The Norwegian Gouvernmental Building Service has an interesting pamphlet realted to your question, advicing on snowloads and dimensioning and allowable loads.
Titel translates roughly as 'When to shovel off (snowloads) and when to run'. (Regrettably available only in Norwegian)
Necessity to evaluate snowloads origins partly from a periode in the seventies with too low and lax dimensioning rules in Norway, and some serious breakdowns especially for 'hall type' constructions.
Resume on some points:
Snowload on buildings should be taken as the 50 year max.
Depending on municipal climate this is now set from 150kg/m2 to 350 kg/m2 in Norway (one value for each municipality).
To dimension locally the load one might assume same dimensional data for load on roof as dept measured on ground over the years.
Weight of wet snow is set at 300 kg/m3, dry 150kg/m3.
Simple structures as halls with framework(factories?) is more suspectible to break down on skew loads and less safe on overloading than more firm and solid constructions.
When measuring actual load (for purpose of evaluating breakdown danger or shoveling load off) take uneven distributed load into consideration. Max, minimum and avarage actual load must all three be considered separately and together. Shoveling must be done evenly not to create uneven loads and great flakes of snow to slide off. The last might create damaging vibrations in the bearing construction.
Your dimensioning should hence be in two parts:
One part giving the assumption of maximum load (350 kg will for instance give about 1,2m of wet snow evenly distributed)
The second part should be an instruction on when to shovel off the snowload including measuring and offloading guidelines, if this not already exists locally.
Instructions should include protecional routines for roof covering not to be damaged by shoveling.
