Wind Uplift Ratings and Single-Ply Roofs

One of the most important considerations in a commercial roofing system is how well the roof will hold up against high winds. This is especially critical in areas that are prone to severe weather, particularly in hurricane “hot zones.” When properly installed, single-ply roofs tend to hold up fairly well in high-wind situations, although some typically perform better than others. This is why developers and roofing professionals need to consider the wind uplift ratings of various roofing systems when determining the best type of roof for a project — especially in areas where high winds are an issue.

What Is a Wind Uplift Rating?

To understand and interpret wind uplift ratings, we must first understand what wind uplift is and how it affects roofing systems. Wind uplift occurs when the wind blows parallel to the roof structure, causing the air pressure above the roof to be lower than the air pressure below. This creates a dynamic of “lift,” where the roof is pressured to separate from the building. The faster the wind blows, the greater the potential wind uplift. (This is almost the same aerodynamic principle that causes airplanes to fly. As wind passes over the wings, the air pressure under the wing gets higher than the pressure above it, lifting the plane into the sky.)

Most roofing systems experience nearly constant wind uplift pressure to some degree since there is rarely a time when there isn’t a breeze outside. As long as the roof is built to withstand this pressure and installed correctly, wind uplift normally causes no problems. However, extreme wind situations can create wind uplift beyond what the roof can withstand, causing roof failure.

This is where wind uplift ratings come in. Manufacturers run a series of tests on their roofing materials to determine how much uplift pressure they can take before they fail. This pressure is typically measured in pounds per square foot (PSF). The wind uplift rating tells you how much uplift pressure the roof can reasonably withstand.

Why Is This Important?

High winds are the greatest natural cause of catastrophic roof failure — and this type of roof failure poses the greatest threat of injury and death to humans taking shelter inside. A roof that isn’t built to withstand high winds is more likely to fail in extreme weather situations. By considering the wind uplift rating of the roofing materials you’re using, you can construct a roofing system designed to handle the maximum anticipated winds of the area in which you’re building, reducing the risks of roof failure.

How Are Wind Uplift Ratings Regulated?

There is no universal system in place for establishing wind uplift ratings. However, several testing standards have become common in the roofing industry, and local building codes often have certain requirements in place for minimum wind uplift ratings based on these standards. Four of the most common standards in use today are:

  • UL 580 —wind test evaluating the general roof structure’s uplift resistance, including panels, fasteners, and substrate.
  • UL 1897 — wind test focused on the roof covering system and its attachment method to the roof deck. (This standard is particularly relevant for single-ply roofing systems.)
  • ASTM E 1592 — a standard primarily used to test the uniform static air pressure difference in metal roof systems.
  • FM 4471 — FM Global is an international insurer for commercial and industrial buildings. Their FM Approvals standards are widely used in measuring many safety protocols, including fire resistance, foot traffic, hail damage, and wind resistance. You can recognize their wind uplift ratings for Class 1 panel roofs as 1-60, 1-90, 1-120, etc., where the second number measures the resistance in pounds per square foot (e.g., 1-90 is rated for 90 PSF).

It’s important to note that regardless of which rating system you’re looking at, none of these test standards are completely foolproof — largely because Mother Nature often deals in surprises. A roof with the strongest wind uplift rating may still become vulnerable in extreme conditions. However, the rating systems provide important guidance so that roofs can be constructed in ways that minimize the risk of damage and failure.

Risk and Exposure Factors That Affect Wind Uplift Ratings

When evaluating wind uplift ratings, it’s important not just to consider the materials you’re using but also the context in which those materials are to be used. Some contingent factors that may affect risk and exposure are:

  • Building height. Taller buildings naturally experience higher winds.
  • Surrounding area and terrain. The presence of neighboring buildings may break wind flow and lessen wind uplift, for example. Buildings in open plains or next to bodies of water may experience higher winds. Hillsides and other terrain features may lessen or worsen wind uplift, depending on the area’s typical wind directions.
  • Structural features of the building. Parapets can reduce wind exposure significantly, while open roofs may be more prone. Irregular geometric designs or features of certain buildings can either lessen or worsen wind uplift. Similarly, open building designs that allow greater airflow can increase wind uplift pressure from inside.

How Do Single-Ply Roofs Perform in High Winds?

The real test for how any type of roof is most likely to perform is in a major high-wind event — like a hurricane. To that end, Building Enclosure compiled some key findings from the Roofing Industry Committee on Weather Issues (RICOWI) as they investigated damage in the wake of three major hurricanes (Charlie, Ivan, and Katrina). The gist of their findings: single-ply membrane systems on low-slope roofs generally held up overall, especially when installed and maintained according to manufacturer specifications. The primary exceptions where damage and roof failure occurred were in cases where installations strayed from manufacturer directives and/or where they weren’t properly fastened.

Challenges and Worst-Case Scenarios for Single-Ply Roofs in High Winds

Despite their general reliability and resistance to wind uplift, here are some vulnerabilities and vulnerabilities in single-ply systems that contractors should watch for:

  • Roof edges and corners. These are the most vulnerable places for single-ply roofs because they are generally the first to encounter high winds. Peeling along the edges can set off a cascading reaction affecting the entire roof. Likewise, corners and eaves are prone to higher levels of wind uplift and should be carefully reinforced.
  • Poorly installed or insufficient fasteners. A lack of stability in mechanical fastening can make the entire roof vulnerable.
  • Inadequate adhesive. In addition to roofing materials with high uplift ratings, be sure to use an adhesive equal to the task and properly applied.
  • Air infiltration below the roof deck. A wind that pushes through openings below the roof deck can increase air pressure underneath. Sealing the deck and the walls are shown to reduce this dynamic.

For best results in selecting the best roofing materials with the right wind uplift ratings for your project, work with a manufacturer representative like RoofSource. Our team of experts can help evaluate your needs and provide options from the most trusted brands to ensure you have the best products for the job. Contact us today to get started.