First floor ceiling and roof

My model consists of two floors, 2.7m high for first floor and 2.7m for second floor. I added a pitch roof which messed the whole model up, so i deleted the roof data from the DDBROOF.NDX folder along with the ROOFPOLY.* and RLIGHTPOLY.* files. The model has become slightly better, but in 3D model view i can see straight down to the GF- so the GF has no ceiling. How do i add a ceiling to the GF. I also want to add a pitch roof with the middle edge elevated 2.25m, but i cant seem to get the datum height correct. Please can someone help me. im assuming the datum height is just 2.7+2.7=5.4m.