Because of myself being a sheet metal worker I set about building my own roll off roof observatory from metal. Lack of space for the roll off caused me to make the roof in two halves. The outer half rolls onto the lower half and they both roll away. I felt that using castors, for example, on both halves would produce excessive gaps. So to keep the roof sections as low profile as possible I made two channels from 2mm thick galvanised steel, the sides about 12mm high and the inside width about 25mm. I then made eight blocks of metal that would run in the channel. They were drilled with a hole which would accept a steel ball bearing. The blocks were fixed to the corners of each roof. So basically the roof runs on captured steel ball bearings which produces minimal gap. Because the roof is an apex style and I have two runners on each side it can bind a little bit but a touch of grease can help. So far it has served me for many years without trouble. The channel shows little sign of wear but I recently changed the ball bearings which helped it run smoother.