Strangely isolated damp problem...

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We have a longstanding issue with a strange re-occurrence of damp spots either side of a chimney breast in an internal party wall of our semi detached Victorian house.

One patch about a foot square just above the skirting, another on the other side about 6" square about 18" higher, and a small patch at the corner of the chimney breast at floor level.

I've dealt with bigger issues before and have an understanding of damp proof courses, condensation, ventilation etc., I cleared the base of chimney breast of soot deposits when upgrading the fire from coal to gas, both areas have been re-plastered at some stage and treated with damp proofing paint prior to papering.
There is no obvious damp, no horror film photos of mould etc., and it takes many months, a year or more before the paper starts to part company with the wall and for dark patches to appear but they always come back.

It transpires that our adjoining neighbour has similar issues and like us are scratching their heads.

Any suggestions/advice welcomed?
 
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Posting pics showing the whole back wall/party wall & chimney breast - floor to ceiling will help?
Pics of the chimney stack will also help?

Is the fireplace in the room above an open fireplace or has it been blocked off?
Does the neighbour have any blocked off fireplaces?
Is your floor suspended or solid?
Have you ever been into the loft space just below the roof?

Typically: "re-plastered" tends to mean gypsum plaster - bad, not lime and sand render - good.
Applications sold as "damp proofing paint" are mostly a waste of money, and can sometimes cause further problems.
 
Aren’t chimneys prone to salt build up?

That then sucks moisture from the room much more easily than anything else; hence the spots.

You could hack it back to brick, neutralise the salts and then lime lender?
 
Posting pics showing the whole back wall/party wall & chimney breast - floor to ceiling will help?
There is nothing to see, other than perfectly dry sound walls apart from the patches.
Pics of the chimney stack will also help?
On the roof?
Is the fireplace in the room above an open fireplace or has it been blocked off?
There's not a fireplace in the room above. We moved in 35 years ago and there wasn't one then.
Does the neighbour have any blocked off fireplaces?
They have a new log burner downstairs.
Is your floor suspended or solid?
Suspended.
Have you ever been into the loft space just below the roof?
Yes.
Typically: "re-plastered" tends to mean gypsum plaster - bad, not lime and sand render - good.
Applications sold as "damp proofing paint" are mostly a waste of money, and can sometimes cause further problems.
Damproofing paint has worked well on the area at the corner of the stack.

Answers within above...
 
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Aren’t chimneys prone to salt build up?

That then sucks moisture from the room much more easily than anything else; hence the spots.

You could hack it back to brick, neutralise the salts and then lime lender?
There is some evidence of damp at the rear corner of the stack where it joins the walls either side. To put it into perspective the room was decorated and wallpapered 4-5 years ago and has only just started to present the dark patches.

The wall to one side may be lath and plaster.
 
Update to thread...
Following a brief burst of torrential real cats and dogs rain a fortnight ago we had just a few drops of rain dripping through the upstairs bedroom ceiling at the edge of the chimney breast. A heavy downpour for the next hour or more did not worsen the flow, a single sheet of kitchen towel being sufficient to mop up. I deduced by this that the rain was traversing the valley but only when experiencing the initial harsh downpour that couldn't 'get away' quickly enough.

However on further investigation and despite their being minimal signs of ingress in the loft, I removed some more insulation and found a soft red bring in the chimney stack had crumbled leaving a hole that a fist would get through. It had clearly been exposed to moisture for some time.

On last inspection the stack, which is topped off with a Cowl, did not appear to need re-pointing and the leadwork looked sound. First job will be to get a professional up there asap.

I'm now thinking that water is getting into the stack, possibly from the adjoining property that shares the valley, and that moisture is getting to the lower floor where the damp patches are appearing, by flowing down behind blown Victorian plaster? That the areas of concern have been patched, have greater contact with the breast, hence the discolouration being isolated to a couple of areas?

I'll let you know how I get on!o_O
 
OP,
Thank you for letting us know the state of play.

However, You originally came on here looking for advice.
Suggestions were made to help you but you appeared to rather follow your own course of action?

Now you wish to rush a paid for professional up on your roof - what trade are you thinking of?
Its common nowadays to ask for any inspection pics or video of possible roofing work.
Perhaps you will post any relevant pics on here?
 
Tell80

What advice have I ignored?

Bristol Ben made a suggestion, which, whilst valid, would in my opinion be an extreme measure at this stage.
You asked a number of questions which I answered to the best of my ability and have only just come back with a somewhat acidic reply!

That you need to ask what paid professional tradesman I am thinking of to look at and confirm or eliminate issues with the roof and chimney breast is odd to say the least.

For the record, I bought this house over 35 years ago, long before YouTube and the plethora of TV house renovation programmes, knocked 3 rooms off, added 6, laboured, dug, plumbed, done a lot of the woodwork and took the roof to felt and batten stage before calling professionals in as required.
In fact, if you refer to my opening post where I state my understanding of damp proofing, condensation, ventilation etc., you may even deduce that I have half a clue as to what I'm doing, that my reason for posting is because the nature of this issue has baffled many folk.
 
Update: Our house is an addition to what was once a stand alone detached property next door. The Chimney stack brickwork is not bonded into the old chimney on one side and it appears that water is coursing down the gap in between the two as the mortar joint is degraded. Some mortar also needs replacing around the Pots and some repointing is required. Hopefully this will solve the upstairs leak.

On further investigation of the patches around the downstairs chimney breast, this again leads to damp in the stack. The end of a beam that sits atop a *sleeper wall (which the floor joists sit on) that runs into the side of the breast is breaking down but what I can't fathom out is the way it's doing so? Instead of cracks and general erosion the degrading timber has formed a pile of timber dust, the 'sculpted out' parts being quite smooth. There are no signs of fungus or woodworm and only the slightest suggestion of moisture in the pile. Any ideas?

*the sleeper wall is 20mm from the main party wall and the gap is full of debris, most of it irretrievable and clearly as old as the house. I'm dragging and hoovering out as much as I can so that it doesn't sit against the timber top.
 

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