Purchased this for a Barbara Gazebo. Whilst the product was fine and undamaged, application to a gazebo with a dished roof and only 100mm overhang is very tricky. I had to cut away a fair bit of the dishing to make space for the outlet.
It hasnt leaked at the moment but I have to say that I dont trust the seals.
Personally, I think you would be better off leaving an overhang at the back and installing guttering.