Building a shelter that survives the winter

There's no heating in there: what warms the shelter is their own bodies. In a large space that heat spreads out and never amounts to anything — an over-roomy shelter is as cold inside as the night outside.

The rule is room for two or three cats pressed against each other, and not a hand's width more. Mine has two snug compartments, and that's why it works.

This is the step you can't skip. The kit came with polystyrene panels cut to size, slotting in between the battens in the walls.

If you're building from scratch the logic is identical: polystyrene in the walls, the floor and the roof. Without it you have a box that cools at exactly the same rate as the air outside, and all it does is break the wind.

It sounds like a detail and it's one of the things that makes the most difference. The front compartment stops the wind coming through the door; the inner one is where they sleep, and that's where their body heat stays trapped.

The two are joined by an opening cut in the divider. Building from scratch, you get the same effect with a simple partition and a hole in one corner.

The size of a cat and no more. The smaller it is, the less heat escapes — and a dog or a fox won't fit through.

In the first version of this guide I wrote that there should be only one entrance, because of the draught. A reader corrected me and he was right: from the inside, a single entrance is a dead end. A colony cat won't willingly go into somewhere it can't leave by another way if a dog turns up at the door — and that's why plenty of well-built shelters stand empty.

What rescue organisations recommend is two entrances on opposite walls, both cat-sized, with a rubber or plastic flap on each. The flaps deal with the draught. Nothing else deals with the dead end.

This year I fixed a clear sheet over the entrance of mine, to break the wind without blocking their view of what's outside. It still needs the second opening, and that's the next job.

Over the wooden floor I lay a large plastic sack — the kind potatoes come in — pulled flat, and only then the straw on top.

It's what stops the wood rotting underneath the bedding. Without that layer the floor slowly takes on moisture and dirt from the straw, and after two winters it's gone — and replacing the floor of an occupied shelter isn't something you do in the middle of January.

Fill both compartments properly. They burrow into it, and it's their own body that shapes and warms the hollow they lie in.

Hay won't do, and confusing the two is the most common mistake I see. Hay is dried grass: it soaks up moisture, rots and goes mouldy. Straw is hollow inside, doesn't get waterlogged the same way, and holds its shape.

If the roof stops flush with the wall, rain runs straight down the wood and in through the joints. It has to stick out on all four sides so the water falls clear.

The first one I had was roofing felt. This autumn I replaced it with a flat sheet, with more overhang than the old one.

Mine stands on three concrete blocks. Air passes underneath and damp from the soil can't climb into the wood.

It's one of the things that makes the most difference and it costs nothing: three blocks, some bricks, an old pallet. A shelter sitting on bare earth takes on water from below and never dries out again.

All the old straw comes out and fresh straw goes in. While it's open, check the roof, the joints and the floor.

A shelter that's built and then forgotten is damp inside after two winters, and damp bedding is worse than no bedding at all. It's one afternoon a year.