I've come across a situation on a project where an owner wants to add a storm drain to a portion of his roof where water is pooling and not draining properly. The coverage isn't very large, approximately 444 sf, however the tricky part is where this water is going to be draining to. The building in question has an upper roof, which has the problem I previously described, and the contractor I'm working alongside has proposed routing the storm pipe down to a lower roof to discharge into a pair of existing storm drains. The drains are 2" and 4" and are currently sized to drain a total area of approximately 3,412 sf of the roof they're situated on. The existing pipes are sloped horizontally at 1/8" / 1' and, given the max rainfall of the area I'm working, they are able to drain a maximum of about 137 gpm of water.
Given the area of the lower roof (3,412 sf) and now adding in the small portion of the upper roof (444 sf) that we're trying to drain, this will lead to about 145 gpm of water that will need to be drained. This means that during the worst possible downpour conditions there will possibly be approximately 8 gpm more rainfall than my existing pipes can handle per the IPC code (1106.2). My question to you all is do you think it would be necessary to upsize all of the existing pipe to accommodate this additional GPM or is the pipe sizing in the IPC conservative where it would not be necessary to upsize?
Given the area of the lower roof (3,412 sf) and now adding in the small portion of the upper roof (444 sf) that we're trying to drain, this will lead to about 145 gpm of water that will need to be drained. This means that during the worst possible downpour conditions there will possibly be approximately 8 gpm more rainfall than my existing pipes can handle per the IPC code (1106.2). My question to you all is do you think it would be necessary to upsize all of the existing pipe to accommodate this additional GPM or is the pipe sizing in the IPC conservative where it would not be necessary to upsize?