How Much Snow Can a Roof Hold? Snow Weight, NJ Design Loads and the Warning Signs

No figure in pounds or inches applies to every house. The roof side of the answer is the framing, its age and condition, the pitch and where the wind puts the snow. The snow side moves just as much, because density changes with temperature, rain and time on the roof.

The risk climbs when conditions stack up: heavy wet snow, rain on the snowpack, drifts against a taller wall, and a flat roof with blocked drains. How often deep snow days arrive is covered in how much snow North Jersey gets in a typical winter; this post is about what the pile weighs.

Depth tells you little; density decides. FEMA puts a foot of fresh snow at 3 pounds per square foot when light and dry and 21 when wet and heavy, and notes East Coast snow tends to be wetter and denser than western snow. NDSU Extension gives it by volume: about 7 pounds per cubic foot for light, fluffy snow, about 15 for average snow, and 20 or more for drifted, compacted snow. The two sources agree at the heavy end and differ at the light end.

Snow does not stay the weight it fell at. FEMA notes that roof snow in sunlight can soften, become denser and then harden when the temperature drops below freezing, and that meltwater can pool and refreeze. Ice is heavier still: a little less than 5 pounds per square foot for each inch, and about 57 for a foot.

Take a 1,000 square foot section of roof. Eight inches of light, dry snow at FEMA's figure is roughly 2,000 pounds; eight inches of wet, heavy snow is about 14,000. A one-inch glaze of ice adds just under 5,000. That is our arithmetic from FEMA's figures for an even layer, not a prediction for your roof: a pitched roof sheds some, and drifts pile more in other places.

Codes start from a ground snow load, the weight of snow on the ground. FEMA explains that the mapped values carry a 2 percent chance of being equaled or exceeded in any given year, so they are a design level, not a ceiling. A designer converts that to a roof snow load using wind exposure, slope, shape, obstructions, use and whether the building is heated; FEMA says wind exposure has the largest effect, and that assuming the roof carries what the ground does is incorrect.

New Jersey does not print a county-by-county number for Bergen, Essex or Hudson. The Department of Community Affairs' Bulletin 19-1 (issued September 2019, revised September 2022) sends designers to the Applied Technology Council or ASCE hazard tool, entered with a street address or coordinates, because a city or ZIP code returns the center of that area and could underestimate the load. Withdrawn 1994 values survive only for a result marked "case study" in six counties, Passaic among them at 40 pounds per square foot; every other county, Bergen, Essex and Hudson included, should generate a number. It is site-specific and can depend on elevation, so the number has to come from your own address. If the tool does not answer, the bulletin says to contact the local construction office.

A design load is not the weight at which a roof fails, and it says nothing about a 60-year-old roof with split rafters. FEMA notes that wood members can split or rot, that drifting loads first entered the BOCA code in 1975, and that an older building may not have been designed for snow loads as they are understood today.

FEMA calls unbalanced snow, deeper in some spots than others, a greater risk to the structure than an even layer. Flat and low-slope roofs hold snow instead of shedding it, though FEMA reports that roofs pitched at only 10 degrees have been observed to shed, and that asphalt shingles hold snow more than metal panels or single-ply membranes. What turns a flat roof from fine to risky is water: meltwater that cannot reach a blocked drain pools, which FEMA calls ponding, a concentrated load. Codes allow for light rain on snow but not heavy rain, and FEMA says a short rain can saturate the snow and add significant weight.

FEMA's checklist before the season and between storms is to make sure drains, gutters and downspouts are clear. Our flat roof page covers how those roofs are built and repaired, and commercial roofing covers larger low-slope buildings. On a pitched house the load collects in predictable places:

FEMA says an overstressed roof deck or framing typically shows warning signs before it fails, and its instruction when you see them is to evacuate promptly and have a qualified design professional, such as a Professional Engineer, inspect the structure. If any of these appear while the snow is still up there, get everyone out first and make the calls from outside.

FEMA's list includes severe roof leaks. A drip during a snowy stretch can be an ice dam or flashing problem a roofer traces, but a leak together with any other sign below means leave first and diagnose later:

FEMA states that removing roof snow is more often hazardous than beneficial. OSHA warns that snow-covered roofs can hide skylights people fall through, and that layers of ice form as the temperature drops. NDSU Extension says that unless an engineer or building official recommends it, the decision to shovel off a roof is an educated guess.

FEMA calls removal prudent when the load is close to what the roof can carry, or somewhat close with more snow forecast. Its method:

Icicles come from the same meltwater that builds an ice dam. University of Minnesota Extension names the conditions: snow on the roof, a higher part of the roof surface above 32 degrees and a lower part below it. That heat primarily comes from the house, it says, and in many homes air leaking through the ceiling into the attic is the major route. Icicles alone do not prove an ice dam, but a heavy curtain along one stretch of eave, or a ridge of ice behind the gutter, is worth a closer look; the fix is on our ice dam removal page.

The structural side is the ice. FEMA says ice dams create a concentrated load at the eaves and reduce a sloped roof's ability to shed snow. Gutters that sag, pull away or bend after a freeze are a gutter repair job.

Do not knock icicles down from a ladder. FEMA warns that an icicle falling even from a short height can cause damage or injury. Keep people and pets out from under the drip line.

FEMA recommends inspecting the roofing once snow comes off, plus a quick structural check after particularly large storms. Photograph anything you find before it is repaired, and read your policy: NDSU Extension notes that roof failure from snow is not covered by all policies. Our storm damage claim documentation page explains what to photograph, and an on-site roof inspection confirms the covering is sound. If it is a pipe boot, flashing or a lifted shingle, that is what we will quote. If it looks structural, we will say so and tell you to call an engineer: (201) 275-9185, HIC reg.# 13VH13970900.

Walk the house from the ground and check:

The figures above come from these sources.