Our neighbour has mice, how can we stop them getting into our house?

We recently bought an attached house in Dublin city and our adjoining neighbour has just informed us he has mice. Is it likely we’ll have them too? What are the best things we can do to prevent them from getting in? We’ve heard of a hardened foam you can buy to fill in gaps.

Dropping winter temperatures mark the en-mass transfer of rodents from their outdoor habitat to indoors. In this case, your neighbour already has a problem and the question is, will rodents be able to travel from one house to the other?

You have not told me the age of your house. In older terraced Victorian dwellings, attached houses did not have party walls above the ceilings. This meant the attic was shared. This gave rodents free rein. If one house had been breached all would ultimately be infested. The advent of airtight houses should make the annual invasion by mice much more difficult. Breaches of the external house envelope will have been identified and repaired following testing. However, the legacy of some poorly constructed housing will mean that there is no absence of opportunity open to transient rodents in older housing stock.

Current building regulations require that semi-detached houses be separated from each other. This includes a requirement to have a smoke-tight party wall. In theory this means that rodents should not have an easy access route into your home from your neighbour’s.

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In my experience of houses built prior to the improvement of building regulations, the party wall may not be constructed all the way to the roof and there may be gaps and spaces in the party wall and in particular at eaves level. While this has implications in terms of the spread of fire, it will also mean that mice can easily travel from one property to the next.

To prevent the ingress of rodents you should first check the external walls. Look for cracks externally at low level and check for openings around waste pipes where these penetrate from inside to outside. These are favoured entry points. Rodents can enter through the smallest of gaps. Once you have identified any weak points and carried out repairs you should then tackle the attic.

The attic is where it is easiest to find evidence of mice if they are in your house. Typically, droppings will be seen around warm down lighters or above the hot press. Look out also for shredded pipe insulation as this is another tell-tale sign.

Next study the party wall. Any gaps should be filled. Do not use expanding foam as this lacks the fire stopping qualities required. Use sand and cement instead on blockwork or gypsum plaster if the party wall is timber framed. Ensure the eaves box at the front and back of the party wall is filled with a fire stopping quilt.

If mice have entered your house you will need to arm yourself with some traps. Butter is usually the best bait as it cannot be removed easily. I discovered this after a week-long battle with a mouse that managed to remove a hard chocolate bait each day without triggering the trap. Humane traps are available if you don’t mind dealing with relocation of the trapped live mouse. There is never just a single mouse. Typically, a family of mice will have moved in or bred as the gestation period is short.

If all apparent external gaps and openings are filled and the party wall is intact you should be free from intrusion. You should be aware however that mice are very opportunistic and will capitalise on an open external door. In the spirit of Tom and Jerry, perhaps a family cat may be a practical solution, or deterrent given that poor Tom never had much luck.

Noel Larkin, chartered building surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie