Is there a special ladder or some specific way to repair glass panes on a greenhouse roof? Also I am thinking about replacing glass with clear PVC panels. Any tips on this?

 

Clear PVC (or standard plexiglass, if that is what your are thinking of) won't stay clear for long. What you can use is UV resistant lexan or polycarbonate.

Since you are asking about some specialty ladder, I am lead to believe that the entire roof is glass. Don't know of any such ladder. You will probably have to start at the outside walls and remove panes one at a time, all the way to the ridge. To install new ones, start at the ridge and work to the outside walls.