We install a full-length rear drip trim to all of our garden rooms. This distributes the roof surface rainwater across the full length of the rear of your garden room and clear of your rear cladding. This is an effective method for managing rainwater fall in most circumstance.
As an option:
We can install a full-length rear gutter profile, downpipe and a water butt to collect the roof surface rainwater runoff. A water butt is always required with a gutter and downpipe to prevent concentrated water flow onto the ground unless there is a drain present.