Full replacement (16mm -20mm thick) requires the removal of all existing timber boards before fixing the replacement board to the structurally sound rafter ends or suitable packers fixed to the sides of them.Refurbishment/capping boards should be fixed to good quality, existing timber boards or newly introduced backing boards (min 12mm marine ply). Never cover over rotten timber - cut out any suspect areas and replace with treated timber or replace the full length with a ply backing board.