Board and batten is the most common style of barn siding and as a result you will see some barn doors built in that style. One thing to note is that board and batten is a style developed, so that one can build with green lumber that has not been kiln dried. This works great for the siding, but when covering an opening such as a doorway the shrinkage that occurs with green wood can become a major headache. By using tongue and groove, which is always kiln-dried before it is sold, one can be assured that the door, which is being built, will not shrink over time. This is of special advantage if you live in a place that gets lots of weather.
Inspect the doorway to make sure that the frame is in good order and that everything is square. You can use a framing square to check this. If there is a jamb present make sure that it is true and straight.
Decide on the dimensions for your door. If you are building a doorway with doorstop or if you are going for a more rustic entrance with a door that swings off the trim, you still will need to know the exact dimensions.
Set up a pair of sawhorses and cut each board to length. We will deal with the width and backing for the door, shortly.
Cut and fit each board together by placing the tongue inside the groove. Keep adding boards until the over all width is at least 2 inches greater than the calculated width of the opening. Make sure the tongue of each board fits tightly into the groove of the next board. You can tap on one end with a wooden mallet to get a tighter fit. Never use a framing hammer unless you have a wooden block to soften the blow.
Cut the tongue off the first board with a circular saw. Make sure the edge is square and that all the tongue is removed.
Attach two cleats to the backside and make sure each tongue and groove board is attached at least once to each cleat. Also be certain that the tongues and grooves are tight. Place the cleat at the very top and bottom of the backside of the door. You can attach the cleats with short wood screws (1 ¼ to 1 ½ inches long).
Cut the door to width.
Attach the braces to the back in a Z-pattern. The top and bottom of the Z should go just inside the two cleats, then you can place the diagonal piece and remove the two cleats. It is best to remove the tongue for any board that you use as a brace.
Add hardware and hang the door.
If you can afford it, you can cover the whole barn in tongue and groove. This door can be built with the boards running horizontal instead of vertical, although vertical is most common.
Be careful with the power tools. If the opening is wide, you might need a pair of doors that swing open from each side of the doorway.