I have recently cut out the cast Iron pipe in a house remodel and am getting ready to redo the Drainage and vent system in PVC. The Cast Iron pipe was 4" pipe. I will be tying into the existing cast iron that leaves the house for the street with a no hub coupling and then reducing down to 3" pipe to do my main drainage and vent stack. My question is since I had 4" vent pipe exiting the roof into a rubber boot on the roof that is still in good shape, can I, at the end of my main vent stack go back to 4" PVC to exit the roof so that I don't have to mess with the rubber boot and shingles on the roof.

Thanks,

Jeff

Shouldn't be a problem. You are not restricting water flow it is just for venting

Yes, I also think it should not be a problem. Only one question come to my mind: why don't you run 4" all the way to the roof ? What's the reason you want to reduce to 3" ?

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Originally Posted by Milo Dolezal

Yes, I also think it should not be a problem. Only one question come to my mind: why don't you run 4" all the way to the roof ? What's the reason you want to reduce to 3" ?

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Thanks for the quick response and helpful answers! I want to do the majority of the runs in 3 inch pipe because it will save me a little bit of money and will fit in between my new joists and subfloor support modifications that I had to make specifically underneath my bathroom where the main vent stack will make its vertical run and the majority of my drains will be meeting.

I agree with Milo and biguy it shouldn't be a problem, good luck.

John