On Sat, 03 Oct 2020 22:12:15 +0100, Pamela wrote:

A short wall dividing two flat roofs absorbs rainwater, which leaks from
the brickwork into the roofspace below. The top and sides of the wall
were treated with bitumen paint which worked for a few years before
water got in again. This is the wall:

https://i.postimg.cc/sx1Q95B7/roof-wall-20201003-A.jpg

Remove railings, cap stones and first course of bricks. Cover exposed
top of wall with sheet lead that comes down the wall and over the top
the roof covering fillet, both sides. Relay new clean bricks and cap
stones, replace railings.

As a bit of lead to cover that width of wall and down the sides won't
be cheap you could probably get away with just lead flashing along
the edges and DMP immediatly above it. Ensure the lead and DPM have a
slight fall to the outside. The wall top covering needs to be
contiguos to stop water penetrating the wall. If there has to be a
joint the edges along the joint need to have a vertial bit of about
an inch and an inverted U cap over the top.

"Sealants" will just fail, again. Properly done lead/DPM will cure
the problem for good.

--
Cheers
Dave.