New plastic to old iron waste connection

Hello all,
After much rummaging last year I have been unable to find the answer to my latest quest in DIY.
In replacing a bath with a shower and swapping a toilet around I will have to interface new waste and soil piping into the existing cast iron soil stacks. My question is basically what is the best way to do this.
For the shower, the outlet will now go through the wall far lower than it did for the old bath. Even though one seems to be able to get away with a far shallower fall angle with plastic, I'll still hit the waste pipe way low. So, what is the best way to connect this new plastic to the old iron, and what is the best way to seal up the old hole? This is the lower flat with one storey above, so it's not straightforward to just lift the whole stack off and replace segments.
Likewise for the loo. There is a leg comes straight through the wall from the stack at the moment. Can I cut this off and seal it, then plaster over, or is that a no no? As with the shower, what's best for making the new join.
Many thanks,
Mark
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It's best if you can remove all the old cast iron branches and replace them further down the soil pipe. A mix and match of different pipes can and will cause problems in the future.
Cast iron soil pipe is just as cheap to buy as plastic, so don't think it's going to break the bank to the job properly.
You can buy things called speed clamps that make a proper joint in cast iron soil pipes, so you only need to replace the branches and the small sections of straight cast iron that you cut away. One section of 2 mtr 4'' cast iron and another two proper branches for the smaller pipes to enter in to. One branch will be 4'' equal on all ends, but the branch for the shower and wash basin outlet should have a 3'' branch from it to take the smaller size pipework. One 4 '' speed clamp fitting should make the shopping basket complete.
Try to cut the cast iron as straight as you can, usually with a big angle grinder and 9 '' metal cutting disc. Make one cut in the main stack just below where you want the new loo branch to be, about 6 or 7 inches should be enough. Once you've made this first cut it allows you to swivel the old branches away from the lead poured joints. This is an easy job as long as you take your time and don't take to large a swing at a time. Swing the pipe back and forth a little at a time until you feel the joint loosening at the spigot ends. You may have to pick some of the lead out as you go, but this should be easily done with your fingers.
Once you get the old branches off the main stack you can start to fit the new ones where you need them. Begin with the larger loo outlet branch as the lowest one, because this needs to keep flowing if the wash basin and shower get blocked. Place the branch on top of where you cut the pipe and hold it in place with a bracket if possible. Place the other branch on top of it and get that one held in place somehow as well. Now place the new straight section of cast pipe with its spigot end upward on to the old section of pipe above the new branches.
Make sure the new cast pipe is tightly fitted on to the old pipe and then mark a cut line on the new pipe where the pipe touches the lower ring of the upper branch spigot end. This gives you enough pipe inside the new branch to make a good seal on the joint. Remove the pipe and the branches again, then you can start placing them in reverse order this time. Fit the new straight section on to the upper part of the older pipe. Now fit the shower branch in place. Finish off with the loo branch on the bottom.
You should now have the bottom of the loo branch nearly, as in a few millimetres, above the cut you made in the stack. Move the branch out of the way and place the rubber insert of the speed clamp over the two ends of the pipe and the lower branch. The rubber insert has a rim inside that should fit snugly in between the two part of the pipe work. Wrap the cast iron two-section ring around the rubber and tighten the bolts up at each side. Tight enough to stop the pipework wobbling around.
When you pulled the old pipework apart you should have been left with a rope type thing stuck to a lump of lead. This is called caulking rope and teeped (or tamped in some places) joint. You should be able to use the rope and lead again on the new pipework if you wish. Just make sure everything is dry and tight before pouring the lead back in to the spigot joints. Replace all the brackets, or fit new where needed, and the jobs a good 'un.
Now you have the whole thing in place you can connect what ever kind of pipes you want to the new solid stack. Remember to reduce the branch for the shower and basin outlet further down to suit 40mm in to 75 mm with a 40 mm push fit straight joint. These are an ideal size for the cast pipe, and allow just enough of a gap to fill with gutter seal or the likes.
Try not to use plastics on the main stack unless you're replacing the whole thing. New plastic won't take the weight of the old cast iron above it.
Good luck with it.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.