19 Jun 2009
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United Kingdom
I am currently refurbishing a ground floor and first floor flat and have the oppoortunity/misfortune of having to create a new soil stack. The current set up means I have to remove the existing soil stack, but I can use the existing drain, but the pipes are only about a foot below ground level.

1. Is the soil manifold allowed to have an immediate 90 degree bend underground or could this cause me problems. For example, I'd quite like to have a soil manifold for the ground floor doing a 90 degree bend before going through the wall below ground then hitting a branch so that I can attach a 110mm pipe to vent away outside?

2. With the soil vent outside, it will hit the tiles - Do people use a coupld of 135 bends to vent above the tiles?
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Is the manifold for the ground floor on a separate connection to the drain or do you intend to connect the stack to it? Regs do not allow any connection lower than 450mm above the invert of the tail of the bend at the foot of the stack. This bend needs to be a long radius bend or 2x45 ° bends to allow smooth flow around the bend from the upper floor appliance discharges.

If manifold is on a seperate connection and will just serve the ground floor bathroom then should be fine, if not then you may need to look at alternative arrangements for the ground floor.

For the offset at the top of the stack around the tiles, purpose made 45 ° bends are offered, usual practice is to glue these together to suit to prevent any movement in adverse weather conditions. Make sure you've got it right before glueing, there's no going back! ;)
Thank you.

OK - so I'd have to take an accurate look at the depth of the underground pipework to establish whether I could even go horizontally through the wall from the downstairs toilet into the stack, and still retain the 450mm using a 200mm radius bend?

Also, I notice that the rainwater pipe and another soil pipe feeds into the drain. Is it ok to tee into the rainwater pipe underground rather than the soil pipe?
Provided the system is a combined sewer (i.e. foul and rainwater go into the same drain) then should be ok to connect as you wish. You do not want to be connecting foul to a surface water drain for obvious reasons!

Whatever you do, make sure all parts of the system are accessible for rodding, shallow inspection chamber is sometimes required rather than a junction to provide required access. Work is subject to Building Regs, so best to run your ideas past the Building Control Officer prior to commencing work. You dont want to do any unnecessary digging and certainly dont want to be burying new drain to be told to change it afterwards!

Ground floor WC can be directly connected to the drain provided the depth between the top of the WC outlet and invert of the drain is less than 1.3m. (Shouldn't be an issue if your drains are shallow!) Main reason IIRC for the 450mm from bend is to prevent 'blowback' in the ground floor WC when a discharge is made from an upper floor WC.
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Thank you. OK, I will end up having about a 3m run of soil pipe from the toilet to the outside wall on the ground floor. In respect of the 40mm and 32mm waste, can I simply boss into the horizontal (fall of 18mm per meter) 110mm soil pipe?
Yes, provided correct fall is achieved on the wastes as well. May be wise to fit an AAV at the top end of the WC branch or you could find flushing the WC 'pulls' the appliance traps though.

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