Q: “…will a fire explode out of the sides of the house rather than through the roof when a metal roof is on a home?”
A: Good question. Most asphalt and fiberglass shingles get a Class A fire rating. Metal while it is extremely fire resistant to small burning embers that may land on it, still has a melting point. To obtain Class A, many metal roofs need a special fire resistant underlayment. My point of this is that metal is resistant to outside fires but, if there is a major fire originating inside the home, there’s a good chance it could actually melt through the metal roof faster than it would burn through asphalt shingles.
Additionally, fire fighters are trained on how to cut through a metal roof for venting an internal fire.
Now, all that said, there have been some concerns raised over the years when metal is installed over old wood shakes. The old shakes become like kindling as they age and there has been concern that an attic fire could spread quickly across the old shakes, faster than it would melt through the metal. This has not been a problem in my experience. I have heard only a few anecdotal stories about it in all my years and you never know what all factors were involved in anecdotal stories.
The melting point of aluminum is about 500 degrees lower than steel so, if someone has a concern and wants to install metal over their old wood shakes, they may feel more comfortable with a product like aluminum that has a lower melting temperature.
I hope this helps some. The bottom line is … metal has been used successfully on homes in the US for over 100 years at this point and, for the reasons stated above, there have not been problems with it containing home fires.