Shallow drainage floor slab design

7 May 2019
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United Kingdom
I am building an extension. The crown of the existing foul drainage exits the existing wall 225 below DPC and enter the manhole 295 below DPC.

I need to change the floor slab to accommodate. I think I want to propose the floor construction to be:
45mm bonded screed
100mm concrete
80mm insulation
1200 guage DPM
compacted Type 1

This keeps the pipe below the DPM (just) with the insulation resting on top, the pipe within the level of the Type 1 for the whole run, probably in pea shingle. However, I'd like 100mm insulation and 65mm screed.

So alternatively, it could be:
65mm screed
100mm insulation
1200 guage DPM
100mm concrete
compacted Type 1

This has the pipe running through the concrete and the Type 1.

Also, does the pipe need to be in pea shingle or encased in concrete?

I'm not sure what might be considered acceptable by BC.

The architect was supposed to do this, but they aren't answering the phone, or email.

Any help gratefully accepted!
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I would say youve got to get the pipe below the concrete slab (which will probably want some reinforcing mesh).

Or you need to have a suspended timber or concrete beam floor and run the pipe below or parallel in between.

Whoever is doing the building regs should have a solution.........
In the first case if your insulation is under the slab you don't need all that screed unless you're embedding ufh or something. Surely you can just use levelling compound or similar to get a decent finish?
As John said - position the insulation beneath the slab. Your pipe could then share the space between the insulation and the Type 1. Fiddly but doable.
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