Plumbing in the Home/venting


I am going from a 1 sink vanity to a 2 sink. I have to move a vent stack that is venting a tub and the vanity sink. Can I tie into my shower vent stack and would it work to vent 2 sinks, a tub, and the shower on the 1 vent stack.

HI Robert,
This should not be a problem. There are few things to consider here though…

1. Venting a double sink vanity must be done properly. Code does not allow you to have two traps feeding a single drain line. This can be handled by using a special "Double Sanitary Tee" in place of the single tee that is in the drain line now. This way you can trap each sink separately and feed them in from either side of the drain into the new double Tee.

2. Any vent line that runs near horizontal must have a minimum slope of 1/8 inch per foot back towards the drain line it is serving so be sure to allow for that in your tie over to the shower vent. When you do run the horizontal lines, make sure they do not "break over" from the vertical lower than 6 inches above the flood level rim of the lavatory sinks or the fixture that vent line is serving.

Per code, as long as the overall aggregate area of the venting equals the aggregate area of the house drain line, you are in compliance. You will need to refer back to your early math training to calculate the area of a circle. The typical drain line on a house is 4 inches.

Just in case you forgot: A = Pi x r squared. For a four-inch diameter drain line that works out to be 12.56 in.˛ As long as the aggregate area of vent lines exiting through the roof equals at least 12.56 in.˛, you're good to go.
Good luck,

Plumbing in the Home

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Dana Bostick


Pretty much any residential plumbing questions. For "item specific" details such as a specific model of fixture, I will need to research and there may not be any useful information available. Note: I live and work in Southern California. We do not, as a rule, use hot water or steam heating systems, oil fired boilers or private water wells so my knowledge in those areas is pretty limited. There are others here on AllExerts that can probably answer those questions better.


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