While removing a cast iron stack may look like simple manual labor to the untrained eye let me assure you that attempting to do so without a thorough knowledge of the procedure is at best a disaster waiting to happen. 4" diameter cast iron hub & spigot pipe weighs 30lbs per 36" length. When you have a stack beginning at the basement floor and rising through the roof on a 2 story structure, that said stack will average about 36 vertical feet. 36 divided by 3" per length equals 12 lengths x 30lbs = 360lbs. We must then add in another 20% for the additional hubs on fittings therefore the pipe and fittings will weigh approximately 1.20 x 360lbs = 432lbs. We must then add in the weight of the lead used to make the joints. Typically CI hub & spigot pipe required 3/4lb of lead for each 1" of pipe diameter per joint. .75lbs x 4" pipe = 3lbs of lead per joint. 3lbs of lead per joint x 12 joints equals 36lbs of lead so the stack has an estimated weight of 432lbs of pipe + 36lbs of lead = 468lbs. (Slightly more than the average weight of a V-8 automobile engine). At best the stack may have some wooden blocking or steel strapping holding it in place but often these old stacks were installed as a self-supporting structure. Even if it has blocking in most instances that blocking is simply two or three pieces of 2x2 that were haphazardly nailed in after the fact and generally will break out at the first sign of strain. Assuming for the moment that this stack may also have steel perforated strapping we must also keep in mind that the said steel strapping was initially installed in the 1940Âs and is most likely rusted beyond use by now. Any attempt at cutting and removing a section of this pipe from the bottom may cause the remaining portion of that stack to come crashing down instantly and without warning. Even if you are fortunate enough to avoid severe personal injury you may rest assured that in the process of the stack crashing down it will rip the horizontal branches out of the walls and may even result in causing the ceilings to collapse on the first and second floors as the lateral lines are torn from their hangers. Please do not take this lightly; about ten years ago I had a close friend who was killed by a falling stack. Before you begin removing a stack you should open the cleanout at the bottom and install an inflatable test plug or a large wad of rags tightly in the line to prevent any shards or bits of broken pipe from falling down into the line below the cleanout. You then begin at the top of the stack and work your way down in sections. Cast iron pipe can be cut with a circular saw fitted with a metal cutting blade providing of course that you have enough clearance to get the saw into the point were you need to cut it, and keeping in mind that the saw will shower the whole area with a shower of hot sparks. When one considers that in a 70-year-old house the plumbing chases are made of wood that is by now tinder dry and generally filled with highly flammable cobwebs this is not a good practice. CI pipe can also be cut with a reciprocating saw, but you can expect to spend 20 to 30 minutes and burn up two or three blades per cut. There are two simple alternatives. Contact your local tool rental company and you can usually rent a "Cast iron snap cutter" for about $15 to $20 a day. (Far less than what the blades for your reciprocating saw will cost). You then use the snap cutter to make the critical cuts where you will need a good square end on the pipe to tie in the new pipe. For the remaining cuts the solution is very quick and easy. Cast iron is a very brittle material. If you hold a 3lbs hammer tight against the pipe wall on one side will striking the pipe on the opposite side with a second 3lb hammer the pipe will shatter. You can easily break the pipe out in manageable sections working from the top down. When you get to the next to the last section in the basement stop breaking it out and use the snap cutter to make a neat finished cut on the bottom. After you make the final cut at the bottom reach down in the pipe and remove any shards that may have fallen down against the test plug, the remove the test plug and your ready to begin installing your new pipe. Although your may not have noticed it when you first looked you can be sure that the stack is rising straight up through your attic to the roof.

Find am experienced plumber. A new piece of no hub cast iron may be the most cost effective fix. While there are now requirements for intermediate supports for cast iron stacks, they did not always exist. I have seen 4 story stacks holding their own weight up with no other supports. It becomes a real chore to even attempt to make repairs on them.

As far as I can determine from your post, there is no need to tear out the cast. Properly support the cast before doing any digging. Make sure you vent the bathroom properly.

The sink/tub drains must be above the WC drains. Code prohibits any fixture arms below the WC fixture arms.