Originally posted by ulzgoroth:

That part's relatively clear, but what you have to do to make the chimney protect the fire from rain is weird looking and unintuitive.

How come? Real chimneys also have to protect fireplaces from rain. The game is pretty forgiving in that any roof directly over a fire seems to guard it from rainfall. I'd probably agree that the amount of space you need to allow for smoke to escape is unintuitive though.