On Sun, 2 Sep 2012 11:49:33 +0100, Phil L wrote:

If I do slab it, how about 1/2" cement board? (I have some, and it's
lighter)

No.
It needs to be heavy duty, if you put anything flimsy up there, the
elements will destroy it in short time or it will end up in next doors
backyard once the first gust of wind licks it

Agreed and take note of the last bit. The 3 x 2 slab on the barn stack
was moved by the wind here and it only has a small over hang for the wind
to gat hold of. It now has some large screws into the stone/brick work
below. The OP's place looks as if it might be a bit exposed to SW'ly
gales coming up the Bristol Channel.

You may need to hire a roofer and/or a cherry picker, failing that,
scaffold.

Scaffold is cheap, I got the impression from some of the other photos
that the ground sloped by the base of the relevant wall so a cherry
picker might not have suitable access and it looked quite a way up as
well, cherry pickers wobble...

Also to easily point the the new slab onto the stack you need all round
access.

--
Cheers
Dave.