DPM & DPC Problems!!

23 May 2013
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West Glamorgan
United Kingdom
Hi All,

I'm really sorry if this is in the wrong thread or if it's been posted before. If it has, please feel free to point me in the right direction :)

Ok.. We're renovating a house and (now wishing we'd just left them) dug up the existing concrete floors in order to put a DPM and insulation in.

We've dug out to 13" and have the following setup planned:

100mm (4") Gravel (already in & compacted)
50mm (2") Sand (already in & compacted)
50mm insulation (the BCO has said this is fine.. I know now that it should be more..)
100mm (4") Concrete

We're not laying a screed, but we are laying slate. When we refit the skirting, if there's a bit of a difference between previous and current floor levels, we'll move the skirting to account for it!

We're about to put the DPM in and I can't find consistent answers to the following questions. For the record, our BCO is known (infamously) for being a stickler, so I really don't want to get this wrong. We're in Wales (Carmarthenshire) if that helps anyone with regards to regs.

1- How far up should the DPM come?
Obviously enough to come up the side of the concrete, and I know that in ideal situation, the DPM would lap into the DPC.. Which leads me on to:
2- However, we're in the valleys and the existing DPC is slate (in some places, in others it's non existent). So where do we lap the DPM in to? Or do we just mortar over it onto the walls (a bit like render and flashing externally)?

The DPM is 1200 gauge and we have some tricky corners; currently we're just origami-ing them into a fold and then taping to the wall or DPM in the alcove under the premise that we don't want to cut it and the weight of the concrete will keep it in place once it's poured.

I've attached a pic of the room (nicely drawn in paint!) to show the alcove area. My husband has cut a square piece covering the room and then an insert piece which laps under the main square piece in order to fit in the alcove. This seems to be overlapped several times and we've made sure it feeds under the main DMP so that in the very unlikely event of damp coming in, it would feed under the main DPM and not on top of it.

If that doesn't make sense, let me know and I'll try and take a photo (or better still another paint scribble!).

Thanks in advance for your help :)


I'm not sure if the pic will show here but it's in my album otherwise!
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Am i correct: you've removed all the (solid) floors in the house?

If so, you should really research pouring slab floors on here.
See, for instance, my post in "Kitchen slab advice" mon april 22 2013 at 4:16pm.

You should have insulation at all perimeters ie. edging insulation located inside the membrane.

Section drawings/sketches should be made noting all previous FFL's and exterior GL's, and in-place DPC. Any variation with the new FFL's to the previous FFL's will bring various difficulties, eg. door thresholds.

Just flash the membrane up the wall above proposed FFL. There are sealers and tapes for joining membranes.

Thanks for your reply. Believe it or not I have done a fair bit of research but it's not consistent.

We have two ground floor rooms which were both laid with concrete but it was decaying and had no DPM etc. We had a damp survey carried out and were told a lot of faff about bad damp in the property- there is actually no apparent damp, nor evidence of it, other than the roof and chimney needed repairing. It was very much a case of being sold a story for us to pay the company about £17,000 in damp proofing.

Anyway, we have dug the floor out to 13". This allows for all of the infill plus tiles and adhesive. If there is any discrepancy between the former and current floor levels there shouldn't be an issue- it should only be 10mm at the most.

With regards to the exterior GL only one wall is an exterior wall- there is an extension built to the side of the property which now forms the exterior walls of the property. We are going to install drainage of some kind to the exterior- possibly a French drain as the exterior GL is slightly higher than the interior in some places.

It's all a bit up and down really with steps up and down internally and externally.

I know what I'm doing with the slab itself but is there a requirement for a certain length of DPM over the level of the new floor? Does it need to be 2" or so extra, or is it just so long as it goes past the floor level?

I'll have a look at your post now :)


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Do not cut the DPM 1200 gauge until after the concrete has gone off, I fix it in place by nailing to the wall well above the finish height of the concrete, then after I've laid the screed or whatever go around with a Stanly knife at finished floor level.

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