Yellow staining on wall and damp meter

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We recently moved into a new to us (built in the 1940's) house.

One bedroom has a yellow stain on the chimney breast around the area where it joins the ceiling.

Work has been done before we moved in to re-tile around the chimney directly above where it passes through the roof and the lead work has been replaced.

The wall in the stained area does not feel wet.

However, i went and bought a cheap damp meter

http://www.bamr.co.za/downloads/Rapitest Damp Checker.pdf

Checking the stained area gave readings between 22-28%

Working away from the stain the readings got lower and eventually went to zero.

Which made me think that there must still be a leak from the roof above.

In the roof space where the chimney passes through the roof there was no sign of any water ingress that i could see. So with my damp meter i started checking the mortar joints in the chimney. 22-28% again.

I checked the surrounding woodwork, 22-28% again.

Odd, so i started checking random bits of wood in the loft area 22-28% again. I checked the mortar where the tiles are back buttered on to the laths (steeply pitched roof, no felt) which felt dry as a bone, again 22-28%.

So, i've come to the conclusion that my damp meter is a pile of cack, i'm not using it properly (which is unlikely because its not exactly rocket science to touch two prongs onto a surface) or my whole house is riddled with damp and its about to fall down.

On a serious note, can anyone tell me how i should actually determine whether this stained area is still damp or just and old stain or even a soot stain from the hydrocarbons that have seeped through the chimney.
 
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its not exactly rocket science to touch two prongs onto a surface

The rocket science is interpreting the results to determine if there is a damp issue or not. And meters are designed [ie calibrated] for timber not masonry or plaster

A yellow stain is a leak from the roof.

Its an old stain if it does not get bigger or wet after rain. If it does, its a leak. No damp meter required
 
Having inspected the chimney in the roof space directly above the stain i'm pretty confident its not coming from a roof leak.

But i would like to know if there are elevated damp levels in that area that might be coming from condensation in the chimney etc.
 
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Condensation within or on a chimney would not normally spread across a ceiling. It might, but it would have to be so severe as to be visible
 

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