### 7.1 Symbols

 Ce = exposure factor as determined from Table 7-2 Cs = slope factor as determined from Fig. 7-2 Ct = thermal factor as determined from Table 7-3 h = vertical separation distance in feet (m) between the edge of a higher roof including any parapet and the edge of a lower adjacent roof excluding any parapet hb = height of balanced snow load determined by dividing ps by γ, in ft (m) hc = clear height from top of balanced snow load to ( 1) closest point on adjacent upper roof, (2) top of parapet, or (3) top of a projection on the roof, in ft (m) hd = height of snow drift, in ft (m) ho = height of obstruction above the surface of the roof, in ft (m) Is = importance factor as prescribed in Section 7.3.3 ls = length of the roof upwind of the drift, in ft (m) pd = maximum intensity of drift surcharge load, in lb/ft2 (kN/m2) pf = snow load on flat roofs ("flat" = roof slope ≤ 5°), in lb/ft2 (kN/m2) pg = ground snow load as determined from Fig. 7-1 and Table 7-1; or a site-specific analysis, in lb/ft2 (kN/m2) pm = minimum snow load for low-slope roofs, in lb/ft2 (kN/m2) ps = sloped roof (balanced) snow load, in lb/ft2 (kN/m2) s = horizontal separation distance in feet (m) between the edges of two adjacent buildings S = roof slope run for a rise of one θ = roof slope on the leeward side, in degrees w = width of snow drift, in ft (m) W = horizontal distance from eave to ridge, in ft (m) γ = snow density, in lb/ft3 (kN/m3) as determined from Eq. 7.7-1

### 7.2 GROUND SNOW LOADS, pg

Ground snow loads, pg, to be used in the determination of design snow loads for roofs shall be as set forth in Fig. 7-1 for the contiguous United States and Table 7-1 for Alaska. Site-specific case studies shall be made to determine ground snow loads in areas designated CS in Fig. 7-1. Ground snow loads for sites at elevations above the limits indicated in Fig. 7-1 and for all sites within the CS areas shall be approved by the authority having jurisdiction. Ground snow load determination for such sites shall be based on an extreme value statistical analysis of data available in the vicinity of the site using a value with a 2 percent annual probability of being exceeded (50-year mean recurrence interval).
Snow loads are zero for Hawaii, except in mountainous regions as determined by the authority having jurisdiction.

Location pg Location pg Location pg
lb/ft2 kN/m2 lb/ft2 kN/m2 lb/ft2 kN/m2
Adak 30 1.4 Galena 60 2.9 Petersburg 150 7.2
Anchorage 50 2.4 Gulkana 70 3.4 St. Paul 40 1.9
Angoon 70 3.4 Homer 40 1.9 Seward 50 2.4
Barrow 25 1.2 Juneau 60 2.9 Shemya 25 1.2
Barter 35 1.7 Kenai 70 3.4 Sitka 50 2.4
Bethel 40 1.9 Kodiak 30 1.4 Talkeetna 120 5.8
Big Delta 50 2.4 Kotzebue 60 2.9 Unalakleet 50 2.4
Cold Bay 25 1.2 McGrath 70 3.4 Valdez 160 7.7
Cordova 100 4.8 Nenana 80 3.8 Whittier 300 14.4
Fairbanks 60 2.9 Nome 70 3.4 Wrangell 60 2.9
Fort Yukon 60 2.9 Palmer 50 2.4 Yakutat 150 7.2

### 7.3 FLAT ROOF SNOW LOADS, pf

The flat roof snow load, pf, shall be calculated in lb/ft2 (kN/m2 ) using the following formula:
(7.3-1)

### 7.3.1 Exposure Factor, Ce

The value for Ce shall be determined from Table 7-2.

Table 7-2 Exposure Factor, Ce
Terrain Category Exposure of Roofa
Fully Exposed Partially Exposed Sheltered
B (see Section 26.7) 0.9 1.0 1.2
C (see Section 26.7) 0.9 1.0 1.1
D (see Section 26.7) 0.8 0.9 1.0
Above the treeline in
windswept mountainous
areas.
0.7 0.8 N/A
where trees do not exist
within a 2-mile (3-km)
0.7 0.8 N/A
The terrain category and roof exposure condition chosen shall be representative of the anticipated conditions during the life of the structure. An exposure factor shall be determined for each roof of a structure.
aDefinitions: Partially Exposed: All roofs except as indicated in the following text. Fully Exposed: Roofs exposed on all sides with no shelterb afforded by terrain, higher structures, or trees. Roofs that contain several large pieces of mechanical equipment, parapets that extend above the height of the balanced snow load (hb), or other obstructions are not in this category. Sheltered: Roofs located tight in among conifers that qualify as obstructions.
bObstructions within a distance of 10h0 provide "shelter," where h0 is the height of the obstruction above the roof level. If the only obstructions are a few deciduous trees that are leafless in winter, the "fully exposed" category shall be used. Note that these are heights above the roof . Heights used to establish the Exposure Category in Section 26.7 are heights above the ground.

