"Rising Damp" at Slab Floor Perimeter

[BumblingBees]

We have just lifted up the floor in our dining room to investigate the cause of rotting skirting boards in our new home...

What we have found is a floating floor construction consisting of a plastic sheet, concrete slab, insulating foam, and then chipboard beneath thick underlay with carpet on top. The rotting skirting boards have been caused by moisture rising up the two inch gap left at the perimeter of the plastic and concrete layer. The chipboard directly above was completely rotten and the bottom of the skirting board is starting to rot too.

The walls are sandstone which have been wicking up the groundwater - but with gypsum plaster on top of the lime plaster and this concrete slab floor, all the moisture is being pushed to this perimeter gap. The question is... how to easily and cheaply rectify the issue?!?

If we add more plastic and concrete into this gap, we understand that this will create efflorescence in the walls with the water pressure build up. Could we look into removing the gypsum (we have done this in a room upstairs already and found good quality lime plaster behind it) and then add limecrete into the gap to create a hybrid floor? Would the gap need to be bigger for this?

Please advise!!!!!

 

[JP_]

When you say new home, do you mean new build, or new to you?

I am no expert, but that plastic sheet does not look like a proper damp membrane - the ones put in my extension were thick black sheets that went up the walls a bit.

 

[BumblingBees]

Hi jonbey, the house is new to us but was actually built in the 1840s! We did think it was strange that the sheet stopped before the wall, and it does seem quite thin. If it's not a proper damproof membrane, do you think it's best to take the whole lot up or could we add some type of epoxy layer? What do you think of the gap where the groundwater is rising up?

 

[JP_]

I can't say really. I'd get a good builder in to have a look, if really old then I might be an idea to dig out floor and get it brought up to today's standards, with full damp membrane, insulation and screed. But a big messy job!

 

[foxhole]

Do you have a water meter , have you checked for leaks , old property may still have lead service pipes which leak easily .

 

[JohnD]

What we have found is a floating floor construction consisting of a plastic sheet, concrete slab, insulating foam, and then chipboard beneath thick underlay with carpet on top. The rotting skirting boards have been caused by moisture rising up the two inch gap left at the perimeter of the plastic and concrete layer. The chipboard directly above was completely rotten and the bottom of the skirting board is starting to rot too.

What you describe would perfectly well fit a house where there was a leaking water supply pipe under the floor, and the builders have tried (unsuccessfully, as usual) to cover it up.

I agree with the lead pipe and water meter idea, though iron pipes also perforate, especially at an elbow.

Also:

station a young person with sharp hearing in that room (and also in the kitchen) late at night when all is quiet. Turn the outdoor stopcock off and on. A motorcyclist, builder or gunner is unsuitable.

The noise of a leak is very quiet, just faint white noise, but they will notice when it stops and starts.

If you have no young person, press the tip of an engineer's stethoscope against the cold water pipe in various places.