Assuming the system below (solid is waste, dotted is vent).

Do I need vents for the Sink C, Toilet C and Toilet B? They are connected to the primary waste line which is vented at the end of it.

Sink C and Toilet C are in the first floor, all the rest in the second floor.

enter image description here

The distance between the toilet C and the junction with the main vertical waste is less than a 1 meter. Here is a top view from the system: enter image description here

  • 1
    I believe vent requirements are based on the distance from the trap and vertical drop. The type of drain (toilet vs sink) and diameter of the pipe may also be relevant. Many locations allow a wet vent where a vented drain line from one fixture can function as a vent for the next fixture. – BMitch Oct 2 '13 at 20:50
  • Note, the OP has listed their location as Araraquara, Brazil. Make sure your answers are relevant or quality what locations your answer applies to. – BMitch Oct 2 '13 at 20:51
  • @BMitch, my main concern is Toilet C. When waste goes down the main vertical waste to the sewer, can it create enough back pressure to break the trap on Toilet C? Remembering that the main vertical waste becomes the primary vent and everything else is vented. – Luiz Borges Oct 2 '13 at 20:56
  • What is the box on the left in your diagram, and what size is the drain pipe? My suspicion is that you don't need a dry vent for toilet C since the stack will act as a wet vent and be within the maximum unvented distance. But I don't know your local building codes or even my own local requirements to give you a firm answer. – BMitch Oct 2 '13 at 21:03
  • In Brazil that is called an "inspection box". It is used on junction of waste lines. Looking at the top view: the top of the box will receive a waste line from the kitchen, the left of the box will receive the main waste line from bathrooms, and the bottom of the box is the exit to the sewer. The drain pipe from toilets and from the main vertical waste is 100mm. – Luiz Borges Oct 2 '13 at 21:12

I'm answering my own question since I changed the design based those comments here and on some other research (code and recommendation).

Sink C now has it's own pathway to the "junction box". The vent on Sink C joins the main vent stack above all and acts as a wet vent to the junction box too.

Toilet B doesn't need a vent due to distance to the main stack.

Toilet C is of a little concern because it is "wet vented" by the main stack, but since the piping there is 4 in I doubt that it could create enough back pressure to break the Toilet C trap.


Yes you do. Add another dotted line to the first part of your dotted line on you main stack. Also my local inspector would not let me share a drain for shower, sink, and toilet.

  • Hum, I can add vents to Sink C and Toilet B, but I can't add a vent to the Toilet C without breaking the concrete slab on the floor. Take a look at this drawing, see if it is ok now. About the shared drain, my drawing is not right, it is a drain from the toilet going to the main waste, and another drain from shower/sink going to the toilet waste line (the toilet waste is below than the waste from the sink/shower), is that ok? – Luiz Borges Oct 2 '13 at 20:14
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    I was under the impression that wet vents were fairly common, which would require sharing the drains. Though I suspect the preferred order is vent -> sink -> shower -> toilet -> stack. Without distances, I'm not sure you can say that shower A, toilet B, sink C, or toilet C need a vent. – BMitch Oct 2 '13 at 20:59
  • Sharing vent space is fine depending on distance. Toilet C and Sink C can share a vent that is higher than Sink C. Toilet B needs its own. My comment was more of a reflection of Toilet A could back up into Shower A or Sink A. I would never be able to get this passed in residential in my area. – DMoore Oct 2 '13 at 21:05
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    Allowable wet venting also depends on the pipe size and the fixture load draining into it. I don't know Brazilian code or the sizes involved, but just the physics involved of draining 2 additional bathrooms through a typical sized toilet vent could very likely cause trap seal breaks, especially if both upper toilets happened to flush near the same time. – bcworkz Oct 3 '13 at 2:58
  • There shouldn't be a problem sharing a drain between shower, sink, and toilet (as long as the drain is the proper size). In fact, I would think that would be quite common. Each trap would need a vent, and they would all have to combine above the flood level of the highest fixture. Maybe your inspector won't let you share a vent between a sink, tub, and toilet. Sharing a drain, shouldn't be a problem. – Tester101 Oct 3 '13 at 10:26

This is what's called a "wet vent" in the USA at least. But local rules vary even here, and have been changing.

"Wet vents are pipes that serve as a vent for one fixture and a drain for another."

One of the worries with a wet vent is that a momentary flood of water from an upstream fixture could result in an air pressure imbalance, sucking water or pushing water out of a trap. The U shaped traps must stay full of water to prevent sewer gases from coming into the home.

This is a non-answer: you'll have to look up wet venting rules in your local area.

  • As far as I know, most places that allow wet vents, require the toilet be the last fixture drained into the wet vent. They also do not allow toilets from more then one floor be drained into a wet vent. The original diagram violates both of these requirements. As for the concern with air pressure imbalance, wet vents are sized differently then vents or drain lines in order to ensure that they are large enough to compensate. – pdd Jan 31 '14 at 1:55
  • There are three ways to answer this question: what will work in a retrofit, and what is code legal in the USA, what's code legal in Brazil. – Bryce Jan 31 '14 at 2:08
  • I agree with you. I just wanted to clarify your concern about wet vents and that "a momentary flood of water from an upstream fixture could result in an air pressure imbalance, sucking water or pushing water out of a trap" -- this is why the toilet needs to be the last fixture to drain into the wet vent. – pdd Jan 31 '14 at 16:05

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