gloom; pest experts agree that with careful design and good site practice, the threat of a vermin invasion can be minimised.

Out of the drains

The vermin problem can often start before the first brick has been laid. And the increase in development on brownfield sites may lead to an increase in the number of rats entering buildings as they surface during building work.

Chris Bath, a technical manager at pest control company Terminix, explains: “In urban areas, most rats live in drains. When you disturb these drains – on a brownfield site, for example – the rats will move somewhere else. This then puts pressure on the existing population at the rats’ new home, which could lead to some rats being forced to find alternative accommodation above ground – and that could mean neighbouring buildings.”

Bath says it is also possible that if a building goes up quickly, before the site’s rat population has had a chance to relocate, they could establish themselves in the building during construction, although he admits that “he has never actually witnessed this happening”.

Contractors themselves can contribute to a building’s pest problem, way before construction is complete. “It is not unknown for pests, particularly rodents, to infest new buildings directly from contractors’ mess huts,” says Wingrove. “And of course, there is the perennial problem of the subcontractors’ discarded sandwiches and orange peel left above false ceilings and behind partitions, which provide a ready source of food for mice or insects once the building is occupied.” Even discarded cigarette butts are used by mice to insulate their nests.

During construction, holes left in the building’s envelope provide easy access for mice and rats into a building. “Young mice can pass through holes as small as 6 mm in diameter, while a hole of only 9 mm can be big enough for a rat to squeeze through,” says Geraldine Lea, a pest expert at the Building Research Establishment. Wingrove agrees: “If you can push a fountain pen through a hole, a mouse can climb through it.”

Cladding does not always provide the expected barrier against vermin entering a building. Thin corrugated steel is one of Bath’s pet hates. He says it is difficult to ensure that corrugations are sealed at the base of the cladding, where it meets the brickwork, and to seal holes in the cladding. Also, with lightweight cladding, holes are often punched through the building’s skin to allow services to pass into the building and they are rarely sealed until the building is completed. The same is true of doors. Bath explains: “Brushstrips are almost always fitted to the base of doors at the end of a contract, and not when the door is being installed.”

IT problems

Rentokil’s Wingrove blames the ubiquitous computer for the increase in vermin-related problems. Communication trunking is often described as a “mouse motorway”, he jokes; in fact, it is a network of motorways that run through office floor voids, connects one office to another, and passes from floor to floor through riser cupboards. This forms an efficient vermin distribution system with outlets throughout the building.

“The same effect happens in older buildings with the retro-fitting of fire alarms and central heating, which lead to lots of interconnecting holes,” says Clive Boase of The Pest Management Consultancy, Pest experts regard anything that leaves a void or gap inside a building as contributing to a building’s potential vermin problem. Stud partitioning and raised floors should be avoided, as should suspended ceilings. “If these are unavoidable, then they should be totally sealed so that it is impossible for anything to enter,” says Wingrove. “However, access panels and inspection hatches should always be fitted so that regular inspections can take place.”

Studwork comes in for particular criticism from Terminix’s Bath, particularly where older buildings are refurbished. “There are massive issues with studwork walls being used to cover old walls,” he says. “It creates a two-inch gap with wooden ledges and you never know what’s behind it.”

Alan Muddiman, Mowlem’s group health and safety manager, agrees that studwork could cause a problem but says that “the Construction (Design and Management) Regulations make the use of studding inevitable for a lot of projects”.

So, what’s the solution?

For contractors that install the rats along with the services, Muddiman says they can prevent this with a little good housekeeping. “We don’t allow people to take a break outside the canteen,” he explains, “and all our rubbish bins are fitted with lids.” For contractors constructing or connecting to sewers, the remedy is surprisingly simple, according to the BRE’s Lea. She says: “Keep all connections closed and ensure manhole and inspection covers are removed only when work is in progress.”

Once the drains have been taken care of, holes and voids are the next problem.

External and cavity walls should be constructed so that there are no internal or external holes larger than 5 mm, and movement joints should be filled with easily compressible and resilient materials. Internally, partition cavities and ceiling voids should be effectively sealed to prevent access to vermin, and risers should be sealed from all connecting voids to stop rats and mice using them as a ladder to adjacent floors.

Should pest experts be involved in the initial design? “It would be nice to get involved with architects to design out problems from the drawing stage,” says Bath. “Failing that, the best way would be to put together CPD material or for us to lecture at colleges.” With the rat population set to boom and all the sewers full, it could be time well spent.

10 ways to keep out rats

1 Good housekeeping on construction sites The site should be kept as clean as possible, all food should be eaten in a designated canteen and rubbish should be disposed of carefully. 2 Connection to drains and sewers Disused drains and sewers on brownfield sites should be excavated and removed, and the holes filled with well-compacted rubble. During construction, connections to sewers should be made in a single day or the ends of sewers should be temporarily stopped. Manhole and drain covers should be lifted only when work is in progress. 3 Holes in external walls There should be no holes in external walls larger than 5 mm. Any holes bigger than this should be sealed immmediately. 4 Service entry points Seal all service entry points as soon as possible. Air bricks should not be knocked through as an easy way to bring pipes or cables into the building. 5 Doors The gap beneath doors should be no more than 5 mm and brushstrips should be fitted with the door. 6 Internal risers Risers should be sealed completely at every cable entry and exit point and at the junction of all ceiling and floor voids. 7 Internal partitioning Internal ceiling and partitioning cavities should be sealed, with access hatches provided for inspection. Any holes in the partitioning for wiring and services should be carefully sealed. 8 Skirting boards Spaces behind architraves and skirting boards should be avoided or filled, and plaster should be taken down to floor level behind the skirting. 9 Services Pipes, ducts and trunking should be sealed where they pass through a wall or partition – in particular through the outer wall. Drainpipes should be positioned far enough away from the building to prevent rats using the gap between the pipe and the wall to scale the wall. 10 Grounds Shrubs and trees should not be planted next to a building; they provide ideal hiding or nesting places and access points.