I'm fitting a woodstove in the dining-room, and want to fit a mantle
above the opening.

I've got a chunk of elm (about 60mm by 1540 by 300) and I want to
attach it floating (i.e. bracklet-less).

So far I've used 4 expansion bolts to fasten threaded rod (M10) into
the stone behind the mantle-shelf; these are rock solid.

My plan was to drill (with a pillar-drill) into the elm, ending up
about 30mm short of the outer edge, then slot the shelf onto the
supports.

I was thinking of drilling 10mm holes in the shelf, then whacking the
shelf on, but would I be better with 12mm, and podging car-body filler
in around the rods?

Haven't drilled the holes yet, but just resting the shelf on the rods
and it "bounces" - it
won't fall down, but there is movement in it.

So, any ideas on how to improve this without any visible supports? It
won't be holding a huge weight other than the shelf itself (vase,
pictures etc.), but I'd rather it didn't flex!

The rods are being plastered around tomorrow, but I could remove them
and re-drill with more substantial bolts, and M12 rod?

thanks,
Nathan