Riku Ylipelkonen, Owner, Standard Building Advisors and FRSA Technical Advisor
As building code revisions are being implemented in Florida, the number of comments on wind uplift testing seems to be increasing. With more attention focused on the performance requirements of roofing materials, there is a desire for product testing to correlate better with the actual behavior during a wind event. Wind uplift behavior and test standard development, has been a long understood and applied behavior. A considerable amount of empirical wind tunnel testing applied to the ASCE 7-22 design standard was conducted in the 1970s. Even back then, we were learning how wind events affect buildings and their components. The development of wind uplift testing in the years following have used the latest technology, new materials and new installation methods to maximize the correlation between cost-effective test standard results and actual behavior of tested roof components in wind events.
Air is a predictable fluid. We have extensive scientific data and an understanding of air as it speeds up, turns, carries moisture and all the other variables considered in a hurricane. As this understanding has developed, vacuum chamber testing has evolved to mimic wind performance for non-permeable sheets like steep-slope underlayments and low-slope roofing assemblies. When wind creates air movement, air moving from one place to another will create a vacuum in the space the air left, causing new air to move into its place. With current steep-slope underlayment layers, low-slope single-ply membranes, liquid-applied coatings and built-up roofs, there is a consistent layer of material creating a membrane that will not allow air to pass through it. This is handy for waterproofing, but also a consideration for wind uplift. Since the material doesn’t allow air to flow through it, the air on top of the membrane that moves out of its original position must now be replaced by air next to it on top of the membrane. If you think of air as these moving cube units, the void left behind creates a vacuum because air wants to always be in all available cubes. The faster the air cubes move, the stronger the vacuum force is that wants air to move into the vacated space. This is how vacuum cleaners work. Vacuum cleaners have a spinning motor that sucks air from the vacuum cleaning head. This resulting low pressure picks up lightweight dust and dirt and hurls it through a filter to capture the dirt and let the air cycle back into place. The same thing happens to these waterproofing membranes during a wind event. Since the air can’t get through the membrane, the moving wind creates a vacuum above the roof that wants to suck the membrane off the roof. The main reason the membrane doesn’t get sucked into the moving air stream is the resistance of the application method to release the membrane into the moving air stream. Having described all this, the design of the vacuum chamber test method makes sense for air impermeable membrane assemblies. However, this kind of test isn’t applicable when you have roof assemblies that allow air to pass through the assembly, like shingles or tile.
When you have a completed shingle or tile roof, you can’t really apply a vacuum chamber test apparatus to this roof because the tiles and shingles do not form a seal preventing air movement. Air gaps occur not only between courses of shingles and tile because of the stacking shapes of shingles and tile themselves, but the entire roof assembly allows air between the gaps of shingle pieces and interlocks of the tiles. Because of this, mechanical testing is more reproducible and consistent to predict the performance of shingle and tile roof components. This is why the lap sealant on shingles is critical to performance of shingle roofs and tile attachment methods become the critical test method for tile uplift performance. Tile uplift test methods have been developed to mimic the air pressure exerted on the specific exposed area of the individual tile. Developmental testing showed that tiles will pivot at the top of the tile by the lugs or top corner of the underside surface of the tile when wind uplift forces are applied. Since we know the size of the tile, we can use mathematical formulas to interpret a mechanical uplift force from a single point on a tile from the wind uplift forces created during a wind event. The location of the mechanical pull is dependent on the size of the tile and the test is performed on a single tile because each component must perform to the tested level to consider the entire assembly at the same performance level. Variables like weight of the tile and slope of the roof are factored in because they affect the performance of the roof. Of note, the uplift resistance the tile experiences from the interlocked tile next to it is not considered because if the test tile is experiencing an uplift force during a wind event, so is the tile next to it and they are not able to keep other tiles in place when they are themselves being displaced.
Considering all this, the current test methods in place have been under development for as long, or longer, than the building codes themselves. They are acceptable methods of testing roof materials and have a high degree of certainty to predict the performance of roofing materials on the roof. Are there better methods possible? Of course. But we don’t use micrometers to measure our roofs because tape measures get the job done in a cost-effective and timely manner.
Riku Ylipelkonen, Owner, Standard Building Advisors has been in the roofing industry for 15 years working for Polyfoam Products. When Polyfoam Products was acquired by 3M and the name changed to ICP Building Solutions Group. Riku worked at ICP as Technical Services Manager until March of 2023, when he left to begin his own company. Riku is an engineer and is working as a consultant with FRSA. He is a member on FRSA’s Codes Committee, Codes Subcommittee, Tile Committee and on the FRSA-TRI Manual Rewrite Committee. Riku is also a member of the American Society of Civil Engineers (ASCE).
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