soil pipe fall quick question

[essgee]

A quick question- the Building Regs Part H give 'gradient limits' of
18-90mm per 1m length for soil pipes and 18-44mm for 40mm sink etc waste pipes.

So does this mean that if the fall isn't vertical then the slope has to be within these two figures? As otherwise they would give a minimum, not two figures!

So what they really mean is soil pipe falls must be greater than 18mm per M and LESS than 90mm per M?

Is this Maximum fall figure to do with protecting the water seal in traps?

Any clarification would be most appreciated!

 

[studips]

well, on a vertical stack the effluent will tend to run down the walls of the 110mm pipe, pipes in a singel stack system are expected to run 50% full, an excessive flow can cause scouring of pipes and siphonage of the trap seal. a wc connection should be swept to reduce the risk of siphoinage on wastes connected beneath it & waste connections should be at 90 deg.

 

[Nige F]

So what they really mean is soil pipe falls must be greater than 18mm per M and LESS than 90mm per M?

Is this Maximum fall figure to do with protecting the water seal in traps?

!

Yes and yes

 

[essgee]

Thanks Nige!
Building Regs are written so badly on so many fronts! Very happy to have that clarified, and back to my soil waste diagram again.

I'm currently reading the paragraph on running boiler condensate, I know it isn't complicated, but you still need to read it at least twice because it is written so badly.

 

[Richard C]

A quick question- the Building Regs Part H give 'gradient limits' of 18-90mm per 1m length for soil pipes and 18-44mm for 40mm sink etc waste pipes. So does this mean that if the fall isn't vertical then the slope has to be within these two figures?

Yes.

So what they really mean is soil pipe falls must be greater than 18mm per M and LESS than 90mm per M?

Yes.

Is this Maximum fall figure to do with protecting the water seal in traps?

Yes; for the same reason there is also a maximum length for unvented branch lines connecting to the stack.

 

[rojaca]

the fall is also to maintain a self cleaning velocity. Too fast and trap loss. to slow and suspended debris becomes just debris

 

[SLPPM]

the fall is also to maintain a self cleaning velocity. Too fast and trap loss. to slow and suspended debris becomes just debris

Yeah, too much of a fall gradient can result in fast moving liquids. Fast fluid flow reduces the fluids ability to carry solids down a pipe, often resulting in a blockage over time.

 

[Dan Robinson]

Damn those stubborn turds. !

 

[Hugh Jaleak]

As said, the falls are designed to maintain a 'Self Cleansing Velocity'. Not so much trap loss in soil pipes, its about effectively carrying the solid waste. Too steep a fall, and the water will run away, leaving solids behind. Too shallow a fall and the solid matter will sit in the pipe whilst the water slowly disperses. Both eventually causing a blockage.

Wastes, poor design will result in traps self syphoning, and too shallow a fall can lead to build up of detritus, again causing a blockage.