flat roof leak cavity closers

[dozydave2]

hi all,
I have detached house built 1972 in very exposed seaside area.
In high winds the rain literally comes at us sideways and up.
We have built a new flat roof extension (as the last one leaked).
After new roof it still leaks?
We had cavity wall insulation (good idea at time!) 4 years ago.
it appears that the rain driven in under tiles then runs down into cavity and tracks across until it comes out through render in the new extension.
The felt under the tiles is ripped so we are going to replace felt and retile first 4 x rows of tiles.(when weather allows).
Should we close the cavity at roof height? or as there is no cavity closer above the flat roof can one be cut into block work like a flashing?
Any help or suggestions grateful.

[welsh brickie]

I think you need to install a damp tray if water is entering the cavity.Damp proof course is laid on the internal course of brickwork or and then laid on the outer course 2 bricks below forming a slanted piece of plastic to divert the damp to the outer skin.Small plastic vents are then inserted inbetween the bricks to allow water to escape.