Damp Proofing

Mortar lines showing damp through new plaster

Anonymous user 15/03/2024 - 2.41 PM

Hi Everyone. In short, we have a problem of mortar lines showing through a freshly plastered (now dried) wall in one area only. Here is the whole scenario: We are in the process of renovating a house, we think it was built in 1950s. It’s semi detached and our issue is with the back corner of the house on the side not attached to next door (so where 2 external walls meet). This area is our downstairs toilet which had tiles half way up the walls when we moved in, however the tiles had “blown” in one patch and so we removed them and had our plasterer plaster it. He was plastering onto brick & mortar and there may have been some tile adhesive still attached in some places but most of it came off with the tiles. We noticed once the plaster was dry that every time it rains the mortar lines of the brickwork show through - it first appears in the patch where the tiles had blown (perhaps explaining why they had blown in the first place) then if the wet weather continues it spreads along the wall. So, some things to note: there is an air brick outside that matches up with where it starts (this was blocked up when we moved in and we unblocked it under advice from our damp proofing people); we had damp proof injection in that wall and the adjoining wall approx 3 months ago; the patches dry out and disappear pretty much completely when the weather is dry; we do not get the same showing through on the adjoining wall or on the rest of that back wall; only in the downstairs toilet; we have cavity walls which we believe were insulated around 1996. Recently we got our damp proofing guys back to have a look and they recommended we try Stormdry so I have applied 2 coats of it to the wall externally but this has not made a difference. I have also attached a vent cover over the air brick outside, thinking it might stop any rain bouncing off the patch outside up into the wall through the air brick while still allowing the air brick to vent. I have read online about “ghosting” but it is not wet to touch, no condensation usually in that room or on the window and there is not a collection of surface dirt so doesn’t seem to match what people say about that. Any thoughts would be much appreciated! Tempting to just paint with a damp seal before painting final finish but would rather get the underlying problem sorted. I have photos from when we took the tiles off and also of what it looks like now when it rains but there’s no facility for adding them on here so let me know if you need to see them and I can maybe email them across.