I have experienced a leak as a consequence of a fractured vertical cast iron pipe which acts as an internal waste for our upstairs bathroom and downstairs shower & WC
My house is approx. 100 years old so I assume the pipe is of a similar vintage. There does not seem to be a straightforward reason as to why the pipe failed.

My plumber has suggested it could be corrosion or as a consequence of temperature changes (hot bath water flowing into a cold pipe).

Any ideas?

100 years old is your answer. They eventually fail and this is not a DIY for most folks. The stack can weigh a ton so get 3 bids on removal and replacement with PVC.

Bob gave you some very good advice. My home was built in 1954 and the builder used substandard cast iron. Over the years cast iron flaked off in one of the stacks and clogged the base. A few week ago 40 feet of sewer main under the slab let go. The bottom was completely ate out. I was lucky! There was enough of the cast iron left for me to get a reline job. While relining 40 feet of 4" cast iron wasn't cheap the hassle of tearing up walls and floors to replace the pipe was well worth the expence. To say nothing of the labor and material ninvolved. Relining saved me a bundle and took a lot of concern out of my life. I can't say enough good things about it. Regards, Tom

Age is the reason... Just like everything else in this world, pipe will deteriorate with age

There was enough of the cast iron left for me to get a reline job. While relining 40 feet of 4" cast iron wasn't cheap the the hassle of tearing up walls and floors to replace the pipe was well worth the expence...

Tom - I've never heard of this. How is it done & is it that common? I am getting nervous of the cast iron pipe between our house and the sewer main. THe house is 65 years old and the pipe runs under retaining walls, concrete patios and other things that would make it difficult to replace.

Thanks
David

RTW, its not cheap and not a DIY project but it is less expensive than tearing up you lawn and that retaining wall.. Sewer Line Replacement, Trenchless Pipe Replacement Roto-Rooter (http://www.rotorooter.com/residential/sewerlinereplacement.php)

Be careful: Re-lining will "copy" your existing sewer pipe. If you have sagging main - than the new pipe will copy all low points. Meaning, you will have new pipe - but with the same low point as before.

Do video inspection first, make sure existing sewer pipe has no Y's in it, it is not pushed down by retaining wall and / or trees. Proceed with relining only if your existing sewer is a straight line with proper slope.

make sure existing sewer pipe has no Y's I don't understand Milo.
What do fittings have to do with a reline job? Are you under the impression that to reline a pipe it must have no fittings coming off from it? If so, you're mistaken. My master bath is wyed off to the main and Rooter-Man cut the wyes discharge directly into the relined main.
Let me explain a bit more about my drainage problem and how I repaired it. 54 year old house. The builder used sub standard cast iron in the drainage. The 4" house main was completely ate up on the bottom of the pipe , blocked with roots and welling up in my living room floor. I was sure that the main had broke in two and I would have to jack hammer up the floor and tunnel under the foundation to transition to PVC, pick up the back bath and move the main outside the house but I had to know what was going on under the slab so a friend suggested running a camera down there. There was no distinct break in the main, however,it was completely ate up on the bottom. That's when Rooter Man of Sarasota and Gulf Coast Florida suggested relining the main, (see image). It took a day to clear the roots and pressure jet clean the main. Next day they came back and relined the main with epoxy. I now have a sewer main that will out last me, (50 year guarantee) and the best part is that I didn't have to, tear up my house and the cost was under half of what I figured to take the main outside around the house. Another bonus was that it was done without tearing up or disturbing my house. When they were finished I kept a piece of the epoxy liner to check it out. It was white and had the same thickness of Schedule#40 PVC.
I was amazed at the equipment RooterMan had outside my place. The equipment he had cost upwards of $100,000.00. Hi tech stuff that took a technician to run it. There were 3 men and 2 trucks to do the job. And it was completed in two days. Lottsa difference from the old days when I went out on a sewer call with a Ridged K-60 Sewer Machine and a ladder to get to the roof vent. I just wanted to point out that there's another option besides replacing drainage that the years have ruined. If your pipes are a candidate for a reline job I strongly suggest you look into it before you decide to replace the drainage pipes. I did and saved a bundle in addition to keeping my home intact. Just thought I would share that with you all. Regards, Tom