The metal will heat up and twist and the OSB will burn.

Obviously if a fire is going that good then a simple sepaeration will only delay it slightly but that slight delay may make all the difference to that side of the roof burning and the metal twisting and collapsing. If the pull down stairs burn thru it would only be minutes difference before the drywall burned thru.

Personaly I always built with a min of 5/8/x drywall between the house and atic from the garage and just about always in the entire garage. That gives things a few more minutes even if it is not an actual fire seperation.

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Ted Menelly
Castle Home Inspection Services
www.inspectmycastle.com