Snow Load Exposure Factor: Ce, Ct, and Is Explained

The flat-roof snow load equation, Pf = 0.7 x Ce x Ct x Is x Pg, has three factors that give first-time users the most trouble: Ce, Ct, and Is. Each one is a simple table lookup once you know which condition describes your building, but picking the wrong row is an easy, invisible mistake that changes the final number by 10 to 30 percent. This is a full reference for all three factors, straight from ASCE 7-22 Chapter 7 and Chapter 1, plus a worked example that threads them together on one building.

Exposure factor (Ce): full reference table

Ce accounts for how much wind exposure removes snow from a roof before it can accumulate to the open-ground value. It combines two inputs: the surrounding terrain category (how much the landscape itself blocks wind) and the roof's own exposure condition (how much nearby trees, taller buildings, or terrain features shelter the specific roof). ASCE 7-22 Table 7.3-1 sets the value from that combination.

Terrain Category B covers urban and suburban areas with numerous closely spaced obstructions, or wooded areas, the default for most residential neighborhoods. Category C covers open terrain with scattered obstructions generally under 30 feet, common in suburban edges and rural areas. Above the treeline in windswept mountainous areas, the values drop further still. 'Fully exposed' means a roof with no shelter from terrain, higher structures, or trees; 'partially exposed' is the default assumption for most buildings when the actual condition is not obviously one extreme or the other; 'sheltered' means the roof is tucked among trees or taller structures that block wind on all sides.

Thermal factor (Ct): full reference table

Ct accounts for how much heat escapes through the roof deck and melts snow from underneath, reducing the load compared to an unheated surface. ASCE 7-22 Table 7.3-2 sets several conditions. A continuously heated structure, meaning normal occupied space kept above roughly 50 degrees Fahrenheit through the winter, uses Ct = 1.0, the default for the overwhelming majority of houses, apartments, and offices. A structure that is heated just enough to stay above freezing, or one with a cold, well-ventilated roof separated from the heated space by a high level of insulation, uses Ct = 1.1; this covers many garages, some barns, and cold-roof assemblies over insulated attics.

A fully unheated structure, one with no heat source at all, uses Ct = 1.2; this covers most sheds and detached, unconditioned outbuildings. A structure intentionally kept below freezing, such as a refrigerated warehouse or freezer building, uses Ct = 1.3, the highest value in the table, because there is no melting at all to reduce the accumulated load. At the opposite end, a continuously heated greenhouse with a low-resistance roof, one that leaks heat freely by design, uses Ct = 0.85, the only value below 1.0, because the roof stays warm enough to shed snow aggressively.

Importance factor (Is): full reference table

Is scales the entire load by how serious the consequences of a roof failure would be, using the same Risk Category system ASCE 7 applies to wind and seismic design. Risk Category I, low-hazard structures where failure poses minimal risk to human life, such as minor storage sheds and agricultural buildings, uses Is = 0.8. Risk Category II, ordinary structures not otherwise classified, which covers the large majority of houses, apartment buildings, and commercial buildings, uses Is = 1.0.

Risk Category III, substantial-hazard structures where failure poses a substantial risk to human life or where large numbers of people gather, such as schools and buildings holding more than a few hundred people, uses Is = 1.1. Risk Category IV, essential facilities that need to remain operational after a disaster, including hospitals, emergency response stations, and designated shelters, uses Is = 1.2, the highest value in the table. A building's Risk Category is typically set by its occupancy classification in the building code, not chosen at the designer's discretion.

Worked example: threading all three together

Take a rural fire station in Duluth, Minnesota. Pg for the site is 55 psf. The building sits on an open lot with scattered trees, terrain category C, and the roof is partially exposed, so Ce = 1.0. The station is continuously heated to normal occupied temperatures, so Ct = 1.0. As an emergency response facility, it falls under Risk Category IV, so Is = 1.2.

Pf = 0.7 x 1.0 x 1.0 x 1.2 x 55 = 46.2 psf. Compare that to an ordinary house next door on the same lot: same Pg, same Ce, same Ct, but Risk Category II gives Is = 1.0. Pf = 0.7 x 1.0 x 1.0 x 1.0 x 55 = 38.5 psf. The fire station, despite having an identical roof and identical site conditions, carries a design load 20 percent higher than the house next to it, purely because of its importance classification. That 20 percent difference is the entire purpose of the importance factor: it builds in extra reserve capacity for buildings society cannot afford to lose during the exact storm event the load is designed for.

Which factor moves the roof snow load the most?

Of the three, Ct tends to have the widest practical spread in ordinary buildings: the difference between a heated house (1.0) and an unheated shed (1.2) is a straightforward 20 percent, and the gap to a freezer building (1.3) is 30 percent. Ce has a similar range at the extremes, from 0.7 on a windswept mountain ridge to 1.2 in a fully sheltered site, a spread of more than 70 percent between the two ends, though most ordinary buildings land in the narrower 0.9 to 1.1 middle of that range. Is has the narrowest range for most projects, 0.8 to 1.2, but because it applies to whole classes of buildings by code rather than by site condition, getting the Risk Category wrong is a common and consequential mistake, particularly on public buildings that sit near the boundary between Risk Category II and III.

Common mistakes when selecting these factors

The most frequent Ce mistake is defaulting to partially exposed without checking whether the roof is actually more sheltered or more exposed than that assumption; a roof tucked into a stand of mature trees genuinely qualifies for a sheltered value, and using the default instead understates a reduction the code allows. The most frequent Ct mistake is treating a garage or barn as fully unheated when it actually has a wood stove, space heater, or other intermittent heat source; ASCE 7 cares about the heating condition of the structure as designed and intended to be used, not just whether the thermostat happens to be off on the day of inspection, so a garage with a permanently installed heater should generally use the heated value even if it is not occupied around the clock. The most frequent Is mistake is applying Risk Category II by default without checking the actual occupancy classification in the building code, particularly for schools, assembly buildings, and any structure that houses more than a couple hundred people at once, all of which can trigger Risk Category III.

Insulation and heating choices also interact with these numbers well beyond the roof snow load calculation itself; a properly insulated, air-sealed attic, see DOE's Energy Saver guidance on insulation, keeps a roof consistently cold, which supports a Ct of 1.0 or 1.1 and helps prevent ice dams, a separate but related problem caused by uneven roof deck temperature.