Was slate tanking once the norm?

[michael]

I’ve just been carefully chiselling out a vertical channel in a wall to accommodate a sunken light switch and associated cable.

I say carefully chiselling because the wall in question is below ground on the side of a hill and I was aware that there would be some form of tanking behind the plaster to prevent ingress of water from the ground and the hill.

With my tiny chisel I was able to cut into a small inconspicuous area of the wall to establish the thickness of the lime plaster coating. Quite hard going though. Pretty hard stuff about 190 years old I think..

The lime plaster is approximately 25mm thick and there appears to be a sheet of black slate behind it.

I’ve continued to cut out the vertical channel (about 2 ft long) and the recess for the light switch and the slate appears to be running continuously down the wall with no joins evident..

Was this the normal form of tanking in the early 19th century or am I just privileged to have such a worthy material in place?

 

[chuckey]

I have seen overlapping roof sized slates stuck on the outside of victorian houses below ground level.
Frank

 

[plumbers mate]

Could be slate tanking - it would be effective.
Black slate is more likely to be from the quarries near Barrow (the Burlington Quarries) rather than Honister (green) or North Wales (grey, purple, red, green or blue-grey)
Slate was used for the other sort of tanking ie the sort to hold quantities of water in.
And for urinals of course.