Replace cast iron vent with PVC

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Gilles Guezille

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Nightmare 1960 Ranch home with 4 inch cast iron vent going through new roof.
I want to replace Most of the CI with 3 inch PVC and use a 3” to 4 inch boot to connect the PVC to the stub of CI going through the roof. The CI is obviously very heavy and the toilet flange is resting on cinder blocks in the crawl space , supporting most or all the weight.
My plan was to cut the pipe in the attic as close to the roof as I can but leave enough length to later use the rubber boot. Then I start cutting chunks of CI below until it’s all gone
My concern is that the stub could fall in because of it’s own weight in and leave a hole in my new roof. Is this a legit concern? If so is there a way to prevent this?
Thank you
 

Reach4

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My concern is that the stub could fall in because of it’s own weight in and leave a hole in my new roof. Is this a legit concern? If so is there a way to prevent this?
Yes, that is a strong concern. Before cutting cast iron, make sure the pipes above the cut are well supported.

Why not replace the part through the roof with PVC? Yes, you need to protect that from UV, but you can paint it with latex paint.

To support, riser clamps can be useful. You have to support the riser clamps.

You need to support PVC also, but it is not as heavy.
 

DIYorBust

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I can understand not wanted to deal with the roof penetration as that would add a significant amount of work. However, is there no way to support the cast iron vent from underneath and save yourself even more headache?
 

Gilles Guezille

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Yes, that is a strong concern. Before cutting cast iron, make sure the pipes above the cut are well supported.

Why not replace the part through the roof with PVC? Yes, you need to protect that from UV, but you can paint it with latex paint.

To support, riser clamps can be useful. You have to support the riser clamps.

You need to support PVC also, but it is not as heavy.
Thank you!
I talked to the roofer who did the new install and I will replace the whole CI pipe. He offered to come check the boot On the roof to make sure it is sealed.
I’m now afraid the house will fly in the wind after removing all that weight! LOL
 

Dj2

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Good choice of replacing the entire CI vent.
If you worry about the house blowing in the wind, then bulk up, starting with 6 meals a day (like on the cruise ships).
 
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