After checking that my pallets were not treated MB, I decided to use some to build a home for bats that may live around here (Paris, France).

I like the idea of having such tiny mammals living around and eat all those damn mosquitoes :)

Supplies

Pallet wood.
Stainless steel screws.
Stuff to measure and cut.
Optional: Geotextile to isolate the roof from cold and/or wind
Optional: Bin liner to protect the roof from rain

Step 1: Back of the Box

From what I read, the bat box has to be at least 40cm high and 20cm wide.

The wood should be fairly thick ; at least 2cm so that temperature don't vary to much. My wood was only 1.5cm. So I decided to assemble 2 layers. This should limit temperature variation.

I cut 4 vertical boards of 60cm and 5 horizontal boards of about 45cm. The horizontal boards are screwed to the vertical one. Using proper screw length, those are not visible from what will be the inside of the box.

In the end, I cut what exceeds from the horizontal bits.

Step 2: The Bat Room

According to what I read, the bats need a space from 1.5 to 2.5 cm. Hopefully, I had a remaining board that was a bit more than 2cm thick. So I cut it to get the sides.

The sides are screwed to the back of the bat box.

Step 3: Front of the Box

The same way I assembled the back, I used horizontal and vertical boards to get the front done. This should limit airflow and excessive temperature variation.

There was a hole in one of the boards. So I had to obstruct it.

Step 4: The Roof

To limit water to drop inside the box, I cut the top of the box using an approximate 10° angle.

After cutting another board to put on top, I saw that there were quite a few spaces from here to there. I'm not sure whether this is really useful but I decided to fold a piece of geotextile to limit the airflow. Then, I screwed the board to get a proper roof. In the end, I just cut the geotextile exceeds.

Step 5: Wood Protection

Although bats won't give a damn about it, I decided to decorate the box. After all, that's my garden :)

I read that care should be taken with chemical products used to protect wood. So I used walnut tincture, linseed oil and turpentine. Those seem the most natural things that can be used to ensure wood protection and non-toxic environment for bats. Hope I'm not mistaken...

Step 6: Add More Protection to the Roof

Rethinking of it, I wondered if the roof would be protective enough from rain... So I added a bin liner on top of the roof. This will not let water drops in. The liner was folded like 4 or 5 times and fixed with iron nails.

Step 7: Installation

I attached two hooks in the back of the box. There already is a (thick) wire mesh were the box is going to be installed. The hooks will support the box weight. And I used garden wire to ensure bottom will not move with the wind. The box should be fairly stable this way.

The box was installed at about 3m from the ground. This should be ok, according to what I read. The only thing I'm not sure is whether those Bamboos will be a problem or not. Time will tell.

Now I just hope bats will find this box friendly.