I want to plumb my washing machine in my garage.

There is a soil stack in the garage but the length I’d be joining is of dubious quality (cast I think) but may want to replace anyway. What measures are required to stop the brown stuff going in to my washing machine (washer on ground floor, toilet on 1st floor). I’ve read that a secondary vent (ideally to the height of the gutters) would prevent this, but what do Saniflo systems have to get around this. What are my options – surely an anti-syphon / return valve exists to solve this problem.

Alternatively there is an additional gutter drain at the front of the garage but I’m not sure where it runs. I poured a bucket of water down it and didn’t see it in our manhole so will ask next door if I can look down theirs, but as we’re in a city I’m pretty much certain everything ends up in the sewer.

You’ll need a standpipe trap for your washing machine discharge. If your stack is cast iron you can (if you can find them) get an equivalent of a strap on boss for it which will allow you to take your 1.5″ waste from your trap straight into the stack. If you can’t find one the easiest thing to do is smash a bit of the stack out and put a 110mm plastic boss pipe in using a couple of Fernco/Flexseal rubber couplings to join to the cast iron.

Will the standpipe for the washing machine be enough to stop the poo’s forcing their way in to the washer? Unlikely I know as that would be working against gravity / momentum but its not something I want to get wrong

You would need to carefully cut a section of cast out with a grinder, smashing as per above, WILL end in tears.Also be careful not to drop any pieces down the remaining soil stack. You can buy strap fittings to connect to plastic which will involve drilling the cast stack.Best scenario would be to replace complete stack with plastic if possible.No poo should make its way into w/m.

Replacing the stack with plastic is an option, and not too tricky with rubber sleeves etc., I was just unsure of what was needed on the washing machine side of the pipe work. If a standpipe trap is all thats needed then thats not too tricky.

As long as the wc discharge is not directly opposite the washing machine T you’ll be ok. If the pan comes in higher and drops down past, it’ll be good.

Careful with ‘smashing’ a lump out of the stack to T into as pieces of the stack can drop down inside it and you’ll struggle to retreave them. I grind a slot horizontally in the down pipe and slide a piece of card in it to block off the down pipe whilst removing the upper portion.

I used the word smash a bit too loosely there didn’t I? Use an angle grinder instead.