The classic approach to building a porch ceiling is to use wooden ceiling planks to cover it. This provides the kind of weather-resistance you need, while the lines in the planking provide an interesting visual element. They lock together with the same type of tongue-and-groove system as a wood floor, and can be installed with a standed trim nailer. Modern ceiling planks are very light wood that you can buy either pre-finished, or paint yourself once they're installed.
Instructions
1. Measure the length of the porch ceiling, perpendicular to the ceiling joists (the wood support beams, set every 18 inches or so).
2. Mark out the measurement to a ceiling plank with a pencil. Use a miter saw to cut the plank to size.
3. Hold the plank to the ceiling where you measured, with the grooved side of the tongue-and-groove facing the wall. Leave 1/2 inch of space from the edge of the plank to the wall. (This will allow the wood to expand with environmental changes.) Secure the board by shooting pairs of nails into every joist with your trim nailer.
4. Cut your second board. Set it against the first board, pushing the groove of the second board over the tongue of the first. Use your trim nailer to shoot a nail through the side of the second board, going upward, into each joist.
5. Continue measuring and cutting boards, building your way across the ceiling. For the final piece, use your table saw to cut the length of the board so it fits against the far wall while leaving 1/2 inch of space. Nail the final piece in place. You won't be able to access the side of the final piece to nail it, since the side will be against the wall, so nail the final piece through the face of the board, as you did for the first board.
6. Cut and nail ceiling trim around the edges of the ceiling to cover the 1/2 inch gap. Cut each piece of trim to length on your miter saw, mitering the ends at 45 degrees so they form the corner. Attach the trim by shooting nails every two feet or so through the trim surface and into the wall behind it.
Tags: final piece, second board, trim nailer, against wall, ceiling planks, first board, inch space