The 5 inch trim boards need to be of a cosmetic quality. They need to fit flush and well because they will be the final cosmetic result.
The 5 inch trim boards overlay the 3 inch trim boards and overlap onto the tiny house siding. These boards cover any discrepancies in the fit of the siding and hide any irregularities or imperfections.
Before the 5 inch boards are installed, it is imperative that you caulk the seams to avoid intrusion of water between the siding and the trim. The caulk should be applied right before the 5 inch trim board is installed so that it adheres to the trim board as well.
On the windward side, it is very important to fully caulk the edge between the trim board and siding. If gaps are present, high winds can get under the boards and cause wind damage to the tiny house. By filling everything with caulk, the wind is shunted away and the structural integrity is maintained.
It is also important to order the trim boards properly. The front board needs to overlap the side board so that the leading edge of the side board is hidden in the wind shadow of the front board. When installing the trim board, have an offcut to gauge the placement of the trim boards. This will prevent any mistakes that could be made while you are attaching the boards.
With your depth set, screw the trim boards in and counter sink the screws so that they can be covered with putty at a later point.
With the front board attached, caulk can be applied to the side and the side trim board can be attached behind the front trim board. The 5 inch trim boards cover over the edges of the lap siding and make everything look uniform and complete.