Hi, I have an old prefab concrete garage, with a semi corrugated roof,
possibly made of asbestos.

The original guttering was metal and held in place by metal brackets
which were curved under the gutter and held in place by the bolts that
also held the roof section on. Most of this has all long since rusted
away.

my problem is I'd like to put new gutters on, but the prefab concrete
sections are impenetrable. This things are really strong, my hammer
drill with a new masonry bit doesn't even leave a scratch, so modern
conventional plastic gutter brackets would not seem a viable option as
they require screwing onto a vertical surface. has anyone seen or
improvised a solution of this kind of thing ? I guess some kind of
metal bracket which would slide between the wall and roof