How to Calculate Snow Loads For Metal Roofs

Snow Load Calculations for Effective Snow Retention on Metal Roofs

Snow retention systems are essential in preventing snow from sliding off metal roofs, posing risks to people and property below. Calculating the snow load is a critical step in ensuring that your snow retention system is designed to handle the weight and behavior of accumulated snow. In this guide, we will explore how to calculate snow loads specifically for snow retention systems, ensuring safety and efficiency.

The Importance of Accurate Snow Load Calculations for Snow Retention

Failing to properly calculate snow loads for retention systems can lead to dangerous roof avalanches or system failure. Accurate calculations ensure that snow retention devices are capable of holding the accumulated snow load safely in place, preventing sudden releases and reducing the risk of injury or property damage.

Get Professional Testing for Snow Load Calculations

While basic snow load calculations can be performed manually, the most reliable way to determine snow loads for a snow retention system on your metal roof is to get a snow load test done by an accredited A2LA certified lab. These tests provide the most accurate data, ensuring your retention system is designed to handle the real-world snow loads your roof will face.

Key Factors in Snow Load Calculations for Snow Retention

When designing snow retention systems, the snow load is influenced by multiple factors, including:

• Roof Slope: The steeper the roof, the more likely snow will slide. Calculating snow loads must account for the slope to ensure the retention system holds back the snow.
• Snow Accumulation Patterns: Snow accumulates differently depending on wind, sun exposure, and roof geometry. Drift loads, which occur in areas where snow accumulates unevenly, must be factored in.
• Snow Density: Wet snow is much heavier than dry snow, significantly impacting the load calculations for snow retention systems.

How to Calculate Snow Load for Snow Retention Systems

The goal of snow load calculations in snow retention systems is to determine the force exerted by the snow and ensure that your retention devices, such as snow guards, can withstand these forces. The basic formula for calculating the vector load on a roof due to snow is:

Vector Load = Roof Snow Load × Sine of the Roof Angle

Step-by-Step Snow Load Calculation

1. Determine the Roof Snow Load: The roof snow load is typically expressed in pounds per square foot (psf) and provided by local building codes.
2. Find the Roof Angle: Use a sine chart or trigonometric functions to determine the sine of the roof angle.
3. Calculate the Vector Load: Multiply the snow load by the sine of the roof angle to calculate the vector load, or the load acting parallel to the roof surface.

For example, a roof with a snow load of 58 psf and a slope of 30 degrees (sine of 30º = 0.5) would have a vector load of 29 psf. Over a roof length of 35 feet, the total load at the eave, where snow retention systems are installed, would be approximately 1,015 pounds per linear foot.

Considerations for Snow Retention Calculations

• Friction: Snow retention systems rely on friction to keep snow in place. However, in some conditions, the friction between the roof and snow may be low, increasing the need for robust retention systems.
• Design for Safety: Snow retention systems must be designed with safety factors in mind. The fasteners, spacing, and material strength must all be considered in relation to the calculated snow load.

Types of Snow Retention Systems and Load Calculations

There are two primary types of snow retention systems: continuous snow fences and discontinuous snow guards.

1. Continuous Snow Fences: These systems distribute the snow load across the roof by spanning horizontally across the roof surface. Calculations for snow fences involve ensuring that the fence can withstand the total vector load of the snow blanket.
2. Discontinuous Snow Guards: These smaller devices, placed in patterns across the roof, rely on the snow’s natural cohesion to hold the snow in place. For discontinuous systems, the spacing of the snow guards is critical. More units are typically needed near the eave, where snow loads are highest.

Example Snow Load Calculation for Snow Guards

Using the previous example (roof snow load of 58 psf, slope of 30 degrees), the total vector load per snow guard would need to be calculated. If the calculated vector load is 1,015 pounds per linear foot, and each snow guard has a rated capacity of 180 pounds, you would need approximately six snow guards per panel to distribute the load effectively (1,015 ÷ 180 ≈ 6 guards).

The Best Way to Ensure Accurate Snow Load Calculations

Ensuring your snow retention system can handle the specific snow loads your roof faces is crucial for safety and long-term performance. While understanding how to perform snow load calculations is important, the best way to guarantee accuracy is through professional testing. An accredited A2LA certified lab can provide precise snow load testing, ensuring your snow retention system is designed and installed to meet your specific needs. This level of assurance is key in preventing system failures and ensuring the safety of those around your building.

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Contact us if you are looking for proper snow load calculations.

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