Snow load on gable roofs

[ajk1]

According to the National Building Code of Canada User's Guide, "Snow loads on a sloping surface act on the horizontal projection of the surface".

Does this mean that if I have a gable roof sloping at say 25 degrees (i.e, < the 30 degree minimum slope where snow load can start to be reduced due to sliding), that the "total" (i.e. pounds) snow load on a sloping roof is the same as the "total" snow load on a horizontal roof of length equal to the horizontal projection of the sloping roof?

i.e. a sloping roof of 10 foot sloping length and a flat roof of 10 cos 25 = 9.06 foot horizontal length have the same weight of snow on them?

This seems strange, because if thesame depth of snow accumulates on the flat and the sloping roof, then I would expect the sloped roof to have the greater volume and weight of snow. What is wrong with my logic?

 

[woodman88]

If you have a 5'x5' hole (with 3' wall around it) will more snow fall into the hole if the floor is sloped (say at 25 degrees) rather than being flat?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jimstructures]

Try this thought experiment.

Imagine a uniform load of snow on the roof. Divide it into very small columns of Snow with no shear strength across the columns.

Now allow the roof slope to become a flat roof.

You will now have the calculated uniform snow load on the flat projection of the sloped roof. You do not multiply the sloped length of the roof times the uniform depth to come up with the volume of snow. You take the horizontal length of the slope and multiply it by the uniform depth. That is the volume of snow you use to calculated the load. Keep in mind that gravity acts straight down on the horizontal projection of the roof. This allows you to simplify the problem.

Hope this helps.

Jim H

 

[oldestguy]

You need to look up the word "projection", generally as used for drafting.

 

[ajk1]

The well analogy and the columns of vertical snow both help me to understand it. Thanks very much. Much appreciated.