You go to build mode and click on the roof. A cool new feature in the game, actually, is that you can move the walls by putting your cursor over it and sliding it back and forth. After you could go back and click on the roof and click on something called auto roof and itll fix it for ya =]
Hope this helps
Some people are having problems viewing the roof, even if you have auto and go back to the town you can't see the roofs on some games. It's something that they will hopefully fix in the first patch.
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Hope this helps