Tom Sawyer
In the Trades
I would like to clear up some confusion here regarding the practice of flat venting. I fully realize that in some states it is perfectly legal and acceptable to inspectors. I want to show you why it should not be though, and that just because a thing can be done it is not always the best reason to do it that way. Take a look at the picture here. This is an acceptable ( to some ) method of venting a water closet. The wye fitting which is being used for the vent has been rolled up so that its run inlet is above the center line of the horizontal drain. So far so good. Where this goes wrong is the horizontal pipe that heads back to the wall parallel to the WC closet bend and then it runs up the wall and presumably to another vent or through the roof. At any rate, that horizontal piece of pipe can and will eventually fill with toilet paper, tampons, condoms and poo and it will plug up thereby negating it's intended use. Better that the vent continue to rise vertically until it has reached a point (6" above the highest fixture served which is the lav) and then it can go horizontal because it won't be subjected to all the nasty stuff that can clog it up.
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