"Swingman" wrote:

BEWA Mantles end up being the resting place of expensive "objects
d'art"
(if not for this resident, perhaps for the next) and because of this
fact
you may well be doing yourself a favor to "over engineer" ... you do
NOT
want any unpleasant call backs from "friends".

What material is the wall? This information is important to an
effective
solution.

Self tapping screws can be used into the metal studs, but, depending
upon
the wall material, I would also routinely throw in a few molly bolts
in
between the studs for the wall half of the French cleat.

If I were doing this job, shear load is not going to be a problem;
however, cantilever loading could be.

As someone suggested, the wider the French Cleat, the better. 6"-8"
would be a good starting point, 12" if there is room.

I would attach this cleat using #14, coarse thread, self tapping, S/S,
flat head, sheet metal screws (At least 2-3 per stud) along with
construction adhesive.

Just you're basic belt and suspenders design approach.

Allow the adhesive to cure 10-14 days before using.

Lew