DIY Doctor

Damp Lath and General Damp Query

Postby PJMe » Sun Jan 12, 2020 11:28 am

Hi everyone this is my first post! I hope someone can confirm I’m on the right track with my damp issue! Sorry for the lengthy explanation!

1930s house with most damp in the top corner of the upstairs bedroom chimney but seems to come down to first floor too. There was polystyrene liner on the walls hiding the issue. I have stripped the plaster off the walls and a small area off the upstairs ceiling where it was loose.

The damp is affecting one side of a double chimney; the side which serves the living room below i.e. likely to be the regularly used fire so thinking about soot contaminants have got into the bricks/mortar etc. While I appreciate its not the most accurate the damp meter shows increased moisture levels after rain and down that side of the entire chimney.

I have fully repaired the very poor condition external chimney. Repointed, new flashing, pot covers, reflaunched and damaged roof tiles replaced. There is no sign of rain coming into the loft space. Although I appreciate tracking can be tricky to identify. I have newspaper all over the roof space (resting and stuffed between beams) and it appears to stay dry.

I’ve also repointed the external wall of the house, following the path of the downstairs chimney. The mortar was much softer than the rest. And I've repointed the chimney in the loft; again the mortar was soft and damp.

I’ve had the heating on and a fan heater blowing up the chimney for some time and felt it was all drying out then the rain leads to the damp meter readings going back up.

As I understand it the last resort is to paint the chimney and maybe down the external wall with silicone base water seal as it seems likely the bricks are porous (likely from salts from soot). However the temp needs to be above 10 degrees so I’ll have to wait.

I am intending to internally insulate the solid walls and don't want to progress until I am happy the damp won't build up and become a hidden issue. I knew it was going to rain earlier this week. So I turned off the heating to stop any drying out (mimicking the internal wall insulation stopping heat get to the internal face of the wall) and see how damp the area became.

The photo below shows moisture after rain on an exposed lath upstairs. The lath was completely isolated from the chimney masonry (closer lath to the left had been removed already) and was protected from any roof leaks above (no evidence of leaks) by a board. It seems especially strange the visible damp on the lath is worse at its centre length which is furthest away from the external external wall and is damper on it edges rather than its centre. Could this lath, and others around the damp chimney, have taken in salts when long-term damp travelled from the chimney along the ceiling plaster and the lath where the plaster 'squeezed' through?

Any other thoughts/suggestions would also be welcome.
Thanks in advance
Attachments
IMG_20200107_194439993 - Copy.jpg
From loft looking down with chimney on left and external wall at top of photo
PJMe
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