Help needed with replacing coping stones on a shared firewall with roof flashing, dpc etc

[MiltonPFC]

The recent heavy rain has highlighted a leak for both my neighbour and I in our respective south facing conservatory roofs, which are divided by a brick firewall which is covered in what looks to be very old concrete paving slabs (see photos).

In each case, for both my neighbour and I, the indoor vertical wall underneath the firewall is wet.

The firewall / roof flashing seems okay, along with the roof tiles and membrane, and so I believe that the leak is caused by cracks in the cement under the concrete paving slabs on the firewall, and a lack of dedicated coping stones (to protect the cement from the brunt of any bad weather)

** Update : Zooming in on the pics it looks like the existing DPC could be the route cause of the cracks ? **

i) Looking at the two photos, do you agree?

ii) If I wanted to replace these old slabs with proper coping stones, should I affix them using a cement mix with external PVA added to help waterproof ?

iii) Do I need to consider a DPC ? - or can I just lay 2 - 3cm of cement on top of the existing flashing (after removing the old slabs and cement) and then attach new Coping stones on that?

All guidance / suggestions very welcome

Many thanks in advance,

 

[MiltonPFC]

Any thoughts gents ? Cheers

 

[^woody^]

It needs a DPC, there is one there now. Other than that you can just relay the existing stones and maybe the DPC too, and it will do for another 40 years or so.

 

[MiltonPFC]

Thanks Woody, appreciated

So would a 2cm layer of cement on top of the flashing, then covered with a new DPC, then another 2cm layer of cement and new coping stones, be the ideal ?

Also would you recommend treating the bricks under the flashing with any kind of sealant ?

Sorry for the daft questions but I have only ever done light re-pointing work before

Thanks again