Snow Loads Explained

Snow loads are defined as the load induced by the weight of the snow.  Roof snow loads are generally a derivative of the Ground Snow and several environmental factors, some or all of which are explained below.

Ground Snow Load

Roof loads are typically 80% of ground snow, but will vary depending on roof slope and other conditions. 

Snow Exposure

The snow exposure is needed to calculate what portion of the ground snow is likely to build up on the roof.  This information should come from the Engineer-of-Record. The IBC supplies the following guidelines to determine this condition:

Snow Exposure Condition

Partially exposed: All roofs except as indicated below

Fully Exposed Roof

Open to the elements on all sides without shelter created by surroundings such as taller structures, or large trees. Roofs with bulky pieces of mechanical equipment, parapets extending beyond height of the eaves or gables, or other obstructions connected to roof systems are NOT in this category.

Sheltered Roof

Nested in trees that meet the criteria as obstructions*.

* Obstructions in a distance of 10h, where h is the height of the obstruction above the roof. If the only obstructions are a few deciduous trees, which are leafless in winter, the "Fully Exposed" category shall be used except with wind exposure "A". Note that these heights are above the roof. Heights used to establish the wind exposure are heights above the ground.

Thermal Condition

The thermal condition is a measure of the roofs ability to prevent snow accumulation on the roof due to heat escaping from within the structure. This is primarily a function of the use of the building. This condition should also come from the Engineer-of-Record, but in general "All others" should be used. ASCE-7 provides the following guidelines to determine this condition:

Thermal Condition*

Description

Kept Just Above Freezing:

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.

Unheated:

Kept below freezing Unheated structures, and structures intentionally kept below freezing.

All Others:

All other structures except as indicated above.

* - These conditions shall be representative of the anticipated conditions during winters for the life of the structure.

Snow Drift:

Indicate if the building is within 20' of another structure that is taller than the building. If this condition exists, then review the "Snow Drifting" section of the Supplemental Design Information and provide the required information. For an explanation of what information needs to be specified, along with an example, please see ASCE 7-02, Section 7, section 1608 www.asce.org . If the adjacent structure is lower than the proposed building, then a drifting condition may be imposed on the adjacent structure. ICON CANNOT BE RESPONSIBLE FOR DRIFT LOADS IMPOSED ON ADJACENT STRUCTURES NOT DESIGNED BY THE MANUFACTURER. If this is the case, inform the Engineer/Architect of record.