### 7.3.2 Thermal Factor, Ct

The value for Ct shall be determined from Table 7-3.

 Thermal Conditiona Ct All structures except as indicated below 1.0 Structures kept just above freezing and others with cold, ventilated roofs in which the thermal resistance (R-value) between the ventilated space and the heated space exceeds 25 °F x h x ft2/Btu (4.4 K x m2/W). 1.1 Unheated and open air structures 1.2 Structures intentionally kept below freezing 1.3 Continuously heated greenhousesb with a roof having a thermal resistance (R-value) less than 2.0 °F x h x ft2/Btu (0.4 K x m2/W) 0.85

aThese conditions shall be representative of the anticipated conditions during winters for the life of the structure.
bGreenhouses with a constantly maintained interior temperature of 50 °F (10 °C) or more at any point 3 ft above the floor level during winters and having either a maintenance attendant on duty at all times or a temperature alarm system to provide warning in the event of a heating failure.

### 7.3.3 Importance Factor, Is

The value for Is shall be determined from Table 1.5-2 based on the Risk Category from Table 1.5-1.

### 7.3.4 Minimum Snow Load for Low-Slope Roofs, pm

A minimum roof snow load, pm, shall only apply to monoslope, hip and gable roofs with slopes less than 15°, and to curved roofs where the vertical angle from the eaves to the crown is less than 10°. The minimum roof snow load for low-slope roofs shall be obtained using the following formula:

Where pg is 20 lb/ft2 (0.96 kN/m2 ) or less:

(Importance Factor times pg)

Where pg exceeds 20 lb/ft2 (0.96 kN/m2 ):

(20 lb/ft2 times Importance Factor)

This minimum roof snow load is a separate uniform load case. It need not be used in determining or in combination with drift, sliding, unbalanced, or partial loads.

### 7.4 SLOPED ROOF SNOW LOADS, ps

Snow loads acting on a sloping surface shall be assumed to act on the horizontal projection of that surface. The sloped roof (balanced) snow load, ps, shall be obtained by multiplying the flat roof snow load, pf, by the roof slope factor, Cs:

(7.4-1)

Values of Cs for warm roofs, cold roofs, curved roofs, and multiple roofs are determined from Sections 7.4.1 through 7.4.4. The thermal factor, Ct, from Table 7-3 determines if a roof is "cold" or "warm." "Slippery surface" values shall be used only where the roof's surface is unobstructed and sufficient space is available below the eaves to accept all the sliding snow. A roof shall be considered unobstructed if no objects exist on it that prevent snow on it from sliding. Slippery surfaces shall include metal, slate, glass, and bituminous, rubber, and plastic membranes with a smooth surface. Membranes with an imbedded aggregate or mineral granule surface shall not be considered smooth. Asphalt shingles, wood shingles, and shakes shall not be considered slippery.

### 7.4.1 Warm Roof Slope Factor, Cs

For warm roofs (Ct ≤ 1.0 as determined from Table 7-3) with an unobstructed slippery surface that will allow snow to slide off the eaves, the roof slope factor Cs shall be determined using the dashed line in Fig. 7-2a, provided that for nonventilated warm roofs, their thermal resistance (R-value) equals or exceeds 30 ft2 hr °F/Btu (5.3 °C m2/W) and for warm ventilated roofs, their R-value equals or exceeds 20 ft2 hr °F/Btu (3.5 °C m2/W). Exterior air shall be able to circulate freely under a ventilated roof from its eaves to its ridge. For warm roofs that do not meet the aforementioned conditions, the solid line in Fig. 7-2a shall be used to determine the roof slope factor Cs.

### 7.4.2 Cold Roof Slope Factor, Cs

Cold roofs are those with a Ct > 1.0 as determined from Table 7-3. For cold roofs with Ct = 1.1 and an unobstructed slippery surface that will allow snow to slide off the eaves, the roof slope factor Cs shall be determined using the dashed line in Fig. 7-2b. For all other cold roofs with Ct = 1.1, the solid line in Fig. 7-2b shall be used to determine the roof slope factor Cs. For cold roofs with Ct = 1.2 or larger and an unobstructed slippery surface that will allow snow to slide off the eaves, the roof slope factor Cs shall be determined using the dashed line on Fig. 7-2c. For all other cold roofs with Ct = 1.2 or larger, the solid line in Fig. 7-2c shall be used to determine the roof slope factor Cs.

