How to connect a plastic pipe to a cast iron soil stack
Here is some useful information from one of our jobs.
You should ideally use a cast iron boss when connecting to a cast iron soil stack. If you use a plastic one they are more likely to leak due to differences in expansion and contraction between metal and plastic.
We are using a product called Timesaver strap on boss. They make this cast iron strap on boss. I ordered this from my local plumbers merchant at the time.
Drilling the hole
The most difficult part is drilling the hole through the cast iron down-pipe. This required a 64mm hole. We used a “Starret” hole saw with a corded drill. The progress was very slow with this product and I’m guessing there is a better hole saw for the job.
Apparently lubrication or cutting oil should not be used since it may slow the cutting process further. You should keep the blade as cool as possible using a water spray or dipping it every so often. It is much quicker and easier if you have an assistant to spray on the water while you drill. This was a very slow process and half an hour after starting drilling, the hole was still not through but nearly there.
Amazingly it took a whole hour of drilling with a few breaks in between. It was a tedious job but it was worth it in the end. I’m sure it would have taken more time and money to do it any other way.
Never try to rush the drilling. If you break the cast then its possible you could have to remove the whole top section and replace with plastic. This would be a costly mistake.
Sealing
For a better seal, clean off all the paint from the edges with a grinder.
Use some quality silicone (such as lsx) on both sides of the rubber ring.
Fit the rubber onto metal boss and bolt on to the soil stack ensuring the hole is centered. Do not over tighten as you could break the cast iron strap. You can always tighten a little more later if it leaks when tested. Remember that cast iron is not like steel and has no flexibility.
Fitting is easier with an assistant but not impossible to do on your own.
Put the rubber ‘o’ ring around the pipe reducer.
Then push it in the hole and put the front cover on… this creates a good compression joint.
Insert your new pipe into it using solvent cement to stick and seal it.
Tip: use small sections of 15mm copper pipes to make a spacer for the brackets. This will prevent strain on the connector. Use plenty of brackets to ensure proper stability and prevent bowing which could cause a block. In this instance the pipe was connected to a Saniflo macerator to carry raw sewage to the soil stack.
The finished job before adding external pipe insulation; it’s important to add this to avoid frozen blockages in the winter.
This article was brought to you by BBS Building Services.
Subject: How to connect a plastic pipe to a cast Iron soil stack
We have more building tips at our Previous Projects page
See also Manhole covers
The boss Timesaver boss strap can be found here