DanG wrote:
Install the rods (all thread works well) in the wood. Bend the
rods as required to insure they are square and true - this should
not be necessary if you use care on the drilled holes. Drill
oversized holes in the masonry, install epoxy screens or
umbrellas, and epoxy. You will need to brace and carry the mantle
while the epoxy sets up. Here is what I use:
http://www.ramset-redhead.com/RH/rh_aas_intro.asp


Your technique sounds like a good and easy to execute plan except for
the all thread. The OP stated the wood is 18" wide. I think the all
thread would bend unless the fireplace had a smooth surface like brick
for the shelf to press against. Maybe some rebar if you can get it as
thin as the mortar joints, still bendable but not as flexible as all
thread. Maybe drilling the hole for the rods near the top of the shelf
so bottom portion could press against the wall preventing the rods from
bending. Overall good plan of attack.

shelf rod
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