Decks are required by code to support their own weight, plus at least 40 pounds per square foot (psf) in additional loads – from people, pets, furniture, and snow and ice. Many builders go to a higher standard, and frame their decks to support at least 60 psf – more than the design snow load for roofs in most states (you can find the ground snow load in your area by viewing this interactive ASCE-7 snow load map. )

While the weight of snow can vary a lot, depending on how wet it is, the ‘design weight’ used by engineers is 69 psf, while the actual weight is estimated to range between 36 to 54 psf. For Boston deck builder Jim Finlay, this translates into a solid recommendation: Don't start worrying about snow until in drifts over the top of 36-inch-tall railings.  Read more.