### 7.4.3 Roof Slope Factor for Curved Roofs

Portions of curved roofs having a slope exceeding 70° shall be considered free of snow load (i.e., Cs = 0). Balanced loads shall be determined from the balanced load diagrams in Fig. 7-3 with Cs determined from the appropriate curve in Fig. 7-2.

### 7.4.4 Roof Slope Factor for Multiple Folded Plate, Sawtooth, and Barrel Vault Roofs

Multiple folded plate, sawtooth, or barrel vault roofs shall have a Cs = 1.0, with no reduction in snow load because of slope (i.e., ps = pf).

### 7.4.5 Ice Dams and Icicles Along Eaves

Two types of warm roofs that drain water over their eaves shall be capable of sustaining a uniformly distributed load of 2pf on all overhanging portions: those that are unventilated and have an R-value less than 30 ft2 hr °F/Btu (5.3 °C m2/W) and those that are ventilated and have an R-value less than 20 ft2 hr °F/Btu (3.5 °C m2/W). The load on the overhang shall be based upon the flat roof snow load for the heated portion of the roof up-slope of the exterior wall. No other loads except dead loads shall be present on the roof when this uniformly distributed load is applied.

The effect of having selected spans loaded with the balanced snow load and remaining spans loaded with half the balanced snow load shall be investigated as follows:

### 7.5.1 Continuous Beam Systems

Continuous beam systems shall be investigated for the effects of the three loadings shown in Fig. 7-4:

Case 1: Full balanced snow load on either exterior span and half the balanced snow load on all other spans.
Case 2: Half the balanced snow load on either exterior span and full balanced snow load on all other spans.
Case 3: All possible combinations of full balanced snow load on any two adjacent spans and half the balanced snow load on all other spans. For this case there will be (n -1) possible combinations where n equals the number of spans in the continuous beam system.

If a cantilever is present in any of the above cases, it shall be considered to be a span.
Partial load provisions need not be applied to structural members that span perpendicular to the ridgeline in gable roofs with slopes of 2.38° (½ on 12) and greater.

### 7.5.2 Other Structural Systems

Areas sustaining only half the balanced snow load shall be chosen so as to produce the greatest effects on members being analyzed.

### 7.6 Unbalanced Roof Snow Loads

Balanced and unbalanced loads shall be analyzed separately. Winds from all directions shall be accounted for when establishing unbalanced loads.

### 7.6.1 Unbalanced Snow Loads for Hip and Gable Roofs

For hip and gable roofs with a slope exceeding 7 on 12 (30.2°) or with a slope less than 2.38° (½ on 12) unbalanced snow loads are not required to be applied. Roofs with an eave to ridge distance, W, of 20 ft (6.1 m) or less, having simply supported prismatic members spanning from ridge to eave shall be designed to resist an unbalanced uniform snow load on the leeward side equal to Ipg. For these roofs the windward side shall be unloaded. For all other gable roofs, the unbalanced load shall consist of 0.3ps on the windward side, ps on the leeward side plus a rectangular surcharge with magnitude hdγ/√S and horizontal extent from the ridge 8hd√S/3 where hd is the drift height from Fig. 7-9 with lu equal to the eave to ridge distance for the windward portion of the roof, W. For W less than 20 ft (6.1 m), use W = lu = 20 ft in Fig 7-9. Balanced and unbalanced loading diagrams are presented in Fig. 7-5.

### 7.6.2 Unbalanced Snow Loads for Curved Roofs

Portions of curved roofs having a slope exceeding 70° shall be considered free of snow load. If the slope of a straight line from the eaves (or the 70° point, if present) to the crown is less than 10° or greater than 60°, unbalanced snow loads shall not be taken into account.
Unbalanced loads shall be determined according to the loading diagrams in Fig. 7-3. In all cases the windward side shall be considered free of snow. If the ground or another roof abuts a Case II or Case III (see Fig. 7-3) curved roof at or within 3 ft (0.91 m) of its eaves, the snow load shall not be decreased between the 30° point and the eaves, but shall remain constant at the 30° point value. This distribution is shown as a dashed line in Fig. 7-3.

### 7.6.3 Unbalanced Snow Loads for Multiple Folded Plate, Sawtooth, and Barrel Vault Roofs

Unbalanced loads shall be applied to folded plate, sawtooth, and barrel-vaulted multiple roofs with a slope exceeding 3/8 in./ft (1.79°). According to Section 7.4.4, Cs = 1.0 for such roofs, and the balanced snow load equals pf. The unbalanced snow load shall increase from one-half the balanced load at the ridge or crown (i.e., 0.5pf) to two times the balanced load given in Section 7.4.4 divided by Ce at the valley (i.e., 2pf/Ce). Balanced and unbalanced loading diagrams for a sawtooth roof are presented in Fig. 7-6. However, the snow surface above the valley shall not be at an elevation higher than the snow above the ridge . Snow depths shall be determined by dividing the snow load by the density of that snow from Eq. 7.7-1, which is in Section 7.7.1.

