Main stack vent size ?

[abacadabra]

Hello everyone, I am new here.

I have a 120 year old house that had indoor plumbing added sometime around 1930 (my guess).

It is a 2 story house plus full basement. The sewage is on private septic. There is a full bathroom on every level (including basement), plus kitchen and laundry area. I am in the middle of some extensive remodeling and I've noticed that there are no re-vents anywhere in the house. The bathrooms are all in a line, close to the main stack. So maybe that's why they didn't put any re-vents in. But I'm re-doing everything and want to make it right and per code.

My question for today concerns the main stack vent. The main stack is a 4" pipe and it reduces to a 2" pipe in the attic and then goes through the roof. I don't want to mess with this pipe because it is on a part of the roof that I cannot access safely from outside. But, I could add another pipe if needed, on a part of the roof that is easier to access. Something I read on the internet said that the main stack should not reduce in size at all for the vent, which would mean it should be a 4" pipe going through the roof.

I calculate my # of drain fixture units to be 29.

Can someone please tell me if I need to add another vent pipe through the roof, and if so what size should the pipe be? Or is the 2" pipe that already exists good enough?

Thanks in advance for any advice.

 

[Jadnashua]

Which code does your locale use? UPC, or IPC, and what local modifications are applied? IT also matters where you live. In some places, where it gets quite cold, it is required to make the roof penetration (and some distance below, if I remember correctly) be expanded to a larger pipe diameter to prevent hoar frost from closing it off. So, I'm not sure that there is a single answer that works everywhere without knowing where you are.

Older houses often relied on S-traps rather than a properly vented p-trap. They tended to work when drained into a large main stack. It can be a major effort to rework the drainage system to meet modern codes and you don't have to, unless you are making changes.

 

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[abacadabra]

Thanks for your reply.

I am in Ohio, so it is IPC. You are correct, the house has S-traps and not P-traps (everywhere that I can see).

The drain piping needs replaced anyway. They are cast iron and starting to rust through. The joints in the main stack are starting to leak a little bit and the other pipes have holes in them. I am planning to replace the main stack (and main stack vent) all the way up to the attic with PVC, then use a Fernco fitting to connect to the 2" galvanized steel pipe that goes out through the roof. The nice thing about the house being built originally without plumbing is that everything is easy to access. The main stack on the first floor is in the corner of a room and then they put some paneling around it...so not hard to get to.

 

[Jadnashua]

A whole house repipe may be beyond the forum's capability to guide you properly.

 

[hj]

Some codes require a 'full size vent pipe" through the roof, while others just require that the aggregate AREA of the vents equal the size of the main sewer line. I cannot imagine a 1930s house not having proper vents and having "S" traps, since modern codes were in place by that time.

 

[Jadnashua]

The house I grew up in was built in the early 1950's, and all of the sinks in the house were s-traps.