Cast Iron Soil Stack replacement

Hi,
I have a cast iron soil stack that is cracked badly a length of about 5 feet. The crack goes from the top of one section to the bottom of that section. The other sections seem fine above and below.
I would like to replace with PVC. It is a straight shot from the basement up through the first floor and to the second story bathroom. Ther are no other conections on first floor or basement.
The problem is BELOW where the toilet / bathtub / and bathroom sink meet at the stack, so I just have to replace a strait length of pipe down to the basement.
1. How do I loosen the conncetions between two pieces of cast iron so I can take out this damaged section and the 2 sections below it. Heat the lead and Oakum and twist the sections apart?? Will that get the pipe sections seperated? How is it best done?
2. How do I reconnect PVC to this cast iron piece.
3. Do I need to support it from above? Can I grab ahold of the vent on the roof with a clamp and thus support the stack above the cut?
Click here for pictures: http://trillionfive.com/plumbing.html
Any other advice!?? Thanks guys!!!
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opps here is link. Click below.
http://www.flickr.com/photos/31219867@N00 /
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first if the cast iron carries waste water thru a living area replacing with PVC will make water passages amazingly loud. unreal how loud PVC is
you need to support the cast iron above and below the cut. you DONT want anything to move and not only leak indoors but perhaps create a roof leak, strap and support it well before beginning cast is very heavy better safe than sorry.
you can easily use ferncos to join new PVC or cast iron lengths together. today they use hubless cast iron, basically cast iron with ferncos.
Cast cuts easily with a sawzall or specialized cutter, if the balance is OK just replace the bad area.
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snipped-for-privacy@aol.com wrote:

Echoing what hallerb said.
If noise (dripping/sloshing) will be annoying, forget the PVC. Use new hubless cast iron pipe.
You can not heat the lead out of that top hub (without burning house down). There isn't access above to drill it out either. Consider making a saw cut vertically thru the bell and then splitting it open, like a melon.
Support is vital. If the stack above shifts even a small amount, you may disturb the bath connections enough to have a major leak on your hands.
This is very tricky work; plan, plan, plan...
Jim
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It's very hard to get the joints apart even when one end is free. If I understand your situation, both ends of this section are joined. Even if you could loosen the joint, how would you get the section out? I would suggest renting a chain cutter made to cut cast iron pipe. They are quite heavy and require some clearance around the diameter of the pipe for the chain and cutters to go through. But once you get them hooked on they will allow you to cut the pipe pretty quickly. I would remove the bell on the non damaged section of pipe, it will be easier to install the repair.

They are called Fernco fittings. They are like a piece of stiff rubber hose with two hose clamps. You slide the hose over the cast iron, tighten the clamps.

Yes, it needs to be supported above the cut, but you can't support it on the roof. It is normally supported by a riser support, here's a picture: http://www.pipingtech.com/products/ptpcat/support/clamp/fig90.htm There is most likely a riser support in place that you can't see near the bottom of the stack, but once you remove a section above it, all the weight going up and out the roof needs to be supported. So you need to get another riser in place above your damaged section before you start cutting it out.

It would be easy to do a lot of damage or get hurt doing this. If you have the stack exposed you might consider brining a plumber in to replace it.
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