Concrete raft 1950s - depth for pipe, asphalt over concrete

28 May 2008
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United Kingdom
I need to put a 22-32mm pipe in a floor.

Build is 1950-52.
Construction is 22-25mm flooring grade asphalt (brittle) over concrete raft.
Raft appears to have a ring beam down to at least 24" from digging (concrete extends 24" below brick, so 225mm of brick above that to the concrete floor).

The maximum depth into the concrete will be 25mm, more typically 15mm because there are some height variations.

Speaking to one structural engineer he said he was always negative on cutting a raft, but saw no real problems with it at all.

Another structural engineer said the porch being 10cm lower "indicated no problems at all, you have a deep pour and thin asphaltic screed".

I guess for reasons of insurance they have to be very cautious - but does anyone see a problem?

One neighbour went down 22mm all over an en-suite (fall) and another slightly further away put all their (plastic coated!) pipes just under the asphalt.

I can hire a tool (Elcometer) to check thicknesses, is it worth bothering with?

It is for HW & CW, both plastic coated. One eventually may be a 32mm waste although 22mm is much more likely re 22mm coil already left over from a decade back. Oh, sticking to the 45 degree rule re keeping away from the walls (110-115mm brick on bitumen DPC on concrete).
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