Even easier is the 'arch' tool. When making a brush, look to the right or hammer and you should see this panel.
Click on the arrow and select 'arch'.
Then work like you normally would, covering the entire circular area with the white box. Then hit enter. You should see this menu.
Fiddle with the settings until you get what you want, set the number of sides to the same as you floor/ceiling and don't go over the top, ie 4000000 sides. After 24 sides, there's not much difference. Another advantage of the arch tool is that it always snaps to grid so no need to worry about that (unless you re-size it).