How do different regions handle snow retention?

For practical purposes we can divide the USA land mass into areas of high, medium and light snow precipitation (North to South). Each has its own way of dealing with snow.  

Northern USA – Heavy Snow Loads

Up North, we want snow off the roof, ASAP. If it freezes in place and snows repeatedly, there is a danger of the roof collapsing under the weight. The ground around a house is usually frozen for the winter months so the foundations are not  vulnerable to snow-melt. The snow can be removed with a large snow-shovel and a couple of miniatures. Better yet, you can watch through the window as a snow-plow whisks it away, and drink something a little more sophisticated. So good snow-retention up North is No Retention. This is expedited by means of a “snow pan”, covering the lowest two or three feet of a slate roof. A snow-pan can be a standing or flat-seam metal roof.  The metal of the pan absorbs radiant heat from the sun, which warms the snow and spills it to the ground. These roofs tend not to have gutters, nor do the houses have surrounding shrubbery. Just a good clear run for a snow plow.

Middle USA – Moderate snow loads

In the medium snow range, with a greater repetition of the freeze-thaw cycle, a snow pile is to be avoided around the foundation wall. Soaking the ground can cause subsidence (the gradual caving in or sinking of an area of land). It’s better kept on the roof and allowed to melt off as water, via gutters and spouting. Unlike the North we don’t know much what to do with it on the ground, anyway. Here, snow-retention comes into play. This is achieved with snow rails, snow boards or snow guards. Anything to trap the snow on the roof. This saves gutters from being torn down, parked cars from being destroyed, and homeowners from being buried while casting about for their newspaper.  

Southern USA – Little to no snow

Virtually no snow retention is installed or needed for that matter.  When snow falls, it is as if the apocalypse has occurred.    

Ice Dams – a roof’s worst enemy

An ice dam is a major problem for roofing systems.  It occurs when water or snow freezes at the eave of a roof.  This causes melting snow to build up behind the “dam” and force water up the roof surface.  This can often be the cause for a leaking roof during the winter season and can generally be avoided with the proper snow retention.  
Interior heat, inside the building envelope, will always escape through the attic to some extent and warm the roof. In addition, whatever radiant heat from the sun is absorbed into the roofing material will help melt the snow. However, at the eave, where melted snow from above begins to pile up, the temperature is cooler-no house heat or radiant heat after nightfall- especially on eaves that extend well past the building envelope. The snow and ice can re-freeze, forming an ice-dam. This is the most damaging snow event, trapping melting snow on the roof and if the dam does slide off, potentially causing great damage.