ASCE 7-05 Code for Buildings with Flat or Low Slope Roofs (<= 5 deg. or 1 in./ft.)
for Balanced Snow, Drift, and Rain-on-Snow Surcharge Loadings
|Building Classification||[?]||Table 1-1, page 3|
|Ground Snow Load, pg||psf||Figure 7-1, pages 84-85 and Table 7-1, page 92|
|Length of High Roof, Lu||ft.||Length of Roof Upwind of the Snow Drift|
|Length of Low Roof, LL||ft.||Length of Roof Downwind of the Snow Drift|
|Horiz. Dist. from Eave to Ridge, W||ft.||Horizontal Distance from Eave to Ridge|
|Type of Roof||Type of Roof = Monoslope, Gable, or Hip|
|Obstruction Height, ho||ft.||High Roof - Low Roof Elevations|
|Roof Slope||in./ft.||S = Rise per foot of Run|
|Exposure Factor, Ce||[?]||Table 7-2, page 92|
|Thermal Factor, Ct||[?]||Table 7-3, page 93|
Configuration of Snow Drift on Lower Roof
(Length of High Roof)
|Due to Drifting|
|Balanced Snow Load|
|(Length of Low Roof)|
Determine Value, pd, Along Snow Drift Load:
|pd @ x||psf|
Snow Drift Load
Occupancy Category of Buildings and Other Structures for Flood, Wind, Snow, Earthquake, and Ice Loads
Nature of Occupancy
|Buildings and structures that represent a low hazard to human life in the event of failure including, but not limited to:
- Agriculture facilities
|Buildings and other structures except those listed in Categories I, III and IV||II|
| Buildings and other structures that represent a substantial hazard to human life in the event of failure including, but not limited to:
- Buildings and other structures where more than 300 people congregate in one areaBuildings and other structures, not includes in Occupancy Category IV, with potential to cause substantial economic impact and/or mass disruption of day-to-day civilian life in event of failure, including, but not limited to:
- Power generating stations, water treatment facilities, sewage treatment facilities, and telecommunication centers
| Buildings and other structures designated as essential facilities including, but not limited to:
- Hospitals and health care facilities having surgery or emergency treatment facilities
EXPOSURE FACTOR, Ce
|Terrain Category||Fully Exposed||Partially Exposed||Sheltered|
|B (see Sect.6.5.6)||0.9||1.0||1.2|
|C (see Sect.6.5.6)||0.9||1.0||1.1|
|D (see Sect.6.5.6)||0.8||0.9||1.0|
|Above treeline in windswept mountain areas.||0.7||0.8||N.A.|
|In Alaska, areas where trees do not exist within 2 mile radius of site.||0.7||0.8||N.A.|
The terrain category and roof exposure condition chosen shall be representative of anticipated conditions during life of structure. An exposure factor shall be determined for each roof of structure.
Partially Exposed: All roofs except as indicated below.
Fully Exposed: Roofs exposed on all sides with no shelter* afforded by terrain, higher structures or trees. Roofs that contain several large pieces of mechanical equipment, parapets which extend above height of balanced snow load (hb), or other obstructions are not in this category.
Sheltered: Roofs located tight in among conifers that qualify as obstruction.
*Obstructions within distance of 10ho provide "shelter", where ho is height of obstruction above roof level. If only obstructions are a few deciduous trees which are leafless in winter, "fully exposed" category shall be used except for terrain Category "A". Note that these are heights above roof. Heights used to establish the Terrain Category in Section 6.5.3 are heights above ground.
THERMAL FACTOR, 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 heated space exceeds 250 F*h*ft^2/Btu (4.4 K*m^2/W).||1.1|
|Unheated structures and structures intentionally kept below freezing.||1.2|
|Continuously heated greenhouses** with roof having thermal resistance (R-values) less than 2.00 F*h*ft^2/Btu (0.4 K*m^2*W).||0.85|
* These conditions shall be representative of the anticipated conditions during winters for life
** Green houses with constantly maintained interior temperature of 50oF (10oC) or more at any
point 3 feet above floor level during winters and having either maintenance attendant on duty
at all times or temperature alarm system to provide warning in event of heating failure.
|IMPORTANCE FACTOR, I, (SNOW LOADS)|
For locations where 0< pg <= 20 psf, all roofs with a slope < W/50 (with W in ft.) shall have a 5 psf rain-on-snow surcharge. This rain-on-snow augmented design load applies only to the balanced load case and need not be used in combination with drift, sliding, unbalanced, or partial loads.
Note: Per Section 7.10, the rain-on-snow surcharge need NOT be combined with the snow drift.
Disclaimer: This calculator is not intended to be used for the design of actual structures, but only for schematic (preliminary) understanding of structural design principals. For the design of an actual structure, a competent professional should be consulted.
‘Calculations courtesy of Alex Tomanovich, PE ’