Hi,

One thing I have not seen in previous replies,

If you use a band clamp to attach the mast to the chimney,
water can get behind the metal band and when it freezes, cause
damage to the brick.

In warm weather, the bricks expand and the metal bands
damage the bricks.

I'm not an engineer but another post recommended to use guy
wires so to reduce windload off the chimney.

I don't believe this will reduce windload at all, the guys will
be attached to the mast only and not the actual antenna so
no reduction in windload.

Best to avoid using the chimney and attach the mast to the house
at the eave using long bolts and nuts attached to some pressure treated
(and painted) 2x4's on outside and inside the attic. stronger and
you don't have to worry about the coax getting too hot from the heat
of the chimney exhaust.

mick