Skylights - Do they flex to conform to radius/slope of roof?

I discovered that the skylight on my fw was cracked near one of the screws. The company supplied a new skylight and sealant and paid for the labor at my local rv repair shop. The folks there that did the replacement suggested that I add a metal frame because there was none. I went ahead and paid the extra few bucks for the frame and I think it is more secure.