Thursday, 26 July 2018

Stopping mice entering through the roof eaves ...

In the spring, we noticed gnawing marks on a piece of soap in the cupboard under the bathroom sink, and removing the bath panel revealed a few mouse droppings under the bath.

From the relatively small quantity, we'd luckily spotted the problem early.  We bought a few traps and had caught the first mouse within an hour, and five more over the next few days.  They were common house mice (Mus musculus).

One thing I couldn't figure out was how they were getting into the upstairs bathroom when there were no signs of them anywhere at ground level.

Then late one night, I was in the downstairs toilet which is part of the ground floor extension of the house, and I could hear scratching above my head, near the roof eaves.    So I reckoned they must be getting into the roof space at the eaves and then climbing from there into the upstairs bathroom via the penetrations in the internal brickwork for the plumbing etc.   And of course the roof space is packed with insulation so there's ready-made nesting material for them.

I did a bit of reading up on mice problems, and apparently they can climb a rough vertical wall and easily get through a hole of 10 mm diameter or so, or any gap through which they can squeeze their heads.

The roof over our extension is made from profiled heavy concrete pantiles, and at the eaves the edges just rest on the first purlin, and where adjacent tile profiles interlock there's a trapezoidal gap created of around 60 x 15 mm.

At first I thought it must be an inherent design flaw in the roof construction but no, even now, the UK building regulations permit openings at the roof connections as long as it's not possible to pass a 16 mm diameter ball through, which seems a very large gap to me.

As in our case, gaps are often necessary to permit an airflow into the roof space to prevent conditions arising where the roof structure might rot from a build up of condensation, so it's not just a case of simply blocking all these openings with mortar or similar.

Therefore, I bought a 300 x 400 mm sheet of 1 mm thick perforated stainless steel (£9 delivered), with 3 mm holes on a 5 mm pitch, cut it into 85 x 35 mm rectangles and then bent over the corners of each individual piece using a hammer and a vice.  

perforated stainless steel mouse / bird guard ...

These guards fit quite closely into the pantile gap, and the perforations still allow air movement to and from the roofspace.

the roof pantiles and overlap joint, with the guards now fitted ....

The mesh guards were simply tapped into position and then secured by a couple of screws into the purlin.

close-up of a fitted guard ...

I've already fitted them to all 27 pantile joints on the south side of the extension roof, and I need to cut another ten for similar tiles on a short section of roof to the north.

Sod's law, the remaining piece of mesh is only sufficient to cut 9 more guards - rather than buy another piece of mesh for the extra one I'll need, I'll use a piece of steel plate and just drill a few holes through.

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