What Is Ground Snow Load?

What Does Ground Snow Load Measure?

Ground snow load is the weight of snow on the ground, in pounds per square foot (psf), that the building code expects at your site. Engineers use it to work out the design snow load on a roof. It is a design value, not a measurement of today’s snow.

Ground Snow Load vs Roof Snow Load: What Is the Difference?

Ground snow load is the starting point, and roof snow load is derived from it. The roof value is adjusted for exposure, temperature and building type, so it differs from the ground value.

How Do You Convert Ground Snow Load to Roof Snow Load?

For a flat roof under ASCE 7-16, pf = 0.7 × Ce × Ct × Is × pg. With pg = 30 psf and Ce = Ct = Is = 1.0, pf = 0.7 × 30 = 21 psf. Real projects use the exposure and thermal factors from ASCE 7 and an engineer.

How Do You Find the Ground Snow Load for Your Address?

Use the official finder, then confirm with the building department.

1. Open the ASCE Hazard Tool and enter the address.
2. In states with their own finder, such as the Montana snow load finder, use that.
3. Ask your building department for the figure in its design table. That is the number the inspector uses.

What Changed in ASCE 7-22?

ASCE 7-22 changed how snow loads are set. In summary:

• It replaced the single ground snow map with four maps, one for each risk category.
• It removed the snow importance factor Is, since risk is now built into the maps.
• The maps are reliability-based, and the LRFD snow load factor is effectively reduced from 1.6 to 1.0.
• The minimum flat-roof snow load changed from 20 to 30 psf for Risk Category II.

The 2024 IRC keeps an allowable stress design ground snow load figure, Figure R301.2(3), and also allows values from IBC Section 1608. Some states still use older tables, so check which one your jurisdiction has adopted.

What Does a 30 psf Snow Load Mean?

It means the design assumes 30 pounds of snow on each square foot of ground. The roof load is then calculated from that figure, and a flat roof often carries less, as the 21 psf example shows. How many inches that is depends on how heavy the snow is, since wet snow weighs more than fresh powder.

How Much Does Snow Weigh?

It depends on how wet it is. FEMA’s Snow Load Safety Guide (P-957) gives these figures:

FEMA adds that snow in the western United States is typically lighter and less dense than snow on the East Coast, which tends to be wetter. By those figures, 20 inches of fresh snow weighs roughly 5 psf if it is light and dry, and about 35 psf if it is wet and heavy.

Can a Roof Hold 20 Inches of Snow?

It depends on the roof’s design load and on how heavy the snow is, not on the depth alone. Twenty inches of fresh snow weighs about 5 to 35 psf by FEMA’s figures, so a roof designed for the local snow load usually handles light snow easily, while the same depth of wet snow, or snow with ice or rain in it, can approach or pass the design load. FEMA’s snow load guidance says snow beyond a building’s design conditions can cause structural failure, and lists four common causes:

• Unbalanced snow. Drifting and sliding snow pile up deeper in some spots and concentrate the load.
• Rain on snow. Heavy rain soaks the snow and adds load.
• Melt and ice dams. Blocked or poorly designed drainage forms ice dams that load the eaves, and meltwater pools in low areas on flat roofs.
• Roof shape. Parapets, stepped roofs, chimneys and equipment collect drifts.

FEMA lists warning signs that a roof is overstressed: sagging ceilings or roof members, popping or cracking noises, doors and windows that stop opening or closing, cracked wood members, cracks in walls and severe leaks. If you see them, leave the building and call the building authority or a qualified design professional. If snow removal is needed, FEMA says to use qualified people, leave a couple of inches of snow on the roof, and avoid metal tools and powered equipment.

What Is a Typical Ground Snow Load?

It varies enormously by place, and elevation matters. Local design tables we checked list, for example:

These are local code values and may be revised, so confirm yours with the building department.

What Is the Ground Snow Load Limit for IRC Houses?

The IRC’s prescriptive rules cover houses in regions with ground snow loads of 70 psf or less. Wood-framed, cold-formed steel, masonry and concrete construction in those regions can follow the span tables in Chapters 5, 6 and 8, which are built on ground snow load (R301.2.3). Where the ground snow load is above 70 psf, the house must be designed under accepted engineering practice. Some jurisdictions require that design to come from a registered design professional; Jefferson County, Colorado writes this into its amendments.

Is Ground Snow Load Set in the Same Table as the Frost Line?

Yes. The IRC puts both in Table R301.2, which each jurisdiction fills in. The same row can list ground snow load, frost line depth, wind speed and more. So the figures on your plan sheet come from the same local table. See what is the frost line.

Ground Snow Load by State

There is no single national table, and several western states publish their own finders because snow load climbs fast with elevation. Where a state has none, the ASCE Hazard Tool and your building department are the sources.

Your local design table is still the number the inspector uses, so open the state list and your place’s page for local design criteria.