These questions aren t quite so easy to answer because there are several variables.
How much snow on a roof is too much.
How much snow can my roof hold.
According to the national research council of canada a roof should be able to withstand pressure of at least 21 pounds per square foot psf this number can fluctuate based on regional building regulations but any well built roof should be able to sustain this much weight before it gets too stressed.
But as the chart below shows packed snow with ice can easily overload a roof once it reaches about 12 off depth.
It may not just be one heavy snowfall that causes a problem but perhaps a collection of snowstorms.
Driving winds can change the depth of the snow and snow depth on the ground does not necessarily equate to the snow depth on a roof.
A snow rake with a long extension arm is recommended.
Trying to work out whether the snow load is too much for your particular roof requires quite complicated maths.
Remove excess snow and ice.
How much snow a roof can hold depends on the type of snow roofline and material among other considerations.
Most roof designs can handle snow loads of 20 40 lbs per square foot.
The drifts on the roof were 5 feet deep.