fixing slate to walls

[Norman Watts <Norman_Watts@zzzzzzz>]

Hi all,

I want to mount a piece of slate (81 x 81 x 2 cm) onto the wall of a
(wood) house. The slate weighs about 35 kg. I'm thinking of insetting
four metal plates (10 x 10 cm) in the back of the slate, one near each
corner, and fixing them in place with screws set in marine epoxy. I'd
also apply epoxy between each metal plate and the stone itself. With
holes in the metal plates I can then hang the slate on bolts or heavy
screws set into the wall, like a picture.

This application is to be outdoors and in a part of the country with
temperature extremes of summer heat and lows of -35 or so. The wall
faces north so not too severe daily temperature swings. It is also
protected from moisture by an overhang.

My concern is that the epoxy will not deal well with the different
expansion rates of the slate and the metal plates and that the whole
thing will come off. Its not in a location that can hit anyone but it
would still be a shame to have it come loose. I could make holes
through the stone, leaving room for stone expansion and contraction,
but I'd prefer a hidden installation.

I've also thought about putting several pins into the wall (and into
the studs behind) angled upwards, and drilling similarly angled holes
into the back of the slate, and then hanging it off the pins but this
is a bit tricky I think with a 2 cm thick piece of stone.

Does anyone have experience with the epoxy/temperature/expansion issue?
Any better ideas? I know that 3/8 inch "bolts" fixed into stone with
epoxy (for rock climbing) last for many years even when exposed to
water and freeze/thaw cycles. However a plate of metal would have a
greater expansion than a bolt, even if the expansion coefficients were
the same.

thanks

norman