Taking a shot from an unexpected angle always works. Twitch streamer cdnthe3rd knows this particularly well. When he spots his enemy, he takes a very quick sniper shot through a crack in the roof, leaving his poor opponent wondering what the heck just happened.
This clip proves that even when you think you're safe in Fortnite, you might not be as well protected as you thought. So stay on your toes at all times, and check your roof every now and then -- it might be in need of repair.