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We have just had the shell of an extension built which includes a flat roof. Firring strips have been used to produce a fall for rainwater runoff however this fall is around 20mm over the 3000mm span i.e. 1:150. Everything I am reading (including BS 6229) states that flat roofs should have a minimum fall of 1:80 (i.e. 37.5mm in my case) and really ought to be designed for 1:40 (75mm) to allow for inaccuracies in construction, settlement of materials etc. Unfortunately I have only just spotted that our roof is only 1:150 and yet the roof (a warm roof construction consisting of joists topped with firring strips, OSB sheets, vapour barrier, 125mm solid board insulation, ply and a polyurethane liquid membrane top coating) and all associated cavity trays and flashing etc is all now in place. Bar the roof lantern the roof is complete.Now I'm fretting about possible issues in the future. Am I right to be worried? Should I be seeking this being altered (i.e. deconstructing to joist level to allow steeper firring strips and then rebuilding from there)? I was going to speak to Building Control tomorrow to say what they say but whilst they might agree that the fall is low I can't see anything in the regs that mandates a particular fall and I don't know whether BS6229 is mandatory either (it seems more of a code of practice). For what it's worth I must admit to having a tendency to worry - to the point of anxiety - about detail like this. I also feel really quite sick about the situation as I don't like confrontation and do like the builders; see them most days locally, am friends with a mutual friend of the builder's son etc and so really want to avoid a falling out if at all possible. I also really respect their workmanship. I have previously already make them put a fair bit of effort in to move an airbrick and so they know I'm a one for detail and worry over what someone else might consider to be nothing; they may well see this to be just another case of that but I can't help but feel I should be addressing this now rather than regret doing so in a few year's time once the roof fails due to a fall-related issue. Grateful for your thoughts/advice. P.S. I hope it doesn't come to this but the contract states that should we end up in dispute we agree to jointly fund an independent surveyor and be bound by his view/recommendation. I have yet to pay for completion (the final invoice only arrived today).