>>34409
Well heat you have, then you only need air.
Yeah, in time all the plaster can get deteriorated. He mishandled that house it sounds.
>completely soaked the wall
That needs to be dried out completely before the new plaster, otherwise you won't solve anything.
>were put in at least 30 years ago but they had become rotten in parts
That doesn't sound good. It wasn't cared for. And the house getting more wet with time.
I think the tar needs to be hot, so it sticks to the foundation and to the tiles. It doesn't need to be overly hot tho, just smearable I believe. The base have to be dustless and solid.

>>34417
Indeed.