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We recently bought a new home. The house used to be a two family home. Somewhere around 2000 the second floor was renovated and the house converted to a single family home.

After a recent roof leak I realized that the house has two soil stacks. After some investigation I found that one soil stack services two toilets, two bathtubs and one sink. The second soil stack only services one second floor sink.

Both soil stacks are 4 inch cast iron pipe and go from the roof all the way into the basement and disappear into the concrete floor. They are about 10 feet apart.

Can the soil stack that is only servicing the one sink be cut back into the house and capped with the hole in the roof repaired?

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The International Residential Code requires only one vent per building.

However, all plumbing internal to the building must be tied into the roof vent. This can be accomplished with PVC (depending on your local codes) running from one vertical stack to another at least 6" above the flood rim of any fixture. In this case the attic works fine.

They cannot be cutoff and capped. Then the fixtures being served would not have a vent.

The requirement for a fixture to be within a certain distance of a vent is a different subject than whether that vent goes all the way through the roof. All vertical vent stacks must be tied together and taken to the main building vent or taken through the roof separately. Sometimes it is easier to run another roof vent than to run all the way across the building to the main vent.

As always check with your local inspector to determine if they are using the IRC or the Uniform Plumbing Code which has different requirements.

Good luck!

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Don't count on it. It has everything to do with the vent connection to the sink or toilet the vent services. there can only be so much distance between the vent and plumbing fixture. They may have added the second vent so the walls would not get tore up in the original house to make the connection, or it was simpler to do that than connecting it in the attic. It was most likely a matter of cost.

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