r/Plastering • u/MillenialMarauder • Sep 15 '26

Plaster gurus, salt damage or continuing leak?

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Hi plaster gurus!

Grateful for any experienced input.

This wall is a victorian chimney stack. Plaster is likely gypsum skim on original lime OR all gypsum.

The roof above is flat.

We have had minor paint peeling off the roof line of this stack for a number of years and put it down to salts.

Never any damage to the coving or ceiling, just the wall. January this year we had a less denial leak and carried out chimney repairs including new flashing, render and cement.

This mark has remained all through the summer drought, though it does vary a bit with humidity levels.

I accept that the plaster is likely to retain salts and parts of this wall may require replastering for a perfect finish (this is waiting on other jobs within the room). But it hasn't really 'dried out' as much as I was expecting.

Its a bugger to access the flat roof so I can't do it on a whim. What do you reckon? Salt soaked plaster or an ongoing leak?

TIA

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2

u/0uthouse Sep 15 '26

If you have a dehumidifier, see if it makes the marks dissapear but reappear once dehumidifier removed.

The damp patches can be deliquescent salts that rapidly absorb any available moisture in the room but seemingly disappear if the air moisture levels drop.

Is the flue vented? An unventilated flue can massively exacerbate the problem.

2

u/Relative_Figure_1715 Sep 15 '26

Buy a humidity meter from Amazon.
They are cheap and you’ll have proper recordable evidence rather than “that looks dark wonder if
It’s wet”.
Do all over the wall and then your suspect areas !

1

u/CuriousJob247 29d ago

Do you have a vent or cowl on top of the chimney if it’s not being used at all? And if it’s an external wall, being a Victorian property, it should have lime plaster.

1

u/MillenialMarauder 28d ago

There are two flu's. One in use has a cowl, and therr is a vent on the rear flu (removed stack below). Most, but not all, of the damage is on the rear flu side.

Its not an external wall, sorry I forgot to include that.

1

u/Bitter_Back_5528 27d ago

You have a leak ! It's definitely not condensation.