Finished Floor level way above DPC

11 Apr 2012
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United Kingdom
Hi I am extending at the side of the house up to an existing solid 9" brick wall. The wall has a DPC but due to sloping ground my FFL will be 440mm above it. I will go for 65mm Screed on top of 100mm concrete but that leaves me with 275mm to make up. How should this be done? Do I just use extra thickness of insulation with the DPM running up the side of the wall to FFL?
The ground on my side is currently just about level with the DPC.
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Dig the oversite ground to whatever height allows your hardcore, insulation, blinding, slab,screed. Ie work down from FFL
Yes run the dpm up the wall to connect wall dpc with floor dpm
Outside ground should be lower than wall dpc
Thanks 23CV. I have added another sketch to hopefully show the problem more clearly.
You can see that the new DPM will be above the existing DPC, allowing damp to penetrate into the wall.
I,m not sure of the best way to deal with that to prevent damp in the wall?
The dpm needs to go down and reach the wall dpc, is suspended floor an option instead?
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It's for a utility room and toilet so I wanted a solid floor.
I googled Jablite Hardcore Replacement Boards, basically 100mm EPS with extra compressive strength. Have you come across these, do you think I could build up from below DPC with a DPM under and up the wall?
What was this wall used for before you built the extension?
You could do it as per your diagram but also include a vertical dpm up the wall but you’d have to protect it from being punctured by the hardcore somehow. I don’t see the benefit of making up the difference with loads of insulation and it’ll cost more
The wall was my neighbours garage which finished on the boundary. He has demolished the garage but left the wall in situ. I'm infilling between my house and the wall with a new floor and roof and enclosing each end with new walls.
Do you think a bitumen DPM vertically on the wall would be ok maybe also use a plastic DPM up the wall too?
Potentially yeah, but if it was me I’d just give the building inspector a shout and run it by them before cracking on

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