Many city building codes require a parapet wall around the roof of flat or nearly flat roofed buidings for safety's sake, there is more presumption that someone may be upon that roof for some reason and become likely to fall off the edge, as has happened in many cases. This stems all the way from the Bible, where at Deuteronomy 22:8 the ancient Israelites were commanded to put a parapet around their roofs for safety's sake.
Parapets obviated gutters, as on pitched roofs, but then necessitated either scuppers which drained through holes in the parapet, or roof drains which usually had a raised, pierced dome shape to prevent clogging, though usually through the years they became clogged anyway since no one thought to go upon the roof every year to inspect and clean them out. This is the reason that so many theatres had drastic roof leaks where the stagehouse wall joined the auditorium roof. The drains were not cleared and then the pooled rain water penetrated the roof and poured in on the organ chambers or the proscenium.
Not all cities permitted scuppers especially on taller buildings since they could allow a considerable amount of rain water to spill down upon pedestrians on the sidewalk below, possibly causing injury.
The best way to really 'seal' a roof is to build a "super-roof" above it that is pitched and will therefore spill the rain off the building BEFORE it has a chance to pool and penetrate the roofing membrane. Such is not cheap, but in the long run, the best preventative. It is done by opening the caps to the steel columns in the walls and attaching at least seven foot high aluminum or similar column extensions rated to hold the weight of a new roof of aluminum sheets over the existing flat roof (as well as wind load), such that a man could walk upon the flat roof with enough head clearance that the new aluminum structure would clear him. Such aluminum roof sheets would shed the rain to the sides if the city allows such, and thereby keep the original flat roof essentially dry during a rain. The sides of such a new raised, pitched roof would best be made of pierced aluminum sheets that would prevent the entry of goodly amounts of rain, as well as birds and wind blown debris. It may not be the prettiest way to prevent roof leaks (depending upon how it is handled) but it is the best bet in the long run. Remember how you know a roof is leaking: by looking up at the inaccessable auditorium ceiling and noticing the expensive damage to paint and decor up there which sometimes requires the skills of a restorationist to restore at many times the cost of merely resealing the roof -- if one can find all the point(s) of leakage. This double pronged method of avoiding roof leaks (the original flat roof toped by a new pitched roof that no one walks upon) may be the only real long term protection, though not cheap. This "Super (as in 'Above')-roof" is the province of an architect, not a roofing contractor. Note that the aluminum sheets are NOT sealed to each other like shingles so as to allow for considerable expension and contraction, and therefore a small amount of rain is still expected to come upon the flat roof, which is why it still must be resealed after so many years and its drains, if any, kept clear of debris. In heavy snow load areas, the aluminum will be heavier and more costly and local regulations will restrict the pitch and disposal or fall area of any snow load. Your architect should be familiar with such.
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