I would not go up on a roof if there was frost on it. I did once try to get on a metal roof was wet. It was a pretty flat slope, but I still was not able to get up on it, had one foot slip. Luckily the other foot was still on the ladder. Another time I got caught in a rainstorm when up on a metal roof, now that will make you pucker; the worse thing was that the ladder was on the other side, and that it was done. That was a scarry one to get off of, you had to walk from screw to screw. I was a lot younger then.