Hi Tony,
My observatory is a roughly 6’x4′ run off shed with the entire building rolling off so I guess the overall weight is not wildly different to your run off roof. Also because I have built on a sloping site the rails are supported on short posts not resting on the ground so the rails are taking the weight not the ground. So again there is a similarity to a roll off roof.
For the rails I have used 70mmx45mm pressure treated timber from, I think, B&Q. The wheels run on the 70mm face and the weight is therefore supported by 45mm of timber. There is an important point here however. The shed runs on eight wheels, four on each side, and each rail is supported on eight posts. These posts are arranged such that whenever the shed is fully open or fully closed the wheels sit directly over a post so the load is transferred straight to the ground and there is no opportunity for the rail to sag under the weight of the shed. I would strongly urge you do something similar.
The guides are quite simple. Most DIY superstores sell lengths of 10mm threaded rod and matching nuts and washers. I cut four roughly 25cm lengths of the rod and mounted one at each corner of the shed frame extending down beside the track. From simplybearings.co.uk I obtained four roller bearings. These slide onto the end of each rod, are held in place by nuts and washers and roll against the side of the track.
For what it’s worth the wheels I used for the shed were these.
The observatory has been in use for a couple of years now and so far there have not been any problems.
Regards,
David.