The housetop is the top part of a house — the outside upper surface. Though actually, almost everyone now just says roof. Housetop sounds a little old, or like church English: in the Bible people ‘proclaim from the housetops’ when they tell a secret to the whole town, and that saying is still used. If you mean a flat place you can walk on, younger speakers say rooftop. You will meet housetop in stories and sayings more than when someone is pointing at a building in the street.