New Stud Wall on Flagstone Foundation - Can Anyone Show Me What DPM, DPC, Sole Plate Config Should Look Like

2 Jul 2009
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United Kingdom

I'm framing an old granite room set on a flagstone foundation. There is no DPM installed as this is 19th century construction.

I'm planning to put down a full black plastic DPM before installing the eventual laminate floor but that won't be for some weeks after I frame the walls.

For the walls, I plan to put a strip of DPC under the sole plate(s). If the DPC is wider than the 4x2 sole plate, should the excess DPC be on the outside or inside of the stud wall and when I get round to installing the floor is it right to lay a DPM then simply lap it up the inside (room side) of the stud wall before plasterboarding over top?

I'm confused about how all these should interact in a retrofit situation where no DPM exists under the foundations.


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I think more information is required to give worthwhile advice

You have both damp arrangement and vapour barriers to consider

and perhaps importantly if you are introducing non permeable materials into a historic building you might create long term damp issues

could you do a sketch and do a couple of pics -maybe of the outside of the building
Is there a current damp issue with that existing floor, or are the stone slabs doing their job as the damp proof layer?
The flagstones had been covered in an epoxy resin by the previous owners. When I first chipped/peeled off the resin, the floor was wet underneath. Now that the flagstones are uncovered, they are bone dry as they are breathing again. I’m aware that I’ll be blocking their ability to breathe again but laying DPM, underlay, laminate, thus the DPM. I’m just unsure of how the DPM and doc should meet/overlap/etc., in particular where I’ll be installing this stud wall probably weeks before the floor and I can’t have a full sheet of DPM laid across the room in the meantime.
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Lay a strip of DPM, to be folded and taped with the other DPM later. Seal fixing holes with silicone.

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