board and bat siding?

First of all, it is board and batten. The best way to install it to prevent water intrusion is to install it over felt or housewrap, then install your boards tightly together, then your battens over the gaps. If your battens are at least 2-3" wide, you won't have much worry about shedding rain.

For further protection, you could run a bead of caulk on either side of the gaps in your boards, then install the batten on top of the caulk.

The boards should be the same thickness, or you will need to shim one side or the other to keep your battens flat on both boards.

I have a small barn, about 16 by 12 that is board and batten. The boards are random width rough sawn, between 8 inches and 12 inches wide. The battens are 2 inches wide and nailed to the boards (there are not studs behind every batten). There are cats between studs at the four foot mark. This barn is the driest one I have, never any water getting in regardless of the weather. This is partly because the rough sawn lumber swells a bit with humidity and seals tighter. The same thing happens with the cedar shake roof. You can se daylight through some of the pinholes through it, but when it rains, it seals up tight.

This is true.... Maybe we should build "barn style" anyway...... I have worked on houses and that were wood shingled and you could see daylight in the attic and dry as a bone. All "thatch roofs" are the same...... Worlds renewable resource.....thatch..... Oh and some of these thatch styles have dirt floors....... That is where the word "Threshold" comes from.....I believe they threw thresh on the floor and in the front of the house became known as threshold......