Black dust between block and plaster

[Alex H]

Hi,

I have a corner in a house which I assume is damp (outside path is too high) which I will tackle as the source of the issue before any internal work.

I removed some of the loose wallpaper in that location and the plaster skim is loose too. Upon removing this there appears to be a black dust between block and plaster.

For information the house is concrete block construction with as yet unknown external render, circa 1930 build.

It appeared to be coal dust like, but I wanted to check that it wasn't anything more sinister? Mould growing?

Links to a couple of pics. I welcome your thoughts/ experience. As it's a bit of a project house I'll apologise now for a raft of questions.

https://drive.google.com/file/d/1KCwLlgi2KAhDhn4_x_pQ83LbnjntK06H/view?usp=drivesdk

https://drive.google.com/file/d/1DN3DJFv-mR2EVcadSa6KAagSsfnTnoqG/view?usp=drivesdk

The timber in that area appears to be very solid, whereas I was expecting it to be rotten.

 

[JohnD]

it looks to me like water has run down that wall, so it could be black mildew.

I've also seen black and brown stains when water leaked down a chimney, but it had a sooty smell and the stain was tarry.

 

[Alex H]

Thank you John, so it's probably water damage rather than some sort of bonding material?
The external path has a hole in it at this location, so could well we water/ mildew.

Hole in path
https://drive.google.com/file/d/1jD6zSV35zQLDbfSwwkU8XroPRaar6WW0/view?usp=drivesdk

It was dry/friable.

In any case I will strip that whole wall to bare block and sort the external path.

Thanks for responding

 

[bobasd]

pull the skirting off the walls at the corner. have you had a bit of flooring up to see the joists for any damage?
the outside path has raised ground level and bridged the render an maybe any dpc in the wall. the render should stop short of the ground.
the black stuff could have been used to paint the render and the inside.
after going back to block what are you going to do?

 

[Alex H]

I've not been able to be too intrusive as we have not signed for it.

The joists span in the same direction as this troublesome wall, so there are no joist ends in that wall. There does seem to be evidence of wood replacement of the floor boards in this area. I'm expecting some issues if I'm honest with the woodwork against this wall.

We plan to go back to bare block, wood/wool insulation board and lime render. See if we can let the wall breath...or that was the current thinking. At the moment the wall can't breathe from either side.

Thank you.