Being located in the Midwest, Mother Nature brings us all types of unpredictable weather, especially when it comes to the winter months. It’s not uncommon for us to receive a couple of inches of snow one day and then the next day it’s sunny with a high of 47 degrees.
What’s also true about Midwest living is that we are capable of getting several inches of snow within a matter of hours. Do you remember the blizzard of 2014? That’s a perfect example of a snowstorm that started in the Midwest and worked its way east...dropping over 18” of snow along its way (in some areas it was much worse).
We all recognize that when recent snow is met with howling winds drifting often occurs; we are accustomed to seeing snowdrifts in predictable places across roads or through our yards. However, did you know that drifting occurs in predictable patterns on the roofs of post frame buildings? Recognize that if your pole building wasn’t designed for the proper snow loads, and potential drift loads, your building is more susceptible to failure.
I’m sure the last thing that you want to have happened is the roof collapse on your brand new farm shop or pole barn garage. Not only would that be dangerous for you, but it would cause damage to the building’s contents and cause a disruption to the use of the facility.
If you happen to live in one of the U.S. states that receives the most snow, you know first-hand just how heavy it can be. Your pole barn snow load will vary based on the region you live in and the anticipated snowfall for that region.
Before we dive into the importance of snow load, it’s equally important to understand what snow load is.
What is Snow Load?
The term snow load is the live load due to the weight of snow on a roof, which is included in the design calculations. It’s also best described as the downward force on a building’s roof by the accumulated snow and ice. Engineers use snow loads to help estimate the overall support a building’s roof needs to withstand a region’s snowfall.
The roof snow load is calculated from the ground snow load. The ground snow load is obtained directly from the American Society of Civil Engineers Standard (ASCE 7-10). This is the standard that engineers follow when designing the building structure.
As mentioned above, each region has a different ground snow load. For example, the ground snow load for northern Minnesota is much higher than the ground snow load for the sunshine state of Florida. Regions that experience abnormal weather patterns, mountain ranges or lake-effect areas, are treated uniquely. These regions are defined as “case study areas.”
A case study area doesn’t have a defined snow load rating because of the inconsistency in the weather. If you choose to put up a post frame structure in one of those areas, it’s important to find a post frame builder that can ensure the ground snow load for your building is accurately defined.
Why Is It Important to Choose a Post Frame Builder with an In-House Engineering Team?
An in-house engineering staff will give you a better product because they factor the snow load rating for you. They also take into consideration the structural integrity of your entire structure.
Structural integrity refers to a pole barn’s ability to withstand anticipated structural loads without breaking. Any post frame building, regardless of size and location, must be designed to resist vertical and lateral loads safely.
First, vertical loads act in the up-and-down direction. Popular applications of vertical loads include:
- Lofts
- Deck systems
- Second stories
- Snow on top of the building
- Dead load (the weight of the building itself)
Second, lateral loads act in a direction parallel to the ground, such as strong winds and seismic activity (earthquakes). The pole barn must be designed to withstand loads, both vertical and lateral loads.
All structural loads must be able to pass from the areas in which the load is applied. This transfer of forces through the post frame structure is called the load path. A continuous load path transfers all vertical and lateral loads from one building component to another until they reach the ground.
If the load path flows continuously without interruption, there will be no structural damage. However, one “weak link” is all it takes to compromise the structural integrity of your pole barn. Therefore, each building component must be strong enough to carry out these anticipated design loads.
What Factors Are Used to Calculate the Snow Load for Your Pole Barn?
There are five main factors used to calculate the snow load for your building. Those five factors are:
- Exposure
- Thermal (Temperature)
- Importance
- Ground Snow Load
- Roof Slope/Shape
Each factor plays an important role in the design of your building.
1) Exposure Factor
The exposure factor considers the ease at which snow can be removed from the roof of the building. For example, a pole building built in the middle of a woods is more protected than a building in the middle of a cornfield. Therefore, the building in the middle of the woods will keep the accumulated snow load on its roof for a longer duration of time relative to the building in the middle of a cornfield.
2) Thermal (Temperature)
The thermal or temperature factor considers the ability of the internal temperature of the building to melt the roof snow load. As we know, heat rises when it escapes a building. A good example of the thermal factor is comparing a greenhouse to an insulated farm shop.
