Mystery damp patch on downstairs ceiling

[Joka1965]

Hi. I have a damp patch on the ceiling in our early 20th C cottage. It began to appear over a year ago, very gradually getting bigger, emanating from a corner between an exterior wall and an exposed beam. Readings from a digital damp meter are around 55-60% at the worst point and drop to about 40% at the edge.
I have had a plumber in to investigate upstairs with the floorboards up. He noted some central heating pipes but due to the positioning and no sign of a leak, discounted them as the cause. Also, the damp patch increased during the summer when the heating was off.
He suggested the outer wall being the likely suspect, especially where our chalet style roof meets the wall and there qas some cracks in the mortar. Also we have a large and unruly wisteria that was possibly a factor.
So, I have reduced and tied the wisteria back out if the way and had a builder repair the mortar and apply sealant outside. He's also adjusted and unblocked the gutters. This was in September. Here's the thing. Since then the damp patch has not increased (I drew round it carefully with a pencil). But the damp meter reading is exactly the same. I have used a mini dehumidifier, and tried heating the ceiling under the damp patch with the flooring up in case it was causing condensation.
It's as if the plasterboard is somehow retaining damp against the laws of physics, but without actually getting any damper.
Anyone else have any ideas... I'd be so grateful!

 

[Joka1965]

 

[opps]

Damp meters are great for timber but incredibly unreliable for plaster. When plaster gets wet, the salts do something weird and will often continue to trick damp meters.

You could tape some clear polythene over the area and see if you get any signs of moisture between the ceiling and top of the poly.

Alternatively, dab part of it with powder based wallpaper paste. If the area is damp, the paste will be tacky days later.

 

[Joka1965]

Thanks for your reply. I'd love to believe that the damp meter is inaccurate, but the patch does still feel pretty cold and damp to the touch. I'm prepared and would love to believe it's slowly drying out, but to be honest it's an act of faith! I'll try what you suggested though. Thanks again.

 

[opps]

Thanks for your reply. I'd love to believe that the damp meter is inaccurate, but the patch does still feel pretty cold and damp to the touch. I'm prepared and would love to believe it's slowly drying out, but to be honest it's an act of faith! I'll try what you suggested though. Thanks again.

If you have insulation above, that may well still be wet. You could drill a series of small holes to aid air flow.

On the upside, the stain has not grown.

 

[Joka1965]

Huge thanks!

 

[foxhole]

Exterior pics

 

[Joka1965]

 

[Joka1965]

The mortar on the tiles was cracked and has been replaced, all minor cracks in bricks filled and a coat of waterproof sealer applied.

 

[Harry Bloomfield]

But the damp meter reading is exactly the same. I have used a mini dehumidifier, and tried heating the ceiling under the damp patch with the flooring up in case it was causing condensation.

There is your mistake... A damp meter, works by measuring electrical conductivity/resistance. Some materials, like plaster, once they get damp, remain conductive even once they are dry. A much better way is to simply feel the material, and if it feels damp/ feels cooler than it's surrounds, it is still damp.

 

[Joka1965]

Thanks for the advice. I'm learning!

 

[ree]

OP,
As you have found out for yourself: Damp meters work - they meter damp.
They dont provide solutions - common sense & experience do that. You sorted that bit yourself OP, great.

The outside pic shows that a fillet of sand & cement with cuts of tile is acting as an abutment flashing.
Thats wrong. Installing that S&C fillet is wrong.
So is "repairing the mortar and applying sealant" - it will do nothing except relieve you of your money.
All the fillet should be removed - then why not lift the tiles next to the wall & post pics on here of what you've exposed below the tiles? Damp penetration at the abutment might be shown?
Lead sheet: stepped cover flashing, & abutment flashing over the profiled tiles is needed.

Pics showing a larger context of the roof might help.
Have you been inside the extension roof to look for damp signs present or historic?

 

[foxhole]

The down pipe into gutter arrangement is going to soak the wall with any heavy rain , need extending away from wall and rerouting .

 

[Funday00!]

We had similar. It was the guttering that caused the problem. Not obvious, but it looks badly placed. Over time, the water penatrates.
No need to put any barrier on the wall as all walls need to 'breath'. It is most certainly the gutters misplaced. check all along the building. Also,make sure no leaves blocking.
Is it a pitched roof in the photo? I can't see lead flashing?

 

[Lower]

That guttering arrangement has damp problems written all over it.