Roof Snow Load Calculator
Calculate the weight of snow on your roof and check if it exceeds safe load limits.
Know when to remove snow before structural damage occurs.
Snow Load Is a Serious Structural Concern Every region with meaningful snowfall has building codes that require roofs to be designed for a specific snow load, measured in pounds per square foot (PSF). A structural failure from snow overload can happen quickly and without much warning. Understanding the weight of snow on your roof can save lives and property.
Snow Density Varies Enormously Not all snow weighs the same. Fresh, light fluffy snow can weigh as little as 1–3 PSF per foot of depth — the kind you can blow off a deck with a leaf blower. Packed snow, which has had time to settle and compress, weighs 15–25 PSF per foot. Wet, heavy snow — the kind that falls when temperatures hover near freezing — weighs 40+ PSF per foot and is the most dangerous. Ice formed from rain or melt-refreeze cycles weighs about 57 PSF per foot, nearly 20 times heavier than light fresh snow.
Typical Roof Load Limits Most residential roofs in snow-prone US regions are designed for 20–40 PSF of snow load (called the roof snow load or RSL). This corresponds to a 25–55 PSF ground snow load, since roofs shed some snow and receive less accumulation than open ground due to wind and heat loss from the building below. When the actual snow load approaches or exceeds the design load, roof failure becomes possible.
The Great Northeast Blizzard of 1978 The Blizzard of 1978 caused catastrophic roof collapses across New England and the Midwest. Measured snow loads exceeded 80 PSF in some areas — more than double the design capacity of many buildings of that era. Hundreds of roofs failed, and the disaster led to significant updates in snow load requirements in the model building codes.
Slope Factor Steeper roofs shed snow more readily. The ASCE 7 standard (Minimum Design Loads for Buildings) applies slope reduction factors: roofs steeper than about 5:12 lose some of their snow load to sliding. Above 70°, almost all snow slides off. However, this creates hazards at the eaves — avalanching snow can injure people or damage property below.
Warning Signs If you hear unusual cracking or creaking, notice doors sticking that normally open easily, or see visible sagging in rafters or ridge, evacuate and call a structural engineer immediately. Remove snow carefully from the edge inward using a roof rake — never stand on a loaded roof.