Waste pipe falls

[David E]

What is the best way to create the correct fall for waste pipes (38mm & 32mm) draining baths & sinks under a floating floor (22mm T&G chipboard on 50mm kingspan) do you assume that the run under the floor on top of the slab is level and that the "head" above it is sufficient to cause flow or is the 12mm maximum fall over 2400mm (for a 38mm pipe assuming 38mm is the oa size) sufficient.

Thanks

David

 

[Bahco]

Sorry, I think you need to explain your post again

You have buried your waste and need to know if the fall is sufficient?
Does the water drain away ok when testing?

 

[David E]

Sorry to not have made myself clear. I am about to lay the insulation and chipboard and run the bath/basin waste pipes through the insulation, but given the insulation is only 50mm thick the fall will not be very much given a 38mm waste pipe. Whats the best thing to do? Pack the pipe up to the top side of the 50mm insulation and support it on the best slope I can to the stack or will a 2400mm run on the flat between the bath/basin and the stack generally run ok?

Hope this makes it claerer

David

 

[ChrisR]

2400mm run on the flat between the bath/basin and the stack generally run ok?

In a word,

NO

You need a fall. Normal advice is 1 in 40, more if there are any "horizontal" elbows.
Start to expect trubble at 1 in 80 or shallower.

 

[David E]

Thanks for your reply, slight problem to overcome I think! I'm assuming that over a 2400mm run there needs to be a 60mm "drop" to give 1:40. Does the vertical drop from the bath outlet to the elbow that starts the main run help "push" water along and assist in improving from 1:80 to 1:40????????? or am I being too hopeful?

David

 

[doitall]

waste pipe that long should be 50mm, bath should be at least 40mm anyway.

 

[htgeng]

Does the vertical drop from the bath outlet to the elbow that starts the main run help "push" water along and assist in improving from 1:80 to 1:40????????? or am I being too hopeful?

yeh, as long as you don't mind paddling in water when you take a shower - or take ages to empty bath and risk the pipe being scummed up over a few years.