Selecting products and services can be a difficult proposition. This couldn’t be truer than when selecting construction materials and practices and specifically selecting your new roof solution. Currently on the market, there are tens, if not hundreds, of possible solutions. From traditional long-standing products, like built-up asphalt roofing, to novel new market solutions, such as TPO’s and vegetative roofing, a wide variety of systems, value and performance is available. With more variety, there are more decisions and making the best decision could be the difference between long-term success and short-term failure. Roofing system installation is an expensive proposition and customers need to balance initial cost versus life cycle cost and performance versus economics.
When considering your new roof it’s important to acknowledge a number of factors, including but not limited to:
- Environmental Conditions of the roof: Do they include extreme weather conditions, extensive precipitation, chemical exposure or rooftop traffic?
- Desired service life of the system
- Energy expectations of the solution as a contributing factor to energy savings or as an additional air barrier
- Or potential novel use of the roof as living space or platforms for PV or wind energy
It is important to have detailed discussions and have an extensive planning exercise while making these decisions, because hopefully this is a solution you will be pleased with for the next 30 years.
There are a variety of environmental issues that cause roofs to prematurely fail. This can be classified in a number of ways but for the purposes of this discussion we can look at them in two categories: dynamic and static. We are using these terms loosely, because in some cases static failures involve rooftop movement, but in general they are very slow in nature and involve little impact.
Roofs can dynamically fail from both manmade and natural occurrences, such as equipment drops, hail impact and extreme rooftop traffic. These impacts cannot be underestimated. While masses, angle and speed of impact can vary greatly it is certain that hail (and other types of projectile) damage can work to create tears and rips in rooftop reinforcement. Once tears begin, they have a natural tendency to propagate. Tear propagation is a well understood physical property, this can be evident in simply trying to rip a piece of paper with and without a tear. Once torn – reinforcements can be torn more easily. The use of highly reinforced membrane is a technical fail safe against tear initiation and propagation. By using reinforcing fibers in a matrix configuration, highly reinforced membranes exhibit greater resistance to tear initiation. The cooperative reinforcement of the glass scrim works to eliminate tear propagation by acting as an independent and composite reinforcement of the product. The higher the tensile strength of the reinforcement, the more resistant to impact tear a roof is. Tensile and tear strength properties are critical in all types of roofing systems including modified bitumen products, built-up roofing and single plies.
Static stresses in a roof are managed by the compound coating placed on the reinforcing scrim. Rooftops “statically” expand due to rooftop thermal shifts as a roof cycles from day to night, summer to winter and during precipitation. These cycling movements are accommodated through the use of elastomeric bitumen compounds used on the modified bitumen membrane or polymeric composite formulations in single plies. Each subsequent sheet is welded together to the next through the use of hot asphalt, cold applied adhesives, thermal or radial welding or torching. These welds become the continuous phase of your rooftop matrix and this composite matrix accommodates the static movement. The more flexible the compound the more movement it can accommodate.
In the case of modified bitumen roofing products, the use of thermoplastic elastomers (SBS rubber and similar polymers) flexibility is formulated into the compound. A good way to determine the level of rubber modification is to examine the low temperature flexibility (LTF) of the membrane. The higher the rubber content, the lower the passing LTF. Starting with the lowest possible LTF is critical in maintaining long term rooftop performance as compounds tend to stiffen over time. The thermal window of the roofing compound is defined by the LTF and the compound stability. This window serves as the appropriate window of performance of the roof. Below the LTF the roof may be too stiff to accommodate movement. Above the compound stability, it may be too soft to resist flow. High performance roofing membranes are formulated to have the broadest possible thermal window.
Think of your roof as a continuous composite. The high tensile reinforcement resists impact and tear, the modified polymeric compound accommodates static movement and creates moderate flexibility. One size does not fit all, selecting the proper roof, working with qualified specify professionals and guaranteeing proper details and installation are critical to success. Make a wise choice, not a short term choice. “Nothing comes from Nothing.”