How much snow can my roof hold?

Two feet of snow on your roof can be a non-event or an emergency, because snow density varies enormously:

• Fresh, fluffy snow — roughly 3 to 5 pounds per square foot for each foot of depth.
• Settled snow that has sat for days — roughly 12 to 20 pounds per square foot per foot.
• Wet, heavy snow — 20 pounds per square foot per foot or more. Heavy rain falling onto an existing snowpack can soak in and add weight on top of that.
• Ice — about 57 pounds per cubic foot; even a thin ice layer outweighs a lot of powder.

So “how deep is it?” is the wrong first question. A winter’s accumulated, rained-on, refrozen snowpack is what design values exist to describe.

Every code-built roof was designed to carry a specific snow weight, in pounds per square foot. That number starts from the ground snow load for the site — the standard statistical measure of how much snow weight a location’s worst winters deposit, defined as the value with a 2% chance of being exceeded in any year (often called the 50-year value). The building code then adjusts it for your roof’s wind exposure, slope, and insulation, and for drifting — the snow that wind piles against walls, into valleys, and onto lower roofs, which is how roofs often actually get overloaded.

That is why the honest answer to “how much can my roof hold?” is another question: what ground snow load was your roof built for, and what is the real value at your site? In flat country the two are usually close. In hilly or mountainous terrain, the mapped value your builder used may not describe your specific elevation and surroundings at all.

Get people out and call a professional if you notice, during or after heavy snow:

• Sagging in the roofline, ceiling, or ridge.
• New creaking, cracking, or popping sounds.
• New cracks in walls, drywall, or masonry.
• Interior doors that suddenly stick or won’t latch.

And a safety note that outranks everything else in this guide: do not climb onto a snow-loaded roof. Anything that needs someone up there is a job for a trained professional with safety gear.

The federal government’s snow-load research lab (CRREL) maintains the station measurements and the method — published as SR-20-1 — for estimating the ground snow load at a specific site from the weather stations around it, known as a ground snow load case study. Nivometra runs that method as software: pin your address, and it finds the relevant stations and sets up the analysis. Exploring the site, the stations, and a preview of the report is free; the documented case-study report — the calculated value plus the stations and analysis behind it, ready to hand to an engineer, contractor, or building department — is $29. See a full sample report.