Ground Snow Load – Roof Design
The Town of Webb Office of Code Enforcement tackles some frequently asked questions
Roof-load designs may need elevating in higher elevationsContractors, engineers, architects and code officers are all familiar with the concept of roof loads and design.
Ever since 2002, when the state entered into the family of the International Building Code (of New York), the design criteria is referred to the Ground Snow Load (GSL).
When a roof truss is designed, all sorts of potential conditions are considered.
Things like unbalanced snow accumulations (wind-blown clean on one side with six feet of snow on the opposite side), wind uplift and general wind conditions.
It’s not uncommon to receive a set of construction drawings showing the ground snow load design as 80 pounds per square foot.
That is what the Building Code of New York shows on the chart for this area.
However, unless you read the text and the fine print, all too often the designer misses the increased load requirements for areas having an elevation of greater than 1,000 feet above sea level.
Straight out of the code books:
1608.2 Ground snow loads.
The ground snow loads to be used in determining the design snow loads for roofs shall be determined in accordance with ASCE 7 or Figure 1608.2.
When using Figure 1608.2 for sites at elevations above 1,000 feet (304 m), the ground snow load shall be increased from the mapped value by 2 psf (0.095 kN/m2) for every 100 feet (30 480 mm) above 1,000 feet (304 m).
Site-specific case studies may be made in lieu of snow loads in Figure 1608.2 or ASCE 7.
Ground snow load determination for site-specific case studies shall be approved and 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).
Please SEE ABOVE MAP…
Most USGS maps will show the elevation of your site.
Today, many cars with navigation systems will also tell you the elevation as you’re driving along.
The Town of Webb varies from approximately 1,650 to as high as 2,200 above sea level.
That means the GSL design shall range from 93 to 104 psf.
Don’t forget, these are minimum standards; in some areas it may make sense to increase that because of known snow falls that never seem to get reported.
Now you know to remind your designer to make sure they have adjusted the design of the roof based on elevation.
