Cutting cast iron pipe w/o causing roof leak

I have to cut off a section of cast iron waste pipe in my attic to tie in a vent .My concern is when I cut off the section of pipe, the weight of the upper part of pipe would cause it to possible come loose from the roof flashing and sink, thus causing a roof leak. The last thing I want to do is open up a can of worms by doing this job. I was wondering if I should be concerned or if anyone else has done a job like this, what advice to offer. Thanks
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Mikepier wrote:

I would fabricate a permanent support for the upper section. Heavy steel strap clamp around it tied up to rafters. If you put turnbuckles in the tie section you can preload it before cutting.
If you can get on the roof to work, much better to simply replace the section with PVC and a new rubber flashing boot.
Jim
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DWV PVC as roof vent requires painting, as the material is not made with UV inhibitors?
Cheers, Wayne
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I like Jim's idea, but I also think that even the slightest movement of the pipe could affect the integrity of the seal around the flashing. You'd still want to get up there with the appropriate caulk when the job's done.
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On Thu, 20 Sep 2007 08:34:26 -0700, Mikepier wrote:

You didn't state exactly where you are working on the vent pipe. Chances are you will see very little, if any, movement if you work below existing vents tied into the main pipe. It would still be a wise choice to secure the uppermost part of the pipe with strap supports designed to hold pipes securely.
A reciprocating saw would seem to move the pipe less than doing so with a handsaw.
I did exactly what you are doing several years back. That vent was cast iron. I had to cut the pipe in the basement to reroute tub and stool. There were enough pipes above me to support the vent without moving.
Reflashing or caulking the existing flash when done makes sense.
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wrote:

If all you are doing is adding a vent, you can buy strap around devices. Then cut a square hole in the cast pipe with an angle grinder and attach the "strap on". You dont need to cut the whole pipe. After all, it's only a vent.
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On Sep 20, 2:44 pm, snipped-for-privacy@notmail.com wrote:

Is there a place online I can find out more about this?
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Open your yellow pages phone book and find a plumbing supply store. Go there and ask.
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Oh man, that requires interacting with real people.
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wrote in message

I'm gettin' a headache just thinking about it.
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on 9/20/2007 4:29 PM Edwin Pawlowski said the following:

I hate real people.Cyber people are better.
--

Bill
In Hamptonburgh, NY
  Click to see the full signature.
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wrote:

Possibly..... There are some plumbing supply stores online. However, I'd check your local plumbing supply places. These things are commonly used. I dont know the exact name for them, but I have seen them. It's basically just a device that fits around the cast iron pipe. It fits around about half the pipe, has a rubber gasket, and uses large hose clamps to attach it. There will be a 1.5 or 2 inch threaded hole in the side to attach your pipe. They also make similar devices for water pipes, so it's easier to attach a water source to an ice maker. Those are called "saddle valves", but they have an actual valve built in. Knowing that, these drain pipe devices could be called "saddle" (something). Just a guess.
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