How Much Snow Can My Roof Hold?

Posted by on Mar 23, 2018 in Housing | No Comments

How much snow can my roof hold? Structural designs for snow loads blog post

Prior to 1979, the Ohio Building Code required a snow load of 25 pounds per sq ft (psf) and had a comment to take drifted snow into account, but didn’t say how. In general this was adequate; however, I’ve been called to investigate structures in which the code requirement wasn’t adequate, including a large one-story office and warehouse building experiencing roof sagging after a heavy snowfall. When snow blew across the warehouse roof and landed on the adjacent office roof that was about 6 ft lower, it overloaded the lower roof framing, causing a weld to break in the bar joists and the roof to deflect. There was so much drifted snow it was nearly piled as high as the 6 ft difference between the roofs, and was compacted so tightly that I could walk up the drift to the upper level, sinking only about 1 ft with each step. It was clear that the code was not sufficient.

How Snow Loads Were Calculated

In 1979, Ohio adopted the BOCA (Building and Code Officials) National Building Code. It included provisions for designing for drifted snow with the amount of drift dependent on the height difference in roof surfaces, or the height of the parapet or other obstruction. We now know that the amount of drift is dependent on the length of the fetch of the roof surface (the fetch is the length of roof that snow can blow along before reaching the obstruction or drop in the roof surface) rather than the height of the obstruction or the difference in roof heights. Thus, when the BOCA code was used, it produced conservative values for small buildings with four ft high parapets or four ft changes in roof level, but still had very unconservative values for large roofs with a long fetch.

How to Structurally Design for Snow Loads Today

Today’s International Building Code, which Ohio has adopted, uses snow loads from American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-10). The actual design incorporates a number of variables:

  • The amount of ground snow for the geographic area
  • The importance of a building (fire departments, hospitals and other important buildings require a larger load than a normal office building or store)
  • Whether the building is heated, which allows for some heat to extend up through the roof and melt some of the snow
  • A reduction from the ground snow to account for a certain amount of snow to blow off the structure
  • The effect of drifting snow which is based on the length of the fetch
  • Sliding snow from a sloped roof
  • Imbalanced snow loads (variations in the amount of snow)

For a typical building in most of Ohio, the ground snow is 20 psf; allowing for the snow that blows off, the actual design load can typically be reduced to 14 psf. This is considerably less than the 25 psf that the old codes required.

Snow Loads on Commercial Additions

While most designers remember to take drifted snow into account on new structures, it is frequently overlooked when adding taller additions to existing buildings.

Adding a taller addition that extends out 200 ft from an existing building can create an enormous load on the existing lower roof. The drifted snow would be over 5 ft high, weigh 87 psf at the step in the two roofs and extend out 17 ft (see Figure 1). With fire sprinkler lines and electrical conduits in the way, it is usually very difficult and expensive to reinforce an existing bar joist framed roof to support this additional load on the original roof.

Structural designs for snow loads figure 1, copyright Schaefer 2018

We have used two methods to solve this problem. The easiest is to add a low section of roof the width of the drift, adjacent to the existing building and design this low roof to carry the weight of the drifted snow (see Figure 2). This has the benefit of providing the owner with more usable space at less than the cost of reinforcing the existing roof to carry the drifted snow.

Structural designs for snow loads figure 2, copyright Schaefer 2018

The second method is to design a low cantilevered roof with the capacity to carry the drift extending out over the lower existing roof on the side of the addition (see Figure 3). This is more expensive than the first option, but may be needed if there is a lack of space to add the additional section of low roof.

Structural designs for snow loads figure 3, copyright Schaefer 2018

The code requires that the roof be designed for drifted snow where obstructions on the roof are wider than 15 ft. This includes HVAC equipment and facades. We see many existing buildings that have large facades added above the entrance. If these are longer than 15 ft along the wall and extend up more than a few feet, the existing roof must be designed for the additional snow drift.

How Much Snow Can My Residential Roof Hold?

The Residential Code of Ohio, which applies to one and two-family homes, shows a map indicating the required snow load. It calls for 20 psf throughout most of Ohio, and 25 psf along a north south strip in the eastern half of the state. The 20 psf load is equivalent to 14 inches of snow at the design density while the 25 psf load is 17 inches. There is a special zone in the northeast corner of the state where they get lake effect snow. The code says to see local conditions in these special zones. The building departments there say to use a 30 psf snow load. The residential code does not require designing for drifted snow, but since most homes aren’t very large, there is not a significant length of fetch to accumulate much drift. We’ve found that some building departments in Greater Cincinnati want residences designed for a 25 psf snow load (even though the code says 20 psf) so the engineers in our office use a 25 psf snow load for most homes (unless the home is located in a special zone or has a very steep roof slope, so the snow slides off).

While sometimes it seems that the snow outside will never let up, structural engineers design for snowfall in your location. If you have concerns about snow and ice weight on a structural system, call a structural engineer to investigate.

Leave a Reply