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The system is fully bonded to the substrate, which eliminates issues with billowing, makes it less prone to condensation under the membrane and makes it more energy efficient.Ī fully adhered membrane is considered an air barrier, which is required on the roof per 2012 IECC. The substrate under the membrane is either a cover board or the top layer of insulation.įully adhered systems cost a premium and offer several advantages over their mechanically attached counterparts. Fully adheredįully adhered systems are bonded to the substrate using a contact adhesive applied to both the membrane and the substrate. If a mechanically attached roof system is being considered, it is recommended that an air barrier be added below the insulation layer, which will improve energy efficiency and reduce condensation due to the billowing of the membrane. On projects where the interior humidity is low and energy efficiency is not critical, an owner may consider a mechanically attached system. Mechanically attached systems are installed frequently with varying degrees of success. Typically, mechanically attached roof systems are selected because of the lower cost or when the project needs the roof to be installed in low temperatures, understanding the requirements to keep the membrane warm prior to installation. According to the Steel Deck Institute (SDI) position statement, Attachment of Roof Membranes to Steel Deck, "Under uplift conditions, the attachment of the roofing membrane along lines with large spacing could produce localized loads that can exceed the capacity of the deck, whereas those same loads applied uniformly on the surface of the deck would be acceptable." SDI recommends that membrane seams should always run perpendicular to the flutes, since the loads are much more significant if run parallel to the flutes.įor mechanically attached systems, the structural engineer should always verify that the structure can accommodate increased loads. Lastly, mechanically attached roof membranes put significantly more stress on the building structure since they are only attached at the seams. (Indiana still adheres to the 2009 IECC.) The billowing roof membrane can also create a sound issue for the owner as it flaps up and down.īecause of the billowing effect, a mechanically attached roof membrane does not count as an air barrier a separate air barrier would need to be installed below the membrane (likely below the insulation layer as well).
VAPOR BARRIER UNDERLAYMENT CODE
For states that have adopted the 2012, or later, International Energy Conservation Code (IECC) an air barrier is required on the roof and walls. (For more information, see the Oak Ridge National Laboratory 2013 study: Condensation Risk of Mechanically Attached Roof Systems in Cold Climate Zones).īeyond that, the billowing action is not energy efficient because the building loses conditioned air above the roof membrane. Since the membranes are typically white, they do not gain enough heat to warm the top layer of insulation and drive the moisture back down into the building. Over time, this can wet the insulation which reduces the R-value and can lead to mold. In the winter, this warm conditioned air cools and can condense on the underside of the cold membrane. There are two primary options for attaching the TPO membrane: mechanically attached and fully adhered.
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For example, if the owner expects workers to routinely service rooftop mechanical units, a thicker roof membrane should be considered. The thicker the membrane, the better the long term durability. In Indiana, we typically see 45 mil and 60 mil specified. TPO is typically sold in thicknesses of 45 mil, 60 mil and 90 mil.
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No matter which option is chosen below the membrane, this is how all TPO roofs are sealed at the seams. While it is still plastic, a roller is used to fuse the top and bottom membranes together, forming the watertight seam, typically 1 ½" wide if done by robot or 2" wide if done by hand. A hot air welder is used to heat the membrane above and below, turning it to plastic and making it moldable. Thermoplastic means that it becomes plastic upon heating and hardens upon cooling, and this is how it seals at the seams.