Why Bats Need a Helping Hand

Britain's bats have had a rough century. Old trees with rot holes and loose bark have been tidied away, barns have been converted, and rooflines have been sealed up with modern breathable membranes that give bats nowhere to cling. Add to that a steady wash of artificial light across our gardens and lanes, and you can see why these remarkable animals are struggling. All eighteen of our resident species rely on insects, and a single pipistrelle can hoover up several thousand midges, moths and beetles in a night. A well-placed bat box will not replace a proper roost, but it can offer a genuine lifeline — and if you site it carefully, you may well see it used within a season or two.

Choosing the Right Materials

The golden rule with bat boxes is rough, untreated timber. Bats need to grip, and a planed, varnished surface is like ice to a small claw. Choose rough-sawn softwood or a hardwood such as oak, at least 18–20mm thick for good insulation and warmth. Avoid plywood, which delaminates in the damp, and steer clear of any timber treated with creosote or solvent-based preservative — the fumes linger and bats will simply avoid the box. If you must protect the outside, use a water-based, bat-safe treatment on the exterior only, never on the interior or around the entrance. Fix it together with galvanised screws rather than nails, so you can open the box for cleaning later on. A simple, cheap, slightly rough box beats an elegant one every time.

Building the Box: Dimensions That Work

Most of our bats are small, so a modest box is fine. Aim for a finished unit roughly 400–450mm tall, 200–250mm wide and 100–150mm deep, with a single internal chamber about 350–400mm high. The entrance slot should sit near the base and run almost the full width, with a gap of 15–20mm for pipistrelles and similar small species, or 20–25mm for larger bats such as brown long-eared. Below the slot, extend the front panel downwards to create a landing platform about 100mm deep — bats drop onto this, shuffle upwards and disappear inside. Inside, cut shallow horizontal grooves or staple a strip of rough plastic mesh to the back panel as a climbing ladder; without it, bats can struggle to get a purchase.

• Rough-sawn timber, 18–20mm thick, untreated inside
• Entrance slot 15–20mm wide (small species) or 20–25mm (larger)
• Internal ladder of grooves or mesh
• Overhanging roof to shed rain, with a slight lip
• Sealed joints — draughts ruin the warmth bats need

Siting the Box: Aspect, Height and Darkness

Position is everything. Bats want a roost that warms up quickly in the morning and stays dark, so choose a south, south-east or south-west aspect. A box baking in full afternoon sun can overheat, while a north-facing one may never reach the 20–30°C that nursery colonies prefer in summer. Mount it at least 3 metres up, ideally 4–5 metres, where it is clear of cats and ground predators, and where bats have a clean flight line in and out. That usually means a wall, a gable end or a mature tree trunk with a clear approach. Keep it well away from security lights, porch lamps and illuminated windows — even a few minutes of nightly glare can put bats off a roost entirely. If you can, hang two or three boxes on different aspects, so the bats can choose.

Mounting Safely and Securely

Boxes should sit flush against the surface, with no gap behind that lets rain or draughts in. On masonry, use sturdy brackets or galvanised cable hooks that allow you to lift the box off for inspection. Avoid fixing directly over a window or door, and never place a box where droppings will land on a path or patio. If you mount on a tree, use a strap or a broad, non-girdling fixing that lets the trunk grow — never a nail driven into living wood. As a final touch, fit a smooth metal or plastic collar below wall-mounted boxes to stop predators climbing in, and keep vegetation trimmed back from the entrance.

Checks, Cleaning and the Legal Bit

Once a box is up, leave it alone. Bats are protected by law in the UK, and it is an offence to disturb them or damage a roost, even an unoccupied-looking one. The sensible time to inspect is a mild spell in late autumn or winter, when bats have moved to their hibernation sites. Open the box, remove old nesting debris and droppings, check the fixings and close it up again. If you ever find bats present, stop immediately and seek advice from a local bat group or ecological consultant. Patience pays: many boxes are ignored for a year or more before a colony moves in, and some never attract bats at all. Fit several, keep them dark and warm, and you have given our night-flyers a real chance.