I want to construct a lean to shed against a large brick retaining wall in my backyard as this would better utilise the space I have. Aiming for a 10 x 7 ft lean to shed. How would I go about building the back wall of the shed that goes stuck on the retaining wall? Would I construct a stud wall on the retaining wall and build the shed from this. Also how do I make my shed watertight? Thanks.