Yes I get the same problem - I thought I had done that in TFP V12 but mayby I am thinking of Floorplan V11. Looks like totally flat is not possible. Alternative, set 3 sides to gable (as previously explained) and the 4th to 1 degree. This certainly looks flat even if it technically isn't. If you want it totally flat then you could use another object instead but this seems uncessarily messy when the 1 degree incline will not show.
Allan