Garden room construction flat roof build

We are fighting the constant rain now on this one. This time last year we all had sun tans! We are using 5x2 c16 for the roof and fixing with 90mm Paslode nails and joist hangers fixed upside down with twist nails. There is a 25mm for every 1mtr fall on the roof, which equates to 75mm over this 3mtr build. With this type of roof and no furrings used we can maximise the internal head height of the room and still keep under the 2.5 limit for permitted development. Thanks for watching as always.