Calculating your Snow Loads
Snow loading can be determined from actual ground snowfall records, multiplied by appropriate factors,
In general, if you receive snow but never more than 22 inches in depth, your ground snow load would be 20 pounds per square foot.
Let’s try some examples
The formula to use is: Snow depth x 2.36 – 31.9
Example 1. Snow depth is 36 inches
36 x 2.36 = 84.96
84.96 – 31.9 = 53.06
Then round up, which gives you a 55 pound snow load.
Example 2. Snow depth is 45 inches
45 x 2.36 = 106.2
106.2 – 31.9 = 74.3
Again, rounding up this would mean a 75 pound snow load.
Reduction Factors
Blown out by the snow discussions and calculations? Then wind will be simple
Design for wind load is based upon the basic design wind speed (in miles per hour) and an exposure factor. The design wind speed can be expressed either as a basic design wind speed V (3-second gust) or an allowable stress design wind speed Vasd. Table 1609.3.1 of the 2018 IBC (International Building Code) offers a conversion between the two.
Exposure Factors
Exposure, or “how open is my building to the wind?” changes the degree of application of the wind speed. Your choices are going to be Exposure B, C or D.
From the 2018 IBC Code book:
Exposure B. “For buildings with a mean roof height of less than or equal to 30 feet, Exposure B shall apply where the ground surface roughness, as defined by Surface Roughness B, prevails in the upwind direction for a distance of not less than 1,500 feet. For buildings with a mean roof height greater than 30 feet, Exposure B shall apply where Surface Roughness B prevails in the upwind direction for a distance of not less than 2,600 feet or 20 times the height of the building, whichever is greater.” “Surface Roughness B. Urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger.”
Example – You live in a city or town with structures on all four sides of you.
Exposure C. “Exposure C shall apply for all cases where Exposures B or D do not apply.” “Surface Roughness C. Open terrain with scattered obstructions having heights generally less than 30 feet. This category includes flat open country and grasslands.”
Example: Your building is rural – out in the country, which may have buildings on two or three sides, but if even one side is “open to the wind” – it’s then Exposure C. Also, if the “open” side is not the side the wind usually blows from, it’s still considered Exposure C.
Exposure D. “Exposure D shall apply where the ground surface roughness, as defined by Surface Roughness D, prevails in the upwind direction for a distance of not less than 5,000 feet or 20 times the height of the building, whichever is greater. Exposure D shall apply where the ground surface roughness immediately upwind of the site is B or C, and the site is within a distance of 600 feet, or 20 times the building height, whichever is greater, from an Exposure D condition as defined in the previous sentence.” “Surface Roughness D. Flat, unobstructed areas and water surfaces. This category includes smooth mud flats, salt flats and unbroken ice.”
Example: You want to build a cabin by a lake or large river, which is over a mile across.
Keep in mind Exposure D is most often related to water. So if you are more than 5000′ from any large body of water, you are not exposure D. That leaves you to pick from Exposure B or C. If you live in a town or city, with buildings all around – that’s easy – Exposure B.
Also be aware that just because your building is protected from the “prevailing wind” (direction the wind most often or always comes from), but you have even one side “unprotected” in any way…the site is still classified as Exposure C.
You must be protected on all four sides to be classified as exposure B.
Watching your Wallet
Under the International Codes, the difference in load carrying capacity for a building with a mean roof height of 30 feet, between B and C exposures is approximately 20%. These differences can have a significant impact upon your wallet.
On the other hand, if your site is truly a B exposure and the building department wants you to design for C, you may want to defend your position.
When in doubt – stand on your building site and take pictures in all 4 directions or invite the nice folks at the Building Department to see for themselves and make the determination. Better to “do it right” the first time, as renovations (not to mention building fines!) don’t come cheap.
Not to sound like a broken record – but print out the Planning Guide from our website. Ask your Planning and Building Departments to help you fill in the “loads”, prior to getting a quote on a building – and then you will be well prepared for designing your building safely.
Call 866-200-9657 to speak to a Building Designer today!