I have the knowledge of installing a wood floor under a door by cutting casing and door jamb. But that seems working only for interior doors. This exterior door to the back yard has a thin casing which can't cover the expansion gap, and there is a threshold there, so I can't cut the jamb. My idea is to replace thin casings with thick casings to cover the expansion gap and put a shoe modeling next to the threshold between 2 casings to cover the gap between floor and threshold. I'm not sure whether this is a good way. Any advice?