draining washing machine into cast iron soil stack

14 Jul 2009
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United Kingdom
Hi, I would like to drain my washing machine waste into cast iron soil stack. I intend to drain the washing machine waster pipe into a normal 40mm open ended vertical waste pipe + trap, and then run a 40mm pipe between trap and soil stack.

The w/machine is on the ground floor. The soil stack only collects waste from toilet on first floor.

Is this possble , and if so how do you guarantee that their wont be no leak at the joint in the soil stack ? Also, what incline should the pipe between the trap and soil stack be set to ? Is one trap sufficient to stop toilet waste from back syphoning into wash/machine ?

Thanks in advance
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There is quite a few old threads in the forum archives about Strap-On bosses for cast Iron ...and their £ expensive . But before you spend loads ,I would get a normal plastic one for a 110mm plastic soil pipe ( as your iron one seems similar) then try it for size . Cut the hole and seal the boss to the soil pipe with a hemp grommet smeared with with something like Evo -Stik all weather sealant. No this isn`t a wind-up . I did exactly that ( apart from the evo stuff ) way back in the 70`s and the joint lasted years with no leaks :idea: I know because it was an outside soil stack and I`m so sad I looked across at it every I drove past the house :oops: . Have a look through the forum for the "proper" way too ;)
Blimey, had to get my olde college book out to see what the fall/incline should be on 40mm waste. Most plumbers do it by eye, as its not an exact science. Anyhow the book says between 19 and 90 mm per metre. As I said its not an exact science, use your common/ use other waste pipes as a guide.

Secondly, traps are not there to prevent back siphonage. They will only prevent foul smells coming into the room from a waste pipe connected to a stack.

You may poss find that the force of the brown stuff coming from above will cause problems with your new pipe. You may need a secondary vent pipe on it.
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thx again guys

whitespirit66: when you say that a secondary vent may be required, what exactly is required; a vertical pipe running alongside the cast iron soil stack upto and beyond the guttering ?
Ideally yes, but you'll get away with it terminating above the height of the WC
ok, so it should be a 40mm plastic pipe, connected to the waste pipe using a suitable T connector ?

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