Material List:

two 10' untreated, rough cut cedar 1x12s
waterproof caulk
drywall screws, 1" long

Tool List

circular saw
electric drill with wire wheel attachment
caulk gun
safety glasses

Steps

1. Get some good plans. Plans for the construction of an open bottom Bat House can be obtained from the United States Geological Survey, Bat Conservation International or from your state conservation agency. The plans we used came from the USGS.
2. Preparing the Lumber. Rough-cut Cedar is rough on one side and finished on the other. You'll need to "rough-up" the finished side so bats can gain a toehold and climb up into the box. You can accomplish this in one of two ways. Using an electric drill with a wire wheel attachment, you can "rough-up" the finished side or using a circular saw, you can cut grooves 1/16" deep, 3 inches apart across the board from one end to the other. Whichever method you choose, be sure to wear safety glasses.
3. Cutting the Lumber. Using a circular saw cut eight 2-foot sections. One 3-foot section and one 1-foot section. Three of the 2 foot sections will serve as the front and sides with the remaining five 2 foot sections serving as the slats that fit inside. The three-foot section will be the back of the box and the one foot piece will be the top. Remember, there is no bottom as the open space allows the Bats access. The Slats create crevices for the Bats to climb up between.
4. Let's Screw It Together. Lay out the three-foot section as the back of the box. After designating one of the ends as the top, attach on two-foot section to the edge face. You'll be screwing the pieces together through the back. Pre-drilling some pilot holes helps to eliminate any chance of the wood splitting. Space one inch over and install your next two-foot piece, which becomes your first slat. Now, repeat this step across the width of the board until you have two sides and five slats attached to the three-foot backboard. Using your remaining two-foot section, box in the sides and slats by screwing it across the face. Next, take the one foot piece and use it to make the top. Remember, there will be a one-foot section at the bottom of the backboard that is exposed. This space is a crawling surface for the bats to grab hold of and climb up into the box.
5. Sealing the Bat House. A Bat House must be sealed, so in places where you didn't get a good tight fit, caulk the gaps. This insures the bats a warm atmosphere they need to thrive.
6. Let's Hang It Up. If you live in the northern United States, or a cooler climate, paint the Bat House black or you can cover it with tar paper. Both help to make it warmer. Also, you'll want to hang the house on the south side of a tree or building to take advantage of maximum sunlight. If you live in the South, you don't need to paint the house and you'll want to hang it in a shady spot, preferably on the north side of a tree or building. This will help keep the bats a bit cooler in the heat of the day. The Bat House should be hung at least 15 feet above the ground and ideally within 500 feet of a source for water.