There are fewer and fewer suitable roosting places for bats these days, as barns get converted and towns spread out. If you think there are bats in your area why not do your bit for conservation and invite them to stay in your garden by making your own bat box?
A bat box is made in a similar way to a birdbox, but instead of having a hole in the front, bats prefer to enter through a slot in the bottom of the box.
Rough-sawn wood is the best material to use for your bat box – the rough finish enables the bat to cling onto it. Make sure the wood is untreated – some treatment chemicals can be harmful to bats.
If you don’t have enough spare pieces of rough, untreated wood lying around, you can buy a plank roughly 1200mm long, 200mm wide and 20mm thick. This will be enough to make all the sides.
Use a pencil and ruler to mark out lines on the timber where you need to cut it. The top and side pieces are 200mm wide, but the back and front are only 150mm so you’ll need to make an additional cut for those. Measure and mark out the following dimensions:
Back: 400mm x 150mm
Front: 230mm x 150mm
Top: 230mm x 200mm
Sides: These are triangular, so mark a rectangle first 340mm x 200mm then draw a line diagonally from the top right corner to the bottom left corner.
Assembling the Box
Cut out the pieces and sand the sawn edges so that they will fit snugly together. Bats don’t like draughts, so you need to fit the pieces together well with no gaps. You may wish to glue the pieces to ensure a draught proof seal before nailing.
You need to cut ridges in the back piece to make a bat-ladder, so lay it on a flat surface and carefully saw ridges across the width, 2mm deep and 5mm apart.
Next you need to take the points off the triangles for the sides – this will create the slot at the bottom where the bats will enter the box. Measure 125mm from the acute (sharp) angled end of the triangle, along the longest side; draw a line across to the second-longest side, so that you have a small triangle. Cut the small triangle off and discard it. This will leave a gap that is large enough for the bats to enter but too small for predators.
Now you’re ready to nail the pieces together – use 40mm wood nails. Nail the two sides to the front panel first, driving the nails through the face of the sides into the edges of the front. Use three nails in each side.
Next, hold the top and back in place to ensure you align them for a nice snug fit. Once you have them in place, nail the back board - driving three nails through each side again. Lastly, nail the roof onto the top.
Find an eyelet with a hole big enough to get a nail or screw through, and attach this to the top of the box so that you can fix the box to a tree, post or wall – position it centrally so that it doesn’t hang crookedly.
Bats like to move roosts depending on the time of day and the season, so the best way to keep them happy is to make three boxes and attach them around a tree trunk, as high up as possible (at least 2m off the ground) and avoiding facing due west, as this is the direction that the most bad weather comes from. Wherever you site your boxes, try to ensure that they are in a spot that is sheltered from wind and rain, but if possible make sure they get as much sun as possible so that they can warm up during the day.
Ideally the boxes should be close to a hedge or tree line, as some bats use these for navigating, however make sure there aren’t too many branches obscuring the box, making it difficult for the bats to fly in.
Do not be tempted to try and peek in the boxes – it is illegal to disturb bats when they are roosting. If you find an injured bat, don’t pick it up – you need a licence to handle them. If you do find an injured bat, contact your local wildlife trust and they will tell you what to do.