These are the types of roofs you can build on your house.
You have to create a floor plan below you before you can build a roof on any level other than the ground floor. Do this by using a hammer/mallet on a floor and Plan>Floor below, then Plan>Roof. You cannot build a roof on a house underground.
It can be fiddly to plan a new wall on a tile which has a completed roof structure adjacent. This is due to the model for the roof overlapping the border tile and making it difficult to be selected.