Mysterious damp patches on external wall

[SE13Mummy]

They are mysterious because we've lived here for 12 years, never had any issues with damp, have a roof that is only 6 years old and have tiles, guttering, pointing etc. checked.

What else might be causing these patches? It's a south-facing, external wall of a 1906 house. The first patch appeared mid-January and although they fade in dry weather, they don't fade completely and they definitely get worse in the rain. If we'd not already had the roof etc. looked at by various people then I'd put it down to that but it's frustrating that we can't find the cause - it's DD2's bedroom and had been finished about 6 weeks before the damp patches appeared. The lining paper on that wall seems to be coming unstuck too so there's clearly something wrong.

I've attached a couple of pictures of the patches and one of the wall prior to decoration (is used to be the kitchen, hence the visible pipes).

how does the pointing or plastering look on the outside. maybe repointing or replastering is needed.
other changes like tree removed in front of the house?

how do ou ventilate?
renovating/redecorating means lots of moisture that needs letting out.

The pointing has been looked at by a couple of people and the verdict is that it's fine. It's bare brick outside so there is no plastering. No trees removed from the front or the back of the house (this room is at the back).

Are you on a T-junction? All the houses on my road south facing at the T-junction have penetrating damp this year after the winter when the prevailing wind has funnelled all the rain down the road into that wall of the houses and the walls have become saturated. Lots of scaffolding up at the moment.

Have you checked the gutters when it rains to see if they are working efficiently? Particularly the heavy downpours that we've had a lot of recently.

If they are full of leaves/come apart at the joins/leak or the downpipe is blocked the water could be running onto the wall

No T-junction; a long Victorian/Edwardian terrace near the top of a hill (not an especially high/steep one but we can see the London Eye). In the third picture the backs of the houses behind can be seen. There is a cul-de-sac behind us where, once upon a time, there was an orchard.

Gutters all checked and all were clear. We've studied them during/after rain and they are doing their job.

We ventilate by opening the windows - there are two, large, sash windows in the room and a velux window.

Like notnow's neighbours, we had unexpected penetrating damp this year - enough to blow plaster in rooms with no history of it. Those storms of severe, horizontal, driving rain, that went on and on in January & Feb, were enough to find out the house's weaknesses.

That's interesting ParsingFlatry. How have you dealt with it?

Asked OH to look at your photos.

On your third photo of the bare wall, there appears to be a circle at the top left that looks like it may once have been a vent hole for a boiler or something. Do you know if this is the case? If so, could it be coming through there?
Was the wall dry lined (with plasterboard) or plastered straight onto the wall?
Do you know the construction of the wall - cavity wall or solid wall?
Do you have any photos of the outside wall in the same place?

Sorry, on iPad so can't zoom in properly on the photos to check if the circle is actually of plaster filling in a hole.

Thanks PringleLicker (and your OH!). The circle is from where the boiler used to go out - it has been filled from the inside and from the outside, as have all the pipe/outlet holes although the largest, circle one was only done after the patches appeared.

I'll need to check with DH about the drylining but I'm fairly sure plasterboard was used...I can't find any photos of the inbetween stage on that wall - everywhere else was boarded first though. I'm not sure about photos of outside either, I shall investigate.

Ok, his thoughts so far are it is probably either moisture penetrating through the external wall and tracking through the plasterboard via the dabs of plaster used to fix it to the wall. Or possibly interstitial condensation between the plasterboard and the external wall.
The added complication is the boiler outlet, as any boiler outlets will involve different salts forming than in other areas of the same wall (sorry, I'm probably not translating very well!)

The problem is that both of these things would involve different solutions and obviously wouldn't like to send you down the wrong route. He couldn't tell without seeing the external wall as it is now, and better if someone sees it in person. Do you know any good general builders who could take a look for you?

As far as I understand it, the penetrating moisture would involve work externally and the condensation would involve insulating the wall internally.

Aha! Yes, out worst patch was where an old boiler flue has come out. The mortar had slightly cracked around the patching, and it was just enough for the driving rain to creep in and saturate the wall.

We've dealt with it by pointing the outside, hacking off plaster inside (ours was plastered onto the wall), leaving for as long as possible to dry out a bit, and replastering. We've just made the assumption that the conditions aren't likely to repeat, and are doing everything possible to help the wall dry out as much as possible (we ventilate in summer and dehumidify in winter anyway).

And yes, agree with PringleLicker. Very hard to see what's going on with plasterboard. And you'll need to know whether you have cavity walls or not.

We made the decision to wet plaster in the first place because our walls are solid, and interstitial condensation was a nightmare I didn't fancy dealing with.

What a pain for you OP. Hope you do manage to get to the bottom of it.

if it is drylined, and there used to be a boiler, I would be very suspicious that there might be disused old pipes in the wall. Or possibly a tank or overflow pipe is dribbling in the loft. Presumably there is not a bathroom or radiator above.

You might be able to see something coming up in the loft, or possibly in the room below, e.g. if there is a kitchen or somewhere else with water. Or maybe a radiator pipe.

Drylined walls do not easily get condensation.

If you tap the wall with your knuckles, you will be able to hear where dots and dabs of plaster might be. If they are behind wet circles, then yes, the water is passing through from the structural wall.

If you are sure the gutters and pointing are good then a plumbing leak seems most likely to me. If it was mineral salts in the wall you might see crystals or mineral hairs on or around the damp patches.

p.s.
if it is a 1906 house then I would assume it does not have cavity walls. Measure the thickness of the wall at a doorway or window opening, I expect it will be 9" plus the plaster.

p.p.s.

looking at picture 3, it could be the roof leaking.

post a pic of the outside please.

DH is home. Apparently it's a sort of cavity wall; two skins of brick with a small gap between them that will have some cross bricks. I was wrong about the plasterboard, picture 3 shows the wall as it was prior to lining paper being put on.

There is no loft above the room - the ceiling goes up to the level of the roof (see extra pictures).

The only photo I can find of the outside is prior to the builder filling in the holes from the boiler, outside tap and other kitchen-related holes.

"it has been filled from the inside and from the outside, as have all the pipe/outlet holes although the largest, circle one was only done after the patches appeared."

Hang on, I'm only reading this properly now.

So you had a lovely vent-sized hole right through your wall, all through the rotten weather?

Course you've got a soggy wall! You've had a lovely puddle in there soaking it's way down through all the bricks, and tracking off through the mortar and generally spreading in the plaster.

It should dry out now you've filled the 'ole, but will take a while.

Thanks Parsing, that had been my initial feeling but various people have said that the patches that are nowhere near the original patch, must be due to something else .

I don't suppose anyone knows approximately how long it's likely to take for the wall to dry out....? We have bought a dehumidifier which seems to extract a lot of moisture not that I know anything about what constitutes a lot.

We have patches like this on an external wall. Had them for two years and we've never had a hole through the wall. The pointing and bricks look ok. Ours is an old house with no cavity wall. No damp anywhere and well ventilated.