Cavity wall leak.

ollieu1900

the flashing is pvc melted together like the roof ifself, common on a flat roof apparently

Grenage

Is that a decked area on the roof of the extension?  Can you lift the boards to look beneath?

ollieu1900

Grenage said:

Is that a decked area on the roof of the extension?  Can you lift the boards to look beneath?

I've actually down a few experiments today, I lifted some of the decking, removed all debris and poured close to 6 gallons of water on the flat roof and no leak. 

I removed some of the sealant and poured water on the flashing and the wall itself and used a hair dryer to mimic driving rain and within a few minutes the ceiling was wet!

So now I'm wondering if it's the flashing leaking or is water moving through the wall and running down the back (where there are no cavity trays) 

Any advice? 

stuart45

There's a very similar property here.
Opinions on stormdry/masonry cream — MoneySavingExpert Forum

Grenage

It looks like the same house!