On 01/02/2020 10:51, newshound wrote:
On 31/01/2020 16:34, wrote:
On Friday, 31 January 2020 11:08:51 UTC, Â* wrote:

A cavity wall has a "tide mark" about 200mm above the floor. The wall
was originally an external wall but, as a result of building an
extension, is now an internal wall. The wall has a 1930s DPC and the
floors have modern DPMs.
I assume that the old DPC has failed so am wondering what the best
options are. At the moment I'm thinking of knocking-off the plaster to
about 300mm, coating the wall with something and then
rendering/plastering with an additive.
The question is what to put on the wall: liquid DPM? bitumen? cement
board (hardiebacker)? foil-backed plasterboard?
What does the team think?

Rising damp does exist but is rare.

True, but if you have it it can be dramatic

It is very unlikely that this is what's going on. DPCs are therefore
not often useful. You propose DPC failure - how exactly does a hard
material such as slate entombed in lime or cement fail?

by cracking

Bottom of interior wall damp is usually from condensation. A wet slab
from a plumbing leak can also cause it.

I'd say sometimes rather than usually. And you need plumbing there for
plumbing to leak


Coating with impermeables normally results in the coated area not
drying out, and the matter therefore worsening. I'd look at the slab
if practical, and look to see if ambient RH in the area can be
reduced. Other options are to scrape & paint the area with lime, which
permits it to dry better than emulsion, and use of a dehumidifier. The
last 2 are often sufficient to keep things in check.

Completely with you on not trying to hide with impermeables (but OK to
create new "DPC" with silicone, if you have suitable materials and
construction. You can pour in an awful lot of fluid into a stone wall).


NT



The 1976-built house that I bought in 1991 had had a leaking 28 mm
connection to the baxi bermuda back boiler since new, judging by the
stalagmites (-tites) of crud down the side of the heat exchanger.

Surveyor apologetic but said he couldn't get the access panel off
because of furniture, but added that he had noted unusually high damp
readings on his meter which was unexpected in this type of modern house.
He assumed it had been the result of 'cleaning'.

Luckily BG fixed it with a new heat exchanger as part of 3* service
at no cost to me (which they wouldn't do now).

The water had seeped into the slab and wicked all along the felt-
lagged CH copper pipes in the slab and where they crossed over the
3/4 inch iron gas feed to the kitchen, the latter has almost
corroded through.

I only discovered this in 2004 when I dug out the screed and put
down 70mm celotex between battens on a 2nd dpc, plus hardwood floor.