The plan is to glue and dowel the centre joint the boards will be getting a couple of coats of base coat before been fixed and once fixed will get another 2 coats of top coat, so the outside face will have 4 coats. The roof overhangs 500mm and protects the walls from a lot of direct rain. Also the door is being made right where it is going to be used so the moisture content shouldn't be to different and should be in equilibrium with its surroundings in service , but will have to see over time