Posted: October 11, 2018 | Project Management
Nature has waged war with roofs since humans stepped out of caves and began building shelters. Primitive roofs used native material and proven construction techniques of the area to apply sticks, leaves, straw, clay, mud, turf, or animal skins to protect people from the elements. Each technique had its benefits, but they still were subject to the effects of continuous weather exposure.
Advances in science produced refined petroleum, giving us asphalt and coal tar. We learned to improve the properties of these base materials through better processing, and added polymer modifiers to help them last longer. We learned that we could link polymers and extrude them in sheet form to make durable single-ply sheets. These innovations all had the same underlying goal: to withstand the forces of Mother Nature for as long as possible.
How are we winning the weathering war? To start, let’s revisit the construction of a modified roof membrane and a single-ply membrane in order to examine what is in place to protect against the daily pounding a roof takes.
A typical modified bitumen roof membrane consists of a fabric scrim – usually made up of fiberglass, polyester, or a fiberglass/polyester blend – that is sandwiched between a coating of polymer modified bitumen. The scrim provides strength and reinforcement to the sheet. The primary function of the polymer modified bitumen is to protect the scrim reinforcement, but also to impart flexibility, especially at low temperatures when things get brittle. It also acts as an adhesion medium for the aggregate or slag top, as well as the glass or plastic burn backer or release liner on the back side.
Thermoset membranes are formed from rubber polymers. They differ from thermoplastic membranes in that, once allowed to cure over time, there is a chemical change that improves their properties. And additives improve tear strength, UV resistance, flexibility, fire resistance, and dimensional stability.
Thermoplastic membranes are based on polymers such as polyvinyl chloride (PVC) and thermoplastic polyolefin (TPO). PVCs are more flexible than TPOs due to the use of plasticizers, similar to how butter is used in a bowl of noodles. Blends of ketone ethylene ester (KEE) and PVC form very robust single-ply systems that combat everyday environmental exposure. Scrim reinforcement is usually sandwiched in between plies, similar to a modified bitumen membrane, to provide tensile and tear strength. Like thermoset membranes, additives can impart improved physical properties in a roof covering.
Formulations vary depending on manufacturer, but for the most part, this simplistic construction shows that these 80- to 120-mil-thick membranes – more so if one includes the insulation – are all that stand between the elements and a wet ceiling or floor. So a roof covering must be constructed properly to withstand the destructive forces of nature.
It is a good bet that at least one time during the life of a roof it will be subjected to a hail storm of varying intensity. The National Oceanic and Atmospheric Administration estimates that a hailstone falls from the sky at 20 mph. Combine the inertia of hail with a driving wind, and it contains sufficient energy to knock minerals off a modified bitumen membrane surface. When the minerals get knocked off, and the sun comes out, UV radiation can soon break down the modifier in the membrane of the roofing surface.
Single-ply thermoset or thermoplastic membranes are not safe from hail. Even a single hail storm can cause cracks and punctures that allow moisture to penetrate into the membrane below. Over the years, many codes have been formulated that contain provisions requiring roofing systems to meet minimum impact requirements.
To counter the damaging effects of hail on modified bitumen membranes, mineral retention is important. The more secure the mineