Snow Loads on Roofs

Here is a quick primer on snow loads for flat or low slope roofs, and whether or not you should be concerned:
  • Water weighs 5.2 lbs per inch of depth.
  • Fluffy snow, the type we get during very cold temperatures, is the lightest and is about 3% water.  12” of fluffy snow is equivalent to 0.36 inches of water or 1.9 pounds per square foot
  • Wet snow may contain 33% water.  Using the same math, wet snow 1 foot in depth may weigh 20.6 lbs.
Wet snow is common in our area and 12” of snow weighs 20.6 lbs per square foot.
Two feet of wet snow will weigh approximately 41.2 psf.
In NYC, your roof has a capacity of 40 psf for Live Load.   Large retail spaces are afforded a Live Load Reduction  which results in a capacity less than 40 psf….closer to 30 psf.   (For the purposes of this discussion, snow may be considered Live Load, because of its expected duration.)

What you should be doing:
Ensure all drains are clear and able to dispense any accumulated water.  Check often during the snow/rain event to ensure unfettered flow.
Utilize experienced staff and do not overload the area with humans.  One person is enough.
Mid spans are the worst location.  Accumulations of snow and water cause deflections in the roof structure.  Deflections in the roof structure cause the accumulation of water ponding.  In most cases, drains are located at low points.  Move snow away from drains and if you have to pile it, place near the perimeter of the structure.  If you have 8” of water accumulation, the water weight is 40 psf.
Be conscience of damage to the roofing membrane during snow relocation.  Consider plastic edged implements.