Building Loading - Snow Load

(Revised:  02/16/2024)

The Snow Load tab defines design-related information regarding the Building's (or Shape's) Roof Snow Loads.  This is the load induced by the weight of snow on the structure and may be expressed as Ground Snow and/or Specified Min. Roof Snow.  Snow load is not the same as Live load and requires a separate input to design the structure correctly.  Some building codes specify Ground Snow and define the Snow load to be applied to the roof as a reduced percentage of the Ground snow.  In many cases, especially in mountainous areas, it will be necessary to ask the local building department for the required Roof or Ground snow load.

„  Note to Builder/Customer:

The builder is responsible for contacting the local building official or project design professional to obtain and provide all code and loading information for the specific building site.  Data supplied is assumed to be accurate and is not verified.

 

„  Notes:

Snow Drifting/Sliding conditions are automatically recognized and added to the Building Shape's Load Cases.  Building input that will automatically generate Snow Drifting/Sliding loads are step conditions such as Roof Height Changes and Wall-to-Roof conditions.  Buildings within 20 feet of each other will cause Snow Drift/Sliding loads if step conditions exist.

 

Example conditions recognized by the software:  Below Eave Canopies, Roof Height Change, Wall-to-Roof conditions, parapets, existing buildings, and buildings near each other.

Example conditions NOT recognized by the software: Facades, Inside Corners, and Valley roof conditions.  These snow build-up loads for these conditions have to be user-added by the Service Center Estimating/Engineering Department.

Snow Load Information:

 

Ground Snow Load (Pg)

This edit box is used to define the Ground Snow Load for the Building or Shape.  This Load is measured in Pounds per Square Foot (English).  The Ground Snow Load data along with the Building Geometry will generate factors that convert this Load into the applied Roof Snow, Snow Drift or Sliding Snow Loads.  A non-negative value must be input in this field to be allowed to exit from the Loads and Codes dialog.  If the Local Code does not require snow load, a value of zero can be input.

 

Specified Flat Roof Snow

Input for this edit box is ONLY required by the Massachusetts Code.

 

Rain Surcharge (RS)

This edit box is “read only” and defines the system generated weight of Rain that is required by the Code to be superimposed on the balanced roof snow.  This load is required for Ground snow ≤ 20 psf when roof pitches are small in relationship to the width of the roof.  The Rain Surcharge (RS) value is measured in Pounds per Square Foot (English).

Note:

The Canadian codes require input of the Rain Surcharge field.  Values for this field can be found in the National Building Code of Canada 2015 Volume 1, Division B, Table C-2, Snow Load Column Sr.

 

Specified Min Roof Snow Load (SMS)

This edit box, when enabled by checking the box to the left of the description, is used to define a Specified Min. Roof Snow load for the Building or Shape per the local Code requirements.  This load is measured in Pounds per Square Foot (English).  Values for the Specified Min. Roof Snow load and Ground snow may be different; however, the specified value is typically larger than the calculated sloped roof snow.  (The SMS load will not be applied if it is less than the Calculated Sloped Roof Snow Load.)  The loading defined in this edit box will only be combined with Dead and Collateral gravity loads for frame, purlin, and panel design.  This load will not be combined with any other snow, wind, crane, or mezzanine loading.

 

Snow Exposure

If enabled, use this drop list to select the Snow Exposure options that describe the applicable surrounding terrain conditions that may impact the applied Snow Load.

 

The following defines the Snow Exposure options for each Building Code.

„  Note:     The Snow Exposure options vary in descriptions for each Building Code.  For example:

·         The selections for IBC codes are 1 Fully Exposed, 2 Partially Exposed, or 3 Sheltered. 

·         For NBCC, they are Cw = 1, 0.75 or 0.50.

Snow Exposure Descriptions:

%!JumpId(IDC_CODES_USE_1)   UBC, 1996 BOCA, 1993 BOCA, 1990 BOCA, 1987 BOCA, 1988 SBC, MUBC, 1994 NCST, 1991 NCST, OBBC, LBB, MSY, OKC, PHX, SEA, CONN

%!JumpId(IDC_CODES_USE_1)   ANSI, 1994 SBC, 1991 SBC, 1996 NCST, 1993 ASCE, 1988 ASCE, 1997 NCST

%!JumpId(IDC_CODES_USE_1)   1990 NBCC, 1995 NBCC, 2005 NBCC, 2010 NBCC, 2015 NBCC

%!JumpId(IDC_CODES_USE_1)   1995 ASCE, 1997 SBC, 1999 BOCA, 2000-2018 IBC, 1998-2010 ASCE 7, MAST (8th), 2015 MNBC, 2014 WICBC

NA: MBMA, JBSA, KS, MAST (5th), MAST (6th), MAST (7th), NYST, WIST, CHI. 

       

„  Note:  2014 FL–HVHZ (5th) does not specifically exempt design for snow; however, the defined snow load is zero for the entire state.

 

Thermal Factor

This drop list is enabled with the 1982 ANSI Building Code, ASCE 7 Standards, and IBC / IBC-based Codes.  The Thermal Factor is used to adjust the applied Snow Load based on the relative climate and temperature of the Building or Shape. 

Heated:                                         All structures except as indicated below.

Kept just above freezing:        Structures kept just above freezing and others with cold, ventilated roofs.

Unheated:                                    Unheated Structures and open air structures

Heated Greenhouse:                 Continuously heated greenhouses.

Kept Below Freezing:               Structures intentionally kept below freezing.

 

Example:  The Office could be a ‘‘Heated Structure’’ and the Warehouse could be a structure “Kept just above freezing” or an “Unheated” structure’.

Surface Cond.

This drop list is initially blank when first inputting a project and a selection must be made before the user can leave this Snow Load tab.  It is unselected by default and your selection defines whether or not you have an Unobstructed, Slippery Roof that allows the free sliding of snow off the roof.  Slippery roof surfaces shall include metal, slate, glass, bituminous rubber and plastic membrane with smooth surfaces.

„  Note:

All BlueScope metal roofing systems satisfy the requirements of “slippery surfaces” per ASCE Section 7.4 including smooth surface membrane roof systems such as DeckFrame.

 

This option must be changed to “Obstructed or Not Slippery” for cases where movement of snow off the roof is restricted by the non-slippery roof surfaces or objects located on or along the low eave.  The result may be a larger magnitude system generated roof snow depending on conditions such as roof pitch and Thermal Factor.  Examples of Non-slippery roof surfaces are membrane roofs with embedded aggregate or mineral granules, asphalt shingles, wood shingles, and shakes.  Roof obstructions that qualify as “Obstructed” include, but are not limited to, roofs with Ice Damming fasteners, Facades or Parapets along the low eave, and roof top units uniformly spread over the roof.

„  Note:

Metal buildings are typically not constructed with attics similar to those of residential structures, and therefore may be considered “Unobstructed” without consideration for the insulation R-value used.

 

Calculated Sloped Roof Snow Load (S)

This field is used to display the final Calculated Sloped Roof Snow load for the Project or Shape per the defined Building Code.  This load is measured in Pounds per Square Foot (English).

 

Standard Controls:

§ OK, Cancel, Apply, Help

 

See also:

§ Building Loading - Building Codes

§ Building Loading - Live Load

§ Building Loading - Wind Load

§ Building Loading - Seismic

§ Building Loading - Tornado Load

§ Building Loading - Deflection Conditions