You are correct in assuming you need to go to face mode and just start deleting and then cleaning up the geo. Sort of treat it like you would if you were actually building a house - not everything would be the exact size needed but you couldn't just scale things up or down, you'd need to cut and add sections.
For the roof tiles once you've cut them (and cleaned up the geo) you'll probably be putting some sort of ridge capping on the joins anyway as that's what a roof would have so you don't need to be too precious with your cleanup if it will be hidden (not saying leave it a mess with tris and stuff - just saying don't waste hours on it when it will be covered anyway)