The joints of boards, nailed areas, burrs of the wood used for filling in, and miyoshi are prone to corrode because water soaks through. Therefore, these areas should be covered with copper plates to prevent corrosion.
When exposed to wind and rain, copper is oxidized and becomes coated with verdigris. The verdigris has antimicrobial properties and preserves wood. Copper plates have been used for roofs of temples and shrines since olden days.