On 26/07/2020 15:22, Chris Hogg wrote:
On Sun, 26 Jul 2020 13:42:50 -0000 (UTC), Jethro_uk
wrote:

On Sun, 26 Jul 2020 13:35:58 +0100, Dave Plowman (News) wrote:

Recent rain has shown my flat roof is leaking. It's part of a conversion
built about 30 years ago. Very nicely built - the correct lie etc and
marine ply decking, and lead flashing. Think it is three layer. Size
approx 5x5 metres.

Any of the modern alternatives better? Membrane or fibreglass? And a
rough idea of relative costs would be nice too.

Access is easy, but it's never walked on etc except for any maintenance.

I find it mildly curious that in the 21st century, no one has seen fit to
come up with something that can be poured, finds the crack and seals
it ...

I have done just that with Isoflex liquid rubber. Leaked at the
junction between my and my neighbour's garage roofs, bitumen felted.
Must have been quite a big crack as it took quite a lot, and at one
point appeared dribbling down the inside of the garage wall. Sealed
the leak though. https://tinyurl.com/y2fgfmzo

Joints in flat roofs are almost guaranteed to fail, especially
if both were originally built-up 3 -layer felt and one half has
been replaced with 2-layer torch-on felt. The heat of the sun,
and the dryness, especially the weather we have had this year will
soon make the joint fail.

IS the decking chipboard, plywood or OSB ?
The first will not like ingress of water.

If you are going to treat flat areas their special primer is well
worth using first.

I did my garage roof with an EPDM kit, but that involved redecking
with OSB3 first. 5.5 mtres by 2.8 metres. It was just on the limit
of my ability to get it up the ladder. A 5x5 sheet of epdm will need
2 people.

Flat roofs need replacing at least every 20-25 years. Some really
bad bodge jobs, like the garage roof of my first house was done in
2-layer felt in 1976, before the days of fibreglass reinforced
felt and every house had a leaking garage roof within 10 years.