There’s some disagreement about this subject here in Colorado.
Three years ago we had a storm that dumped 8 feet (that’s measured feet, we had about 5 feet on the ground). Snow sliding off metal roofs ripped signs off of commercial buildings and deck handrails off homes with roof designs that didn’t route slides above the rail.
I had a disussion yesterday with a broker who is dead set against them. His argument was that the big advantage to expensive metal roofs is that they shed snow easily. Preventers defeats the whole purpose. He went on to point out that snow will slide eventually anyway and preventers just force roof accumulation to build up to unsafe levels before it finally slides. These seem like a valid points, at least under some circumstances.
Seems to me it might depend on site-specific conditions. The roof pitch, temperature of the roof and temperature and nature (stickiness) of the snow all should effect the level of accumulation that will build up before the snow slides.
A shallow pitch with no preventers might be very dangerous if wet snow fell on a frozen metal roof. It could build up to unsafe levels before the roof warmed up and it broke loose. A shallow pitch with preventers might not shed snow at all and then why pay all that money for a metal roof?
A steep roof usually sheds snow at fairly low (safe) levels of accumulation, so no preventers seems better in that situation.