My house is more or less on the top of a hill and we are in an area prone to thunderstorms. The house is not the highest object on the hill as there are trees within a hundred metres.
My question is is it safe (for the house) to mount a pole on the chimney under the circumstances I have described? The chimney is brick construction and the rest of the house is timber with a metal roof. If I do this should the mast be earthed or not?
I have puzzled over this for a while and would really appreciate the thoughts/experiences of the forum.
One hundred meters is far enough away from your house that I certainly wouldn't consider those trees to be adequate lightning rods. Of course, you certainly don't want trees too close to your house either.
The house I grew up in for 13 years was on a similar hill. We had an underground well about ten feet from a huge tree and even less from a metal clothesline, not far from our house. The taller tree was never hit, but the well was hit at least twice (one of which I think I witnessed from my school bus about a half mile away).
I agree that brick is not normally a good conductor, but keep in mind that the water that might cover them could be. And if you have any wires connecting the mast to the house itself (rather than mounting a transmitter to your chimney), I'd be even more concerned.
All that being said, I also believe that grounding may actually increase the chances of being hit, but if it happens, you're MUCH less likely to have a fire as a result. Proper grounding is a must.
Having any kind of tall object attached to my house would make me nervous. Others will probably disagree, but if you can find any other location that isn't shielded by your house, I'd strongly consider it. You're not going to get official-quality measurements at your residence anyway (nobody does if there are trees around). The fact that you're on a hill already gives you better wind measurements than most.