You don't need to put a lightning detector antenna on the roof. If you have a non-metallic roof you could mount it in the loft space and it would work just as well. Even if you did mount it on the roof it's no more dangerous than having a TV/radio antenna or satellite dish on the roof. Whilst it's possible to get a lightning strike direct onto an antenna it's quite rare for this to happen. Most of the houses in my town (40-50000 houses) have at least a TV antenna, but I've only ever heard of one fire caused by a lightning strike (which blew the TV up). I once had some computer components destroyed by a nearby lightning strike, but in that case the surge came down the (overhead) phone line.