leaking garage roof - best approach?

depadz

Have a flat roof garage which has been repaired before with a kind of rubbery material (before my time so not sure what/when). It seems the repair is failing and water is getting into the garage - i'd describe it as seeping in rather than leaking in so hard to pinpoint actual point(s) of failure.

What would be the best product to put over the existing repair - would this do the job?

The moss type stuff has been scraped off. Would it be advisable to treat it with some moss clear beforehand?

https://www.screwfix.ie/p/bostik-flexacryl-roof-repair-compound-grey-5kg/46079

depadz

In case any use to anyone else, used the above Bostik product and so far so good after a fairly wet few weeks