Hi; I am building a block and timber workshop in my garden and intend to put on a fibreglass flat roof. I am hoping to fit the roof joists today / tomorrow and before I do I need some advice on the structure to ensure that I can fibreglass it properly later. All of the guides I have seen usually have one side of the roof butted up against a wall however mine will be sitting on top of the tall wall on the right of the pic below, this will be referred to as the back of the building. Her is the partially completed frame (minus noggins and additional wall plates): On top of the frame in the pic I will be fitting an additional top plate. A wall plate will also be fitted to the top of the back wall so that all the plates are level. I will then fit 4 x 2 roof joists running from right to left in the pic above e.g. from the tall wall at the back. This means the drop of the roof will be towards the path which you can just see in the left of the pic. My intention is to have a gutter running along the front of the building (in line with the path) with a down pipe either depositing water down the steps or into a small water butt on the RH side of the building (looking towards the door opening). 1. When fitting the roof joists on top of my wall plates should they end flush or overhang all around slightly? 2. I am using 18mm OSB3 as my base board for the roof, I understand I need a 3mm gap between any joins but should the board overhang the roof joists or be flush?. 3. At the front of the building where I intend to fit the gutter should the roof joists and or board overhang to allow for a downpipe? 4. Do I need to fit an arris rail all around? Looking at Please login (registration is free!) to view this link. http://www.diydoctor.org.uk/projects/building_a_flat_roof.htm everything is flush? I understand that I need to fit furrings on the tops of the roof joists to give a 1:50 drop.