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This is why all master plumbers should be held 1 million percent accountable if their measurements are wrong as they are supposed to know not guess sizes. Not the engineer but the plumber is responsible hopefully and should be liable. We calculate flow rates based on how many fixture units we are dealing with like combined flow and if the municipal storm lines are large enough.
For example we look at local codes and weather charts and by interpolation we find how many feet of drainage will become GPM flow (3 Square feet for example in some codes are considered 1 FU or 7.48 gallons after the 1st 10,000 ft etc. simple stuff huh?). Do you have any sump pumps connected to this system as every GPM of a pump is also considered 1 FU We also have to consider "pitch" of the roof/drainage area's as the more incline the more gallons of flow we will encounter and thus larger piping must be used. The conductors from the roof like leaders and or gutters also have to be properly sized and pitched with scupper boxes placed to protect the roof from possible heavy ponding which can cause a cave in (1 Cubic foot of water weighs over 62 pounds) and most non promenade decks are designed for weather protection (30 pound design weight per Sq. ft) and really offer no load bearing weight factors like normal decking. Normal is 125 per Sq. ft. If you would be so kind as to E mail me with your exact area (I need this for 10 year frequency flow rates) And the square footage of the areas to be drained. If possible the roofing materials (KMM,EPDM,slate,BUR,SIS, Gravel, river bed rock for ballast, Koppers, etc) then by knowing the roofing membrane/insullation type I can figure out the flow restriction factor. Some contractors do install reversed insulation as it is much easier to find roof penetrations without insulation under the bitumen membrane. Is this a pitched roof or flat and if pitched what degree?
 This does not have to be exact as I use a fudge factor incse of leavs or other debris blocking the strainer, leader head etc. Normally a flat roof has a design load of "only" 30 pounds per sq. foot, this of course is for allowing a build up of no more then 3" of water. Ask your plumber at what flow rate did he/she figure for design of the size of the pipe?
The key in this profession is to actually know our job BUT in some cases this is not the normal. That is why we have lawyers as a counter measure for the simple mistakes some contractor make.
 Sylvan LMP (stormwater Master Plumber) "Plumbers Protect The Health Of The World." 
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This is why plumbers have to know when to install a controlled flow devices not to have a flooding or over loading the system.| Sylvan Tieger - [Intro] | [Qualifications] | [Email] | [Website] | [Articles] |
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