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commercial roof drain---venting

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A commercial roofer and a plumber are in disaggreement.  The roofer says a vent in the roof drain should help prevent backpressure on the drains and facilitate the flow of water from the roof.  The plumber says that it will have no effect.  We have too many drains going into a drain and not willing to increase size of drains in building due to cost and amount of work involved.
asked in Plumbing by dc3749 (120 points)
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1 Answer

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Hi dc3749,

The following is a paragraph from the International Plumbing code (http://publicecodes.cyberregs.com/icod/ipc/2009/icod_ipc_2009_9_par002.htm).

901.2 Trap seal protection. The plumbing system shall be provided with a system of vent piping that will permit the admission or emission of air so that the seal of any fixture trap shall not be subjected to a pneumatic pressure differential of more than 1 inch of water column (249 Pa).

In other words, the plumbing vent protects traps against atmospheric pressure differences that could cause them to siphon or blow out. I'm not sure what the plumber means by saying that the vent facilitates the flow of water from the roof… that’s what the gutter is for. Any water  that enters the vent will drain into the soil stack but that’s a natural “side effect”, not a purpose.

answered by darekrudy (21,730 points)