Quote - "[...] but since I've already put a lot of work into making the roads, I’m inclined to preserve them[...]"

With my method you wouldn't waste this work. Your road bitmap serves two purposes right now: 1) they serve to determine where a different material should be and 2) they serve to determine houses placement. With my proposal, your road bitmap would serve only one purpose (usage number 2), but it would still be useful.