There's a Hippopotamus on Our Roof Eating Cake 40th Anniversary Edition - Hazel Edwards - Bog

The imaginary Hippo can do what he likes on the roof. In fact, he does all the things a little girl wishes she could, but is not allowed. When she takes a bath, the hippo takes a shower; when she goes to bed, the hippo watches television on the roof. And when the repair men come to fix the roof, the hippo conveniently disappears, returning of course to eat a very large piece of cake.