If you talk about building a purple martin birdhouse, you really mean you are going to build a purple martin condominium. These little birds love to live in a group setting with neighbors close by. Build their condos with lots of doors, keep it cool with white paint, make sure they have a nice pool to splash in close by, and your little purple martins should move right in, as long as you built their place according to their specifications.
Cut the wood for the floors and walls for the first story. Plan on building at least two levels, but you can add as many levels as you like. A table saw would be great for this, or a circular saw or even a hand saw, if that is what you have.
Drill three 1-inch holes in two of the wall boards. Determine the placement of the holes by dividing the side into three equal sections, mark it with a pencil and centering the hole in each. Drill one 1-inch hole in two of the other wall boards, right in the center. These will be the entrances for the purple martins. Add one hole to each of the triangular end roof walls, right in the center.
Assemble four walls in a row about 6 inches apart. Two will be the outside walls with the three holes and two will be inside. Place the other two walls with one hole on either end, closing the square evenly. Adjust the boards as necessary. Mark their placement with a pencil and then staple them to the floor, starting with the inside and working outwards.
Add the six wall dividers between the walls, forming nine evenly spaced squares. Staple them into position with the staple gun. You now have the first floor of your purple martin condominium finished.
Repeat Steps 1 through 4 for each level you plan on adding. Stack the new layer on top of the finished one and connect with a 1-inch L bracket on each side, screwing it into place with wood screws. This is necessary so that you can unscrew them later to open the birdhouse for cleaning in the late fall.
Attach the two slats flat on the top of either end of the walls with staples. Position the two triangular walls on either side, lining them up with the walls below them. Secure them in place with two 1-inch L-brackets on each side. Lay the roof pieces on top centering until even and staple to the triangular walls.
Paint the house with a bright white paint. Allow to dry overnight and then place on a platform where it has plenty of space on all sides and near a source of water.