We have codes and they are very strictly enforced. So much so that most folks just go the route of using engineered roof trusses to leave the figuring to the engineers. Our trusses are engineered to hold 90 lbs/sq ft.
If you shovel a roof correctly, there is not that great a risk of injury. Sure folks fall off and get hurt, but I suspect they are not paying close enough attention to what they are doing, as I have been up on plenty of roofs and never once had a close call. Many times after the first shoveling the snow is only about 3-4 feet shy of the eaves anyway, so not much to get hurt on after that. I now have snow right up to the eaves of my wood shed, which is the only roof that I really worry about, as it does not use engineered trusses, just my handy work!
My point about roof loads was that with a properly designed and built roof, the load exerted in one particular spot is shared with the entire roofing system. Sure the load will decrease the further you get from the actual point of load, but my weight being on the roof is not confined to just where I am standing, it is distributed throughout the entire roof. So it really is not adding a bunch more weight to the spot I am standing on.
Now, let 4-5 feet of super dense snow pile up on that same roof and the point load is actually being carried by the structure directly beneath it, as you have the same amount of load on the entire roof. The roof structure over the house would probably hold 4-5 feet without problem, but why chance it. Like I say, if you are mindful and careful, roof shoveling is not as dangerous as it is made out to be.
As for metal roofs self clearing. It is true, but only when you have enough of a warm up to break the bond between the roof and the snow. This winter has been a highly unusual one in that we have had almost no thaws and those we have had have typically not been long enough to cause the roofs to slide. In the other years I have been living with the metal roofs, they have self cleared.
Also, when we get to this time of the year, there is the risk of rain falling onto the snow and significantly adding to the weight, so many folks like to get the roofs fairly well cleared now, so that any rains to not cause problems. That is why I did my wood shed last week.
-John
