RB wrote:

To me a chimney cap is a concrete and reinforcing wire mesh placed on
the top surface of a chimney. Among other things this serves to keep
water from getting between the bricks that the chimney is made of and
then freezing and damaging the chimney.

snipped

That must be a local or archaic term. But it does describe it pretty well.
"Frosting" just wouldn't sound right.

However, Googling up "Chimney Cap" gets mostly stuff which looks like this:

http://www.dukefire.com/caps.htm

Which is probably what the OP's after.

Happy Holidays,

Jeff

--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to
place the blame on."