DPC/DPM lapping

27 Oct 2011
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United Kingdom
Please can somebody halp a confused first time self builder. I have just completed the beam and block floor for our extension on our bungalow. The beams were set at 175mm below existing floor level(100mm insulation 75mm s&c sreed). Beams bear down on the outer leaf of existing brickwork. The existing DPC is 75mm above top of beams(existing DPC is 225 below existing floor height. All beams and closure blocks have been laid/bedded on dpc. What method should i use to lap DPC /DPM where external walls become internal walls?
also how do i lap DPM with DPC on new internal blockwork? I am under the impression that it needs to lap with DPC. Cant see how it can be done without laying insulation and screed before walls are built above DPC?

I would really appreciate some expert advice
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Thanks for your reply Woody,

I'm not sure if it should have a membrane or not, the working drawings produced by our architect shows a DPM turned up wall and lapped with DPC. Although some websites say it isnt necessary?
You probably need to speak with your Architect to see what he is on about

Normally, beam and block has no DPM, so the Architect needs to explain his design
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A radon barrier would only be required if the existing house had such a barrier. Building Regs can dictate that a dpm is required f the ground levels are higher or the void cannot be drained or other peculiarity but really these should be avoided and designed out if at all possible.

My money's on an ill informed designer though! :p

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