Snow Load Calculator

Snow load is a gravity pressure, and the arithmetic is depth times density. One foot of settled snow at 15 pounds per cubic foot imposes 15 pounds per square foot on the surface beneath it. Density is the variable that surprises people: fresh dry snow runs about 5 to 10 pcf, settled snow 10 to 20, wind-packed or aged snow 20 to 30, and saturated or ice-crusted snow 40 pcf and above, so two roofs under identical depth can differ threefold in load. Slope matters because steep, slippery, unobstructed surfaces shed. Design standards apply a slope factor that begins reducing load somewhere between about 15 and 30 degrees depending on surface slipperiness and whether the roof is warm or cold, and this page uses a comparable reduction starting near 15 degrees. Typical design snow loads run 20 to 30 psf across much of the northern United States and 50 to 100 psf or more in mountain and lake-effect regions, while mild climates may be governed by the 20 psf minimum roof live load instead. Three limits apply here. This is a measured-depth estimate, not a code design load, which is derived from a mapped ground snow value adjusted by exposure, thermal, and importance factors. It reports a uniform load, whereas failures usually begin at drifts against walls, dormers, and lower roofs where accumulation can be two or three times the balanced value. And rain falling onto existing snow can add several psf within hours.

Snow was not treated quantitatively in building rules until the twentieth century. Early American codes prescribed a flat roof load, often 20 to 40 psf, with no reference to local climate. Systematic mapping followed the creation of a national minimum-load standard: the document first published in 1945 as American Standard A58.1 gathered dead, live, wind, and snow requirements in one place, and later editions introduced ground snow load maps built from National Weather Service station records rather than from custom. The American Society of Civil Engineers assumed responsibility for the standard in the 1980s, and it has appeared as ASCE 7 since. Successive editions added the machinery now considered basic: conversion from ground to roof snow through exposure, thermal, and importance factors, explicit drift surcharges at roof steps and projections, unbalanced cases for gable and curved roofs, sliding snow onto lower roofs, and a rain-on-snow surcharge. Much of that detail was added in response to investigated collapses, including widespread failures during the heavy eastern United States storms of the winter of 1995 to 1996, which demonstrated that roofs rarely fail under uniform snow but frequently fail where drifting concentrates it. More recent editions have moved snow toward reliability-targeted values and defined case-study regions where mountainous terrain makes any single mapped number meaningless.