Student or Learner
The house (which)/(who's)/(whose)roof is red was sold to an American.
How is correct?
In addition, a roof is part of a house, it belongs to the house, so use possessive whose--yes, things like, houses can indeed be modified by whose. Moreover, the word who's is a contraction for who is,
Contracted form: Who's on the phone?
Uncontracted form: Who is on the phone?
Does that help?
Write, Which one is correct?, not How is correct?