Damp V on London valley parapit wall
Hi, I have a damp v appearing on the front of my front wall - (mirroring in front of but going below the London valley roof behind it) The wall has replaced coping stones and the rear re-rendered so I don’t think it’s damp getting behind that.. any ideas?
Was wondering if it could be something to do with condensation?
Hi, thanks for the post.
Where is the internal damp?
Is there an accompanying tea bag stain like discolouration?
London roofs are notorious for condensation as there is poor insulation under the valley, causing heat (thermal bridging). I've have been to properties were water drips off the underside when it is cold and dry, but not when it is warm and wet outside.
Typically the condensation results from high humidity from a poorly ventilated bathroom or drying clothes inside.
If the issues is penetrating damp, there should a tea bag like discolouration.
However, interstitial condensation, that is condensation forming under the valley, but above the ceiling, result in the same discolouration.
Measuring humidity with two inexpensive dataloggers (currently £16 Amazon) will help you track down the source of vapour.
See https://surveyor.tips/datalogger and https://surveyor.tips/laser-thermometer . Armed with the temperature of the damp patch and the relative humidity and temperature elsewhere in the room, you can work out the surface relative humidity using the calculator at mouldpoint.co.uk.
I hope this has been useful. I would need to see more.