I think if fire is close enough to melt your plastic dome it won't matter what material your observatory roof is made of metal or wood your observatory will be destroyed. What's nice about these new plastic domes is you will be able to replace it four or five times before you equal the cost of the metal ones!
Please visit my Observatory Construction pics at: http://community.webshots.com/album/136917818udlQWZ
And check out this place too!
http://www.observatorycentral.com/
http://cleardarksky.com/c/PtnGrdObAZkey.html
