Just wondered if anyone on here had any technical knowledge of rules on drainage of flat roofs! I am asking because a neighbour has built a boxed dormer onto the parapet wall at roof level with a flat roof overhanging the parapet (a shared party wall). Their roof is completely flat whereas I have read guidance on roofing sites/forums that there should be a raised edge to the roof to keep the rainwater from draining off the side. They don't have this and the roof edge slopes downwards slightly to drain the water off the side! I have raised this with them but they are denying any water comes off the side. I even videoed it dripping! They obtained building control from a private building control company hired by the builder but I think they may have made some shortcuts (they didn't bother with a minor detail like observing planning permission for example). I am concerned about the impact on our property of the extra water flowing onto it and eroding the parapet wall. However, I find the building regulations really hard to interpret so wondered if there's a roofing/construction person on this forum who can shed some light!