Use a four foot long 1" x 8" board. Cut as shown on the cutting diagram, using a tilting circular or jigsaw to cut the bevel between the backboard and roof pieces. The smaller angled ends of the triangular sides must be cut off to allow an opening of about one inch at the bottom of the bat house, as shown in the side view. The cutting angle for this will be approximately 62 degrees, but you will need to adjust it to suit the thickness of your wood.
Assemble the box with nails or screws and glue. The glue will provide some weather sealing and help retain heat. If desired, apply caulk at all joints.
The front door pivots at the bottom on two nails. Drill a hole high up one one side and into the side of the door. A loose fitting nail placed into these holes will hold the door in place. Provide additional weatherproofing by attaching narrow strips (1/2" x 1/2") of wood to the inner sides of the house to act as door stops. Attach a small screw to the front of the door to use as a doorknob. The house may also be built with a fixed, instead of hinging, door, but this will make it more difficult to clean out should birds nest in it before it is inhabited by bats. Apply a coat of polyurethane to the outside of the house only.
Place at least ten feet high on a building or tree. Southern exposure is best.
(Use rough-cut lumber or make many small dents in backboard for bats to cling to.)