Why does the vent have to be above the flood level rim?

[tmpusr889]

I understand that vents are needed to prevent vacuums from forming because of negative air pressure and thus prevent the traps from draining. I don't understand why the vent needs to be six inches above the flood level rim. It seems like allowing air in, at any height, as long as it is downstream from the trap, would relieve/prevent the vacuum. Can someone explain this to me in dummy terms?

[Terry]

If the sink backs up, then the vent fills with water and is no longer a vent.
The vertical sections drain away the solids well, but the horizontal sections don't. Therefore, code wants the horizontal unwashed sections to be above the flood plain. It's the long term solutions to keeping the system up and running no matter what.

Imagine if Japan had placed their infrastructure above the flood plain. It just makes sense to prevent water problems in the future. If those sections of pipe are where the sludge and water can't collect, then they will last as long as the home.

There are some cases where inspectors will allow piping with waste fittings below the flood level.

Here is a nice link to Bert Polk's plumbing tips

[Tom Sawyer]

I was just gonna go with because we say so.

[dlarrivee]

If you have to ask, then you probably shouldn't be doing your own plumbing...

[LLigetfa]

There are some cases where inspectors will allow piping with waste fittings below the flood level.

My kitchen sink is in front of a window and the sill is too low to allow the vent to be the required distance above the rim. The inspector made an exception.

Free standing tubs are another case.

[SacCity]

The other option would have been to use a longer trap arm, with the drain running off the side of the window and run the trap arm to under the kitchen sink. I've even seen in some houses where they just run this drain flush along the studs and notch th cabinets around the pipe.....
Michael

[dlarrivee]

My kitchen sink is in front of a window and the sill is too low to allow the vent to be the required distance above the rim. The inspector made an exception.

Usually there's just a longer trap arm and a typical vent/drain beside the window actually...

[LLigetfa]

The vent went vertical up to the window sill and turned sideways to go around the window where it went vertical again. The horizontal section was above the rim of the sink but the inspector noted that it was not 6" above as required by code.

{edit}It was a wide mulled pair of windows, much wider than the sink base cabinet so the trap arm would have had to be much longer and be routed through an adjacent base cabinet. Around here they don't let us do side turns on the same plane so I could not jog the trap arm unless I rolled a pair of elbows.

[jimbo]

............... why the vent needs to be six inches above the flood level rim.?

The take off for the vent isn't the issue. It is that a vent pipe can not be HORIZONTAL below that 6" number, for the reasons explained by others. The vent has to take off, and rise on at least a 45º angle.

[hj]

You only quoted HALF of the requirement. The other part is 'or 42" above the floor whichever is HIGHER'. That requirement is so you cannot put it a 21" for a toilet and then later install a 36" high sink on that line and expect it to be legal.

[Tom Sawyer]

Massachusetts still allows flat venting IIRC

[tmpusr889]

Thanks, Terry. The link was helpful and I forgot about backup issues.

I still have a question though. If the vent branches are at 45 degrees and are allowed to drain and the main vent is above the flood plain, shouldn't that be enough? Why does the initial rise have to continue until it's above the flood plain? What is wrong with this:



?