Thanks a lot Jo_Ann for your kind help, but it didn`t work .. Thers is no flat ceiling anywhere on the loft. Your height zero is 1800 mm and the other knee wall is 1100 mm, same as railing. Can`t really see what I\m doing wrong. ..
One should thing that it would be only to make the roof first , then the terrace area as a room, then select and choose "No roof above " and "No ceiling " but no. But I`ll keep on trying :-)