### 7.6.4 Unbalanced Snow Loads for Dome Roofs

Unbalanced snow loads shall be applied to domes and similar rounded structures. Snow loads, determined in the same manner as for curved roofs in Section 7.6.2, shall be applied to the downwind 90° sector in plan view. At both edges of this sector, the load shall decrease linearly to zero over sectors of 22.5° each. There shall be no snow load on the remaining 225° upwind sector.

### 7.7 Drifts on Lower Roofs (Aerodynamic Shade)

Roofs shall be designed to sustain localized loads from snowdrifts that form in the wind shadow of (1) higher portions of the same structure and (2) adjacent structures and terrain features.

### 7.7.1 Lower Roof of a Structure

Snow that forms drifts comes from a higher roof or, with the wind from the opposite direction, from the roof on which the drift is located . These two kinds of drifts ("leeward" and "windward" respectively) are shown in Fig. 7-7. The geometry of the surcharge load due to snow drifting shall be approximated by a triangle as shown in Fig. 7-8. Drift loads shall be superimposed on the balanced snow load. If hc/hb is less than 0.2, drift loads are not required to be applied.
For leeward drifts, the drift height hd shall be determined directly from Fig. 7-9 using the length of the upper roof. For windward drifts, the drift height shall be determined by substituting the length of the lower roof for lu in Fig. 7-9 and using three-quarters of hd as determined from Fig. 7-9 as the drift height. The larger of these two heights shall be used in design. If this height is equal to or less than hc, the drift width, w, shall equal 4hd and the drift height shall equal hd. If this height exceeds hc, the drift width, w, shall equal 4hd2/hc and the drift height shall equal hc. However, the drift width, w, shall not be greater than 8hc. If the drift width, w, exceeds the width of the lower roof, the drift shall be truncated at the far edge of the roof, not reduced to zero there. The maximum intensity of the drift surcharge load, pd equals hdγ where snow density, γ, is defined in Eq. 7.7-1:
(7.7-1)
 (in SI: γ = 0.426pg + 2.2, but not more than 4.7 kN/m3)

This density shall also be used to determine hb by dividing ps by γ (in SI: also multiply by 102 to get the depth in m).

If the horizontal separation distance between adjacent structures, s, is less than 20 ft (6.1 m) and less than six times the vertical separation distance (s < 6h), then the requirements for the leeward drift of Section 7.7.1 shall be used to determine the drift load on the lower structure. The height of the snow drift shall be the smaller of hd, based upon the length of the adjacent higher structure, and (6h - s)/6. The horizontal extent of the drift shall be the smaller of 6hd or (6h - s).
For windward drifts, the requirements of Section 7.7.1 shall be used. The resulting drift is permitted to be truncated.

### 7.8 Roof Projections and Parapets

The method in Section 7.7.1 shall be used to calculate drift loads on all sides of roof projections and at parapet walls. The height of such drifts shall be taken as three-quarters the drift height from Fig. 7-9 (i.e., 0.75hd). For parapet walls, lu shall be taken equal to the length of the roof upwind of the wall. For roof projections, lu shall be taken equal to the greater of the length of the roof upwind or downwind of the projection. If the side of a roof projection is less than 15 ft (4.6 m) long, a drift load is not required to be applied to that side.

### 7.9 Sliding Snow

The load caused by snow sliding off a sloped roof onto a lower roof shall be determined for slippery upper roofs with slopes greater than ¼ on 12, and for other (i.e., nonslippery) upper roofs with slopes greater than 2 on 12. The total sliding load per unit length of eave shall be 0.4pfW, where W is the horizontal distance from the eave to ridge for the sloped upper roof. The sliding load shall be distributed uniformly on the lower roof over a distance of 15 ft (4.6 m) from the upper roof eave. If the width of the lower roof is less than 15 ft (4.6 m), the sliding load shall be reduced proportionally.
The sliding snow load shall not be further reduced unless a portion of the snow on the upper roof is blocked from sliding onto the lower roof by snow already on the lower roof.
For separated structures, sliding loads shall be considered when h/s > l and s < 15 ft (4.6 m). The horizontal extent of the sliding load on the lower roof shall be 15 - s with s in feet (4.6 - s with s in meters), and the load per unit length shall be 0.4 pfW (15 - s)/15 with s m feet (0.4pfW (4.6 - s)/4.6 with s in meters).
Sliding loads shall be superimposed on the balanced snow load and need not be used in combination with drift unbalanced partial, or rain-on-snow loads.