Most everyone who has used a bathtub caulk has experience using silicones. And everyone who has a car with rubber bumpers and tires knows the virtues of rubber with weathering. Both are excellent – for their own purposes. But which has better properties for roofing?
Let’s begin with metal roofing. Both products have nearly identical moisture absorption standards for days on end in ASTM testing with this type roofing which always slopes. They allow almost zero absorption for durations of weather. This means excellent protection against rust and corrosion. Both also provide excellent protection against UV and other elements. However, that’s where their similarities end.
Topps Seal® is preferred for metal because of far superior physical features like elongation and tensile strength. Elongation of Topps Seal for example is 900%. The best quality silicones typically offer about 200 – 250% elongation. Elongation shows the ability to move with a roof and stay unbroken. Metal roof panels can experience more movement. Under this scenario it means Topps Seal is at least 4x more capable in terms of performance when it comes to the additional elongation your roof may need.
Ultimate tensile strength also is a highly regarded property for which elastomeric roof coatings are considered. ASTM and all other norms will measure and require certain minimums. Silicone coatings generally provide strength only in the neighborhood of 300 – 700 psi (~2 – 5 N/mm2). Whereas standard grade Topps Seal can provide upwards of 1500+ psi (~12 N/mm2). Ultimate tensile strength determines a coatings’ resistance to failure when put under stress. Again, another highly revered quality that’s especially valuable with roofing built of metal panels. What’s more, property retention over years of service stays greater for better outcomes.
100% rubber Topps Seal is a better option for this roof type for various other reasons. The silicone coating presents a particularly hazardous situation on every sloped roof when the slightest amount of moisture is present. To their detriment as we all have experienced they are extremely slippery when wet. Residual rain drops, fog, dew, or even high humidity makes those roofs an even more extreme safety hazard for any rooftop equipment repairman, plumber, roofer or other who has to get up on the sloped roof. Sooner or later it is required. For similar reasons including toughness, more athletic shoe soles are constructed with rubber, not silicone.
While a 100% silicone coating may provide all that’s necessary in some circumstances, their use on metal comes at a considerable expense. They generally are costly. Research shows one can expect to pay nearly double for a good quality silicone alternative compared to the Topps® all rubber option which offers more. This is before factoring in that the physical properties may fall short of your roof’s needs in the first place. Why pay more for a silicone that provides less?
Flat roofs present a better option for silicones, particularly if they hold water for longer periods. The construction of those roofs, not being multiple panels or having much slope, reduces the safety risk considerably, and the potential for leaks associated with roof movement. To the silicone coating’s credit, they can be useful with roofs holding water for extended periods. They also can boast a very high moisture vapor transmission, suggesting trapped moisture beneath plies might escape it. Yet one must consider that silicone products used on bitumen typically require an epoxy primer. Epoxy allows zero vapor passage, so it defeats the moisture transmission value of the coating on top. Because of this, reputable silicone suppliers and roofers won’t coat or provide a warranty over entrapped moisture. The consequence of trapped moisture always is decaying underlayment, loss of insulation, and premature catastrophic roof failure. Virtually all national roofing standards require dry substrates as well. For this reason, infrared or nuclear scans always are strongly recommended before using any roof coating on low slope roofs. If moisture comes in, the epoxy primer most assuredly won’t let it out. This usually makes moisture permeability a moot point.
Finally, there is the issue of shelf life. 100% silicone coatings usually have only a 6-month shelf life kept at moderate temperatures to avoid spoiling. A partial pail that has been reclosed will become totally hard within 24hrs and require disposal. One must factor this into their costs and make certain that they are not using expired silicone coating. Unopened Topps Seal® by contrast has a 2-year minimum guaranteed shelf-life however we have experienced excellent performance even after 7-years of storage. An opened Topps container will remain usable for months when resealed immediately after use.
We have been asked for research about the best roof coating for cooling capabilities, and how silicone roof coatings compared to Topps Seal®. We will address this in a later blog. In general, while they may start off similarly, after only 3 years Topps Seal’s reflectivity exceeds most silicone products by as much as 50%. The retention of physical properties are strong indicators of a coating’s future value.