We have built a 3 x 3.6m pergola on top of our decking but we desperately need to add a roof as there are cherry trees behind our house which is dropping cherries, birds mess all over our decking. My missus wants the roof to be retractable as she wants the sun (when it's available in the uk) but also protection from the rain and trees at other times. The problem I have is that the supporting beams stick out further on all 4 sides than the frame and they are completely level so we have no natural slant. Ideally we would want something like the polycarbonate corrugated roof but not sure how I could make this retractable. Does anyone have any ideas?