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Offline robbie.peacocke

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WC drainage help
« on: May 11, 2017, 11:46:33 AM »
Hey guys,

Im after some advice on the typical clearance required for WC waste pipe installations.

Im working on a project with a 125mm thick floor slab, with a lattice of ground beams below the slab. A WC with a 100 PVC waste sitting over one of the ground beams is shown in the sketch attached/below.

 Currently the structural engineer has allowed 300mm from the top of the floor slab to the top of the ground beam (the measurement labelled ? on the sketch). This allows 175mm clearance within the ground between the floor slab and the ground beam.

   What is the recommended minimum length of straight vertical pipe from the bottom of the WC to the top of the 88 Bend (female & female socket ends)? Or does it not matter?
   Is it better to have the entirety of the 88 Bend below the slab?
o   Similarly, are there any reasons to avoid casting a portion of the socket end of the 88 Bend within the slab?
   What is the recommended clearance above ground beams like this? Is 50mm adequate?

Can you please also offer advice for whether your answers will apply to 40, 50, & 65 wastes penetrating similar slabs too.

Thanks for your help!

Robbie Peacocke
Hydraulic Engineer
Powell Fenwick


Offline Stu Clark

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Re: WC drainage help
« Reply #1 on: May 25, 2017, 01:01:54 PM »
Hi Robbie,

Not sure on the exact regs in the book here but;

There is no minimum amount of pipe to come up from the slab.  Generally plumbers will cut this flush with the floor when it comes to the fit out, but leaving 100-200mm will give you enough to work with.

As for the bend, I'd recommend having the bend below the slab.  The only reason I'd suggest that is so if for any reason the vertical pipe were to get damaged it would be easier to remove it from a fitting that isn't encased in concrete that it would be if it were.  That way you won't also damage the fitting if you had to chisel any concrete surround the pipe if this were to ever occur.

Recommended ground clearance I would assume would be a clips distance, as you'd need to support it somehow anyway.  But don't hold me exactly to that.

Hope that helps,


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