Thought that'd get your attention!

I have finally investigated the rising damp in the extension. (1980's, home built I suspect, and not very well.) Good news is there is a dpc and it hasn't failed. Some plonker left the membrane under the screed an inch short of the dpc, and plastered over the gap. Yup. that's right, the plaster goes down below floor level, under the screed. Hence the plaster bridged the dpc, and sucked moisture up the wall.

I've uploaded some pictures.....

I've dug away enough screed to find the end of the membrane under the floor, and also enough plaster to expose the dpc, and now the wall's drying out nicely. Question is : what do I do now?

a) Mix in some waterproofer with some screed / render and fill it back in again or
b) get a 100mm strip of dpc and tuck it behind the membrane, to cover the gap.

Then what? Screed the floor back in, obviously. Do I leave the new dpc strip long, or cut it off at the dpc level? Replaster over the top? (Will plaster even stick to dpc?) Or leave a gap behind the new skirting board?

All tips appreciated!
Thanks.

it shouldn't be plastered to the floor so leave a gap behind your skirting board.

Ok, will do. But what should I do about the gap between the screed membrane and the dpc?

personally i'd apply a bead of silicon to a length of 4" dpc, tuck it down into the membrane and fill the gap up to screed level, on the screed side (not the wall side) with a 3:1 sand cement mix using sbr as an additive at a rate of 1:2 sbr:water
just leave the sticky up dpc where it is and skirt over it...

nice pic btw.... bossman might nick that and sticky it...

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If it's not a dumb question, does the bead of silicon go on the bottom or top edge of the dpc strip? Am I sticking it to the wall, or to the screed membrane? Do I tuck it down the outside of the membrane? Or the inside?

My brain works better in pictures!

stick the dpc strip to the membrane, its just belt and braces to stop capillary action drawing moisture from the brickwork below the wall dpc through into the screed..
just extending the membrane up over dpc level, dont have to use actual dpc on a roll, can be a strip of 1200 guage membrane, i just thought 4" dpc would be cheaper and easier...
and if you got a really good seal between the two, no real need for the sbr mix, you can just 3:1 sharp sand BUT it'd be a good idea top paint some neat sbr on the old screed edge prior to filling or even better, paint an sbr/cement slurry on the edge... get you better adhesion between the old screed and new filler screed...

some of the old damp jobs we did in the past had no membrane at all, were plastered as yours was (under the screed, bridging the dpc etc)...
solution was to hack off the bridging plaster, fill the gap with an sbr/cement/sand 'bottle cove' and then tank down onto the floor before installing a chemical dpm over the entire floor area....

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A 30m roll of 100mm dpc is £3.50 at screwfix. No worries about cost then!

I think what I'll do is dig a bit more screed up to get a longer edge of free membrane, then poke 100mm dpc down the back and silicone them together in situ. Seems a bit less messy doing it in that order.

Then sbr, sharp sand & cement screed filler, and wait a few weeks before tiling the floor!

Thanks cw.

Hi,
To be honest, as a few posters have pointed out plaster should not go down to the floor there should really be a 50mm gap that is so any damp or moisture can evaporate behind the skirting,
So basically i would chop out the plaster to the level of the DPC let everything dry out and refit the skirting making sure you have the proposed gap and that nothing touches the area underneath the DPC

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