Skylight structural load testing provides proof that a skylight can withstand extreme weather. These tests simulate the environmental forces exerted by high winds, heavy rain, and snow accumulation to create positive and negative pressure loads on a skylight or roof window. Testing these products at this level ensures they can protect people and prevent costly water damage.
Positive design pressure testing simulates the effect of live loads on a skylight. A live load is any temporary weight on a skylight. Snow and ice are the most common live loads that can affect skylights. One square foot of wet snow can weigh up to 21 pounds. Areas like Lake Tahoe, CA and Buffalo, NY see 6 to 8-foot snow drifts. The snow loads on skylights range from 126 to 168 psf (pounds per square foot). This kind of weight can cause glass to flex inward. This adds stress to the sealant and surrounding skylight sash and frame. So, a positive design pressure test determines how much weight a skylight can hold before the seals or glass fail.
Positive design pressure testing follows ASTM E330 testing guidelines. First, the skylight is mounted on a frame in an airtight test chamber. Then, downward force is applied via air pressure. The test begins by applying half of the target positive test load. The load is then increased to the target positive test load. Finally, twice the amount of the positive test load is applied. The target load is predetermined based on the components of the skylight being tested. Applying the positive test load can be completed in a few minutes, but a full certification test sequence can take 1 to 2 hours per product.
Certified positive load ratings, or what is published on the product label, are required to have a 2 times safety factor. This means that if a product is certified to hold 100 psf, it has actually been tested at 200 psf. This additional safety factor ensures homeowners and builders that VELUX skylights will successfully perform under extreme weather.
Negative design pressure tests simulate wind blowing across the skylight. Positive design pressure tests the force applied to a skylight. Negative design pressure tests the force as it tries to lift the skylight. In high-wind hurricane zones, like Miami, FL, these kinds of winds can blow up to 165 miles per hour. This kind of pressure can cause the glass of skylights and windows to flex outward, adding stress to the skylight frame.
A negative design pressure test also starts by mounting the skylight in an airtight test chamber. Pressure is then applied to the inside of the skylight. This simulates the suction that is created by hurricane winds when blowing across skylights. The test begins with half of the target negative test load. After a minute, the target test load is applied to the skylight. Finally, 1.5 times the amount of target negative test load is applied. The testing cycle for each target load is only a few minutes. The target negative test load is predetermined based on the components of the skylight being tested. The entire negative design pressure test can take 1 to 2 hours to complete per product.
Certified negative load ratings, or what is published on the product label, are required to have a 1.5 times safety factor. This means that a skylight with a certified negative pressure rating of 67 psf was actually tested at 100 psf.
Skylight product testing is critical. The information helps contractors, architects, and building code officials determine what’s required to install skylights and Sun Tunnel® skylights in which part of the country. This information also ensures that VELUX products are installed in the correct position on the roof. Local building codes dictate which skylights can be used on homes and commercial buildings. VELUX ensures the skylights available can meet all the local building standards through rigorous testing and certification.
Having the right structural design specifications is just one step in the building process. VELUX can help with some of the other steps. We have CAD drawings and BIM objects for the design phase. Product specifications are available to ensure the correct sizing and dimensions. There are also testing certifications available to submit with projects. All of these are free to download.
For roofers and remodelers looking to improve their skylight installation skills, VELUX offers free training courses. We have in-academy training at our Pro Partner Academies that also includes business development classes. On-site training is available in select areas via mobile training. There are also several skylight installation videos available on our Pro website.
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