Understanding Hurricane Window Ratings

Living in paradise comes with challenges, and for Broward & Palm Beach County residents, that challenge includes being prepared for hurricane season. While securing your home and stocking up on supplies are crucial, one often overlooked element is your windows. Standard windows can shatter from high winds and flying debris, putting your family and property at risk. ​​​​​​

This is where hurricane-rated windows come in. These specially designed windows offer an extra layer of protection against the destructive forces of hurricanes. But with various ratings and options available, choosing the right ones can be confusing. ​​​

This guide will help you navigate the world of hurricane window ratings and make informed decisions to keep your South Florida home safe and secure.

Why Do Hurricane Window Ratings Matter?

Traditional windows simply aren’t built to withstand hurricane-force winds and debris. When they break, they create openings that allow wind and rain to enter your home. Once this occurs, a few negative consequences arise from not having hurricane-rated windows installed.

Structural Damage

When a home or business is built, the structural strength of the building comes from its design as a complete unit. The roof and walls are simultaneously supporting one another. In a hurricane-force windstorm, the wind pressure can be so severe that it will tear off roofs and cause the walls to collapse.

Water Damage

Living in a tropical climate means that there is plenty of rain. When that rain is accompanied by high wind velocities, the damage is severe. Wind-driven rain can flood your home. The resultant dampness, if not taken care of, can cause structural weakness of wooden materials as well as mold growth. The repair work required for water damage is always far-reaching and expensive.

Debris Injuries

Unfortunately, human and animal injuries often occur from hurricanes. The flying debris whether from external or internal sources can cause a lot of damage. One of the worst injuries arises from standard window glass that’s shattered by the storm winds.

Hurricane-rated Windows

Hurricane-rated windows, on the other hand, are built to exceed the building code requirements for South Florida. This is impressive as the building codes for window ratings in South Florida are some of the most stringent. These windows undergo rigorous testing to ensure they can withstand high wind pressures, the impact of flying debris, and water infiltration.

High Wind Pressures

For impact windows and doors the wind pressure unit for measurement is pounds per square foot (psf). These ratings indicate the window’s ability to resist wind loads without breaking or deforming.

Flying Debris Impact

While researching impact windows and doors you may come across the term “large missile rated”. Hurricane-rated windows are tested with projectiles that simulate wind-borne objects, such as a piece of 2×4 lumber, to ensure they remain intact.

Water Infiltration

Even hurricane-rated window glass can crack – the wind pressure is that overpowering. However, when this does inevitably happen, the window system should prevent water from entering your home.

By investing in hurricane-rated windows, you gain peace of mind knowing your home is better protected and you and your family are safer during a storm.

Understanding the Ratings: DP, PG, and Missile Levels

When shopping for hurricane windows, you’ll encounter two main rating systems on the box, namely DP and PG. Let’s look a bit closer to understand a little more about what they are and what impact they have on your decision-making process.

Design Pressure (DP) Rating

This number specifies the maximum wind pressure the window can withstand in pounds per square foot (psf). There are two types of pressure, positive and negative. Positive pressure is the direct pressure from the wind pushing onto the surface. The more destructive pressure when it comes to buildings is the negative pressure or suction. This is caused by the low pressure created when the wind blows over a sloped roof. For Broward, Palm Beach, and Martin Counties, building codes typically require DP ratings based on your specific location and wind zone. For example, Miami-Dade County requires DP 50 in most areas, while coastal Martin County might require DP 70. The DP rating is calculated by considering the following factors:

• Design wind speed for the area
• The terrain surrounding the building site – this is the Exposure Category
• Building height
• Window size
• Location of the window on the building

Once these factors are known, architects and contractors can determine the required DP rating for each window.

There are two tables in the Florida Building Code, 7th Edition (2020), namely tables R301.2(2) and R301.2(3) which provide matrices that allow the calculation of the required DP rating. The matrix in table R301.2(2) contains both positive and negative pressure values. Let’s have a look at a quick example:

We have a home located in Harbor Island, Martin County right along the coast. The building has a mean roof height of 20 feet and the window is located near the corner of the wall. If the area has a design wind speed of 160mph, what would the required DP rating for the window be if the effective wind area of the window was 20 square feet?

Solution:

• Design Wind speed for the area = 160mph
• Terrain surrounding the area = Coastal suburban = Exposure Category D
• Building height = mean roof height of 20 feet
• Window size = effective wind area 20 square feet
• Window location = Corner of building = Category 5

If we use the information that we have collected and fetch the relevant data from table R301.2(2) then we’ll find the positive and negative pressure ratings are 26.4 and -34.5 respectively. We can now apply the multiplier for Exposure Category D for a mean roof height of 20 feet and we’ll see that it is 1.55. The Design Pressure (DP) rating for this scenario would therefore be:

1.55 x 26.4 = 40.92

1.55 x -34.5 = -53.47

Based on our answer, the best fit would be a window with a rating of DP55 or greater.

Performance Grade (PG) Rating

This rating looks at three factors and all must pass for it to meet the Performance Grade (PG) classification. The three factors considered in the PG Rating are wind, water, and structure. Generally, a performance grade rating correlates well with the DP rating. But where PG goes a step further is that it also accounts for water penetration. The rule of thumb here is that if a window has a DP50 rating then the PG50 rating would mean that if the window can stand up to 50 psf of wind force then it could withstand between 15 and 20% of water penetration force. That means a product with a PG50 rating would have resisted water infiltration up to 10 psf in testing.

Missile Level (ML) Ratings

Missile Level ratings further specify the size and type of debris the window can resist. In South Florida, ML C is common for most inland areas, while coastal zones might require ML D or even ML E. The ML ratings allow for size, weight, and impact velocity:

• A = 0.07 (~5%) oz steel ball at 130 feet per second
• B = 2.0 (~0.25) lb 2” x 4” lumber at 50 feet per second
• C = 4.5 (~0.25) lb 2” x 4” lumber at 40 feet per second
• D = 9.0 (~0.25) lb 2” x 4” lumber at 50 feet per second
• E = 9.0 (~0.25) lb 2” x 4” lumber at 80 feet per second

Further Considerations Regarding Ratings

Now that we have a bit of a technical background for how hurricane window ratings work and are calculated, these additional factors can be good to keep in mind when upgrading or installing impact windows.

Materials Used

Laminated glass, laminated vinyl, and impact-resistant acrylic are common options, each with different strengths and weaknesses. When it comes to frame material aluminum and vinyl are popular choices as both offer durability and insulation.

Professional installation

Lastly and probably most importantly, you really should ensure that you get your hurricane-rated windows installed by a reputable local installer for impact windows and doors. Improper installation can negate the benefits of rated windows.

Protecting Your South Florida Home: STS Impact Windows, Doors & Roofing

For over 20 years we have been serving the contractors, home and business owners of South Florida with extreme attention to detail and unwavering personal service. We strive to provide the finest quality products and ensure that our customers are 100% satisfied. Give us a call at 561-600-1018 to get your hurricane-rated windows and doors installed or upgraded.