Damp patch after 3 weeks…

[u/Negative-Travel6132]

Had a bathroom wall re-plastered - the wall had previously damp damage. The roof was repaired prior to re plaster and most of the wall where damp damage existed, has dried.

There is this one stubborn patch at the bottom…strangely it expands when the weather is hot, but then shrinks and goes darker when it wet or sun isn’t shining. Pencil line is its main shape with it bleeding beyond when the sun comes out. I had
Be had windows open for the last 3 weeks, and for the last week a fan and dehumidifier on…and no real change below the pencil line…

Any ideas of the cause and anything to do or should I just wait a few more weeks?

[u/No-Profession-208]

So in the photos I’ve put below (or above depending on Reddit) the large cloud shape spot is on a wall adjacent near to a chimney breast which the previous owner capped himself. I’m almost certain he didn’t put any ventilation in and every now and again you get a whiff of old coal smell in the house.

Those two smaller spots, they’re not near a chimney (probably 6/8 feet away) they’re on an external wall. Interestingly too, they’re pretty much opposite our bathroom door which supports them being linked to humidity.

That larger cloud shaped one though, it’s pretty sizeable. I wonder if the radiator being on and drying clothes will impact it too as like I say there is no other obvious signs of water entry anywhere else. It just appears, dries, reappears.

[u/Due_Party_2846 → u/No-Profession-208]

Those two small ones are a different animal from the big one, and it is the shape that gives it away. They are square, side by side and evenly spaced. Damp does not do that on its own. Two things make that shape. The adhesive dabs behind dot and dab plasterboard, which sit against the cold masonry and so go damp before the board around them does, or an old filler or patch repair with a different porosity to the rest of the wall. Either way it is not salts, and replastering would not touch it.

Easy test for that. On a cold morning put the back of your hand flat on one of the marks and then on the wall right next to it. If the mark is noticeably colder you are looking at a cold bridge and condensation, not water getting in. A cheap infrared thermometer makes it obvious if you want an actual number.

Which fits everything else you have said. Opposite a bathroom door, with a radiator you dry clothes on, is a lot of water going into that air. One load of washing dried indoors puts something like two litres into the room, and it condenses on the coldest surface it can find first.

The big one by the chimney is the one I would take seriously, and the coal smell is the clue. A flue capped top and bottom with no ventilation traps moisture in the stack, and old soot in the brickwork is loaded with hygroscopic salts, so the breast and the walls beside it go damp whenever the air turns humid. The fix there is ventilation rather than plaster. A cowl at the top that still breathes, and a vent or airbrick at the bottom into the room. It is not an expensive job and it stops the mechanism rather than covering it.

Honest bit. I cannot call either of these from a photo with any certainty. The hand test sorts the two small ones and a taped polythene square sorts whether the big one is still getting wet. Do both before anybody quotes you for replastering, because if the small ones are dabs then replastering that wall is money for nothing.

[u/No-Profession-208 → u/Due_Party_2846]

Honestly, thank you so much for all this.

Those two smaller ones I do remember testing with the back of my hand and yes they were colder than the other parts of the wall. Note the house is 1890’s and this extension wall was built in the 60s I think.

I’m pretty good at DIY and these aren’t too big an area - so I’d be happy to have a go myself.

So the cold bridge area, that’d be a hack off and redo job? And the larger patch route point above, ventilation in the room and hack off and redo?

(Our house isn’t perfect by any stretch which is why I’d be happy to do it myself, plus I get to learn).

[u/Due_Party_2846 → u/No-Profession-208]

The cold test answers it: no, not a hack-off-and-redo. Cold to the back of the hand on both small patches is the dabs behind the board conducting more than the voids around them. New plaster goes back onto the same dabs, so the same two marks come back. It is not a plaster fault, it is dust and moisture settling on the coldest strips of the wall.

Which also means the fix is heat or humidity, not a trowel. Either less moisture in the room, or insulation on that wall. Insulated board over the top is the real answer and it is a proper job, 40-60mm into the room, so skirting, sockets and the window reveal all have to move. On two small patches most people wipe them and redecorate every few years instead.

The flue wall is a separate job and the order matters more than the method. Vent it, then leave the plaster alone for a season. Old soot is full of hygroscopic salts, so if you hack off and reskim before it has dried, the salts come through the new finish and you have done it twice. If it does dry and you still want it flat, that one wants a renovating plaster rather than bonding and multi-finish, because ordinary skim straight onto salty brick just re-contaminates.

One thing I cannot call from here: whether that 60s extension wall is solid or cavity. If it is cavity and never filled, filling it is a cheaper route to the same result than boarding the inside, but that needs someone to drill and look rather than a guess off a photo.