"build new floor" grayed out

[RootFarm]

It helps (a lot!) to give plenty of information regarding a problem. In this case we only know that you deleted a roof, and now you can't build a new floor. We don't know which program you are using, nor which version number. We don't know how many floors are already in your plan. We don't know how you deleted the roof. Did you delete the roof planes, or did you do something else? Attic floors are part of the floor below. What did you do to try to delete the attic floor"

You have left us to guess that maybe you are trying to build more floors than your version supports, but we can't really be sure there isn't another problem. That is why you got the answers that you did, in case you are wondering.