Riku Ylipelkonen, Owner, Standard Building Advisors and FRSA Technical Advisor
One of the biggest changes coming in the 2023 Florida Building Code will be the addition of design pressure performance reporting for underlayment assemblies. Steep-slope roofing assemblies require special consideration because they are not currently tested as completed assemblies. The underlayment assemblies are separate, usually from the roof covering. On low-slope assemblies, the combinations are all tested or substantiated as they are published. There is testing and analysis to substantiate each combination on the product approvals. If you have seen one from a large manufacturer, you know these can get cumbersome. Imagine having to coordinate and assemble testing to approve all those different assemblies. This is where the logic comes into steep-slope roofing assemblies. For a manufacturer to get an approval for an underlayment product, there are a battery of physical properties testing, roof covering specific tests and quality assurance testing or certification. Can you imagine how complicated the test program gets if you have to test each tile profile and color individually to make sure they work together? What if you have to add in the attachment method of the tile as a variable? What if you want to use the same assembly under metal options and shingles also? The combinations of assemblies would climb into the hundreds very quickly and would be too costly for a manufacturer to market the product.
Shingle assemblies have some interesting challenges for waterproofing underlayment to contend with. The sheet can be mechanically attached or adhered, bituminous or synthetic and, outside of Florida, the product must also contend with freezing and water damming. In my previous article (Product Approvals, FRM August 2023), we learned that the wind uplift causes a suction force on the roof deck and roof covering assembly. In that condition, you worry about shingles tearing free from the mechanical fastening or the fasteners withdrawing from the deck, releasing the shingles. Those actions cause concern for the
underlayment system. The concept of the secondary water barrier is to counter this kind of damage. This is where self-adhered underlayments add value to shingles. Keeping adhesion to the plywood deck and sealing around existing nail penetrations keeps the water out of the building. This helps maintain life safety and minimizes any water damage or other structural damage to the building interior and contents. While some may argue that this is above and beyond “code minimum” and its intent, the reduction of insurance storm claims that are funded by Florida citizens helps reduce costs for all participants.
Metal roof covers experience similar conditions to shingles with another glaring consideration: HEAT Metal is a good conductor of energy. This means that the sun warms the metal panels on a roof every day and then the panels cool during the night. This heat cycling is more severe for metal roofs, especially if darker colors are used for the metal panels. Heat cycling is also a concern for the underlayment. Materials must be optimized to handle high heat cycling with resistance to degradation at the higher end of the temperature scale. The Florida environment, with high heat and humidity and higher severity of heat cycling, makes choosing the right building materials a real challenge. These are all challenges when you start specifying synthetic underlayments in the Florida environment, especially under a metal roof.
Tile roof covers add another layer of complexity to consider. The addition of the design pressure testing is to ensure the underlayment system performs through the wind event, maintaining life safety of building occupants and, hopefully, maintaining the integrity of the waterproofing to keep the contents of the building intact and undamaged. The tile fastening
method is a consideration when choosing the underlayment assembly for the project. Using mechanical fasteners is common with shingles and metal roofs but tile can also be adhered with mortar or foam adhesive. If the tile is to be installed with adhesives, the underlayment system must be compatible with the adhesive selected and the underlayment system must withstand the wind uplift forces from the wind itself AND the transfer of those forces from the tile. The ideal failure point for an adhered tile is for the tile adhesive to break bond and release the tile. This way, the waterproofing of the building is safe with no failure of the underlayment system. In other words, if your roof experiences a wind event where the tile fails, you want the underlayment to maintain the life safety of the occupants and, ideally, minimize the water damage inside the building envelope.
There are assemblies, like cedar shake, slate tile and others that have specific considerations as well. The important thing to keep in mind is that the underlayment assembly should perform with the roof covering that is installed on top of it.
Riku Ylipelkonen, Owner, Standard Building Advisors has been in the roofing industry for 15 years working for Polyfoam Products. 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) and the Single Ply Roofing Industry (SPRI).
Previous Article