Rain water coming down cavity over extension

[Stangalang]

Hi all,
Hope this is the correct area to post. We moved into a semi with a 1st floor wraparound kitchen extension with pitched roof against side of house, so two external walls are sat on steels. We noticed staining on the ceiling along the edges of the extension a couple of days after moving in and it was diagnosed as a leaking overflow for the toilet which we had fixed.
We thought we'd cracked it until we had heavy driving rain during storm Ciara which caused water to pool on the surface of the ceiling all along the previous stain marks.
Long story short, I decided to move sections of the ceiling where they were worst affected, we had a roofer around who noticed the lead wrapping on the corner of the wall/roof joint needed replacement as there was a gap, this was done and during storm Dennis this week that area of the ceiling remained dry.
The gutters are finlock, but we had them checked and they are aluminium lined and flowing water correctly, the main roof tiles were lifted in that area and the felt was deemed fine. All leading has been checked on the extension and again, this is fine.
All the pipes travelling through are cavity were also checked and are leak free.....it only happens when it rains heavily.
Builder reckons the wall needs waterproof paint as the bricks have gone porous (it's a SW facing wall).
Extension was originally built in 76 and then extended again in 93 with a tiled roof which I have the plans for both, the back leaking walls are sat on two steels, one under each leaf, should there not be a cavity tray over this?
The two steels mentioned above then sit on a single steel which spans the length of the kitchen, this is full width of both leaves.

I have videos of it happening but can't upload.


Sorry for the ramble....

 

[^woody^]

You need a cavity tray

 

[Stangalang]

You need a cavity tray

How difficult/dear is it to retro fit the trays? This one is above my head (no pun intended).

 

[^woody^]

You would take out sections of wall about 4 bricks long by 3 or 4 courses high and insert the tray or DPC. It's not particulalry difficult, just needs care. There are probably numerous threads on here about the process.

A waterproofing coating (siloxane liquid not paint) is a potential remedy, but its not guaranteed and will need a proper assessment first to determine if suitable.