For the roof supports, I thought of various methods to support an extended roof without vertical supports, finally ended up with this design that works by positioning wheel tracks at the appropriate points such that when the beam that supports the roof extends out, it starts off being supported on two bottom castors, but as the COG crosses the outer wheel, the wheel on top takes the load and is supported by the upper track.
I had to offset the two halves vertically as the roof needs to roll off 1.2m, but the beam needs to be longer than that to act as a cantilever.
the offset design allows the use of a 1.8m beam, and the roof will be made with lightweight materials including insulation. each of the castors are rated at 50kg each. I expect the whole half roof to be less than 50kg.
for tracks, I initially thought of the sliding gate track but thought i'd make my own. so routed out a V and a straight, glued an aluminium flat for the wheel to roll on and avoid compressing the timber when the roof is left at one position for a long time.
its not the smoothest, but will do the job as I'm not planning on motorizing it.
As for waterproofing the split, I will add on an overhang on one half so it completely covers the split. the two halves are also sloped slightly to prevent water collecting.
I went with blue cement sheets for the sides.
the sides and the roof will be insulated.
Corners will also have a channel with some render to make it water proof.
Thinking of 6mm marine ply for the roof as its not taking any load and should be light enough.
will be using pine where possible for the roof frame, hardwood at support points.
some pics.
test video of one of the beams
https://youtu.be/JjvHbOsIo7E