Del Voight - Penn State Extension
Snow loads are important to consider during the winter. In most cases the structure engineering should easily handle the small snow falls that we get in the Lebanon area. However, if the snow drifts to certain parts of the roof this is where more than the 4 foot or 4 inch of ice can build and overwhelm the structure of the building.
Check periodically to determine where the snow is beginning to build up.
If you do venture on the roof to clean it off be sure to tether yourself to secure point with rope so you are safe in cleaning it off.
I am not a structural engineer however just checking into some engineering books I found some references that basically depends on the building codes in the area for snow loads. Typical light fluffy snow can wiegh about 20lbs but a heavy snow can wieght as much as -60 lbs of load per cubic foot for most common structures.
It appears that it is not any easy answer unbalanced loads cause issue as well as the slope and design not to mention the age of the structure and if there are other items under the roof in storage adding wieght. Cornell Snow Load calculation accounts for roof type and pitch and other factors. Snow load estimate.
It is important to prevent snow dams that allow water to freeze and dam up on the roof which can be common on L or T structures. Further there are specialized roof rakes to assist in reaching dangerous areas and are worth the purchase versus injury.
Before doing anything consult a local engineer and or your local township building inspector and ensure that removal is the right course of action. Remember shingles get brittle and metal roofs are very slippery so safety first when dealing with this issue.