The roof of a greenhouse is so warm that the snow will melt and slide off. However, the ceiling insulation in an insulated farm shop prevents the warm air internally from reaching the roof, which in turn allows snow to accumulate.
Have you ever noticed icicles hanging from the roof of your building? Sure, they might look neat, because of the different shapes and sizes they may form, but their beauty is deceiving. When icicles form, it’s a sign of a poorly installed thermal package, it shows that heat is escaping the attic and melting the snow.
3) Importance
The simplest way to explain the importance factor is by considering how essential the structure is during catastrophic events. Let’s compare your local fire station to your tool shed. In this example, one building houses the community’s emergency vehicles while the other building contains your lawnmower and other outdoor equipment.
When an expected weather event occurs, which building is of more importance? That’s easy, we’d all say the fire station. The ASCE 7 standard defines a higher importance factor for the more essential structures.
4) Ground Snow Load
As explained previously, the ground snow load is obtained directly from the ASCE standard. This is the standard that engineers follow when designing the building structures.
5) Roof Slope/Shape
Roof slope/shape will also affect the snow load. A steeply pitched roof will accumulate less snow in comparison to a shallow pitch roof.
For example, you build a small hobby shop in your backyard with a 12/12 roof pitch and your neighbor has a shed with a 2/12 roof pitch. Both structures will receive essentially the same snow load from any given snowfall, however, your building, with a 12/12 roof pitch, will accumulate less snow in comparison to your neighbors building with a 2/12 roof pitch.
Roofing materials should also be considered when calculating your roof snow load. This is also described as the “slippery surface.” If you choose shingles for your roof, they are going to hold snow longer than what a metal roof would. In summary, the roof material and shape of the roof affect the ability of the snow to move.
How Do Pole Barn Trusses Affect Snow Load Rating?
Now that you have a good understanding of the factors that are used to determine your snow load, it’s also important to understand the role of your building trusses.
Your engineer will also consider these four variables when designing your trusses:
- Sliding Snow
- Drifting Snow
- Unbalanced Snow
- Step Downs/Building Connections
Based on your building’s geometry and its location relative to other structures, it’s possible for any of these four things to occur. Larger loads will occur in regions that experience these loading scenarios. To help prevent structural damage, the trusses will either be closer together, stronger as individual components, or the purlins spacing will be decreased to accommodate the extra load.
1) Sliding Snow
This occurs when snow slides off an upper roof and lands on a lower roof. A step-down porch on the sidewall of a post-frame building is an example of a roof system that would experience sliding snow off the main building roof.
2) Drifting Snow
Drifting snow loads occur on a building when the snow lands on the roof and is then relocated to drift up against another building, a parapet, or a façade. In most cases, this occurs where a step-down condition exists. Office buildings on the front of a large warehouse and porches/lean-to’s are two common examples.
3) Unbalanced Snow
Unbalanced snow occurs along the ridge of a building structure. Unbalanced snow loads are generated when snow from one side of the ridge is relocated to the opposite side of the ridge. This area, the unbalanced zone, needs to accommodate these potential loads.
4) Step Downs/Building Connections
Are you looking to add square footage to an existing building? Is your desired addition taller than your existing building? Creating a step-down situation while adding onto an existing building generates drifting snow loads, and potentially sliding snow loads, that need to be considered.
If you anticipate adding onto your building in the future, plan ahead! Let your project sales consultant know your long-term plans from the beginning.
Are You Ready to Take the Next Steps with Your Post Frame Building?
As a post frame builder who’s been around since 1958, we want to make sure that our customers are educated before they make a buying decision. Often times, it’s the structural issues (like snow loads) that are often missed. It’s not a natural topic that we think about when choosing a post frame builder.
Our recommendation to you is to make sure you thoroughly understand the importance of structural integrity (at all levels) before you choose a post frame builder.
Do you have more questions that are not covered in this article? If you need help designing and planning, please contact FBi Buildings at 800.552.2981 or click here to email us. If you are ready to get a price, click here to request a quote and a member of our customer engagement team will help you determine the next steps of your project.