I would not advise using the chimney at all. I once had a Mini-Quad on a short mast attached to a brick chimney with chimnely straps. It was a steeply pitched roof and relatively shielded from the prevailing winds. It still nearly brought the chimney down. 73, Don, K2DC
I second Don's concerns. If the chimney is the best location after evaluating all
1. consider mounting the yagi/hex beam only a few feet above the rotator and the rotator only a few feet about the top of the chimney. Your design of 12 feet above the chimney top would increase low-angle radiation for most bands, BUT without guying this structure it would lead to serious problems with wind/ice/vibration.
2. consider a high-performance mini-yagi such as the new series of 2-element N6BT models (http://www.n6bt.com
) OR the mini-hexbeam from http://www.k5bob.com/
3. finally, there are several trap dipoles (or make your own as I did) that could give you 5 band operation and only require a mini-rotator.
Mounting any antenna, especially one that may be hard to handle atop a steep pitched roof isn't recommended. The Rohn tower along side the house may be the better route.
GL, 73, Rich, K3VAT