A free 3D planner for roll-off-roof observatories, from Dark Dragons Astronomy. It needs JavaScript to run.
Set your building's size, wall height and roof, and it frames the whole thing — walls, rafters, track, truck and outriggers — as real lumber with real cuts. It then answers the questions that decide whether the design works: does the roof close over the parked telescope, does the tube clear the pier through a meridian flip, and how much sky do the walls leave you at each compass bearing.
Out of it come a cut list, a buy list, printable rafter templates and a step-by-step build plan. It is not engineering: check your own local code, and have anything load-bearing reviewed before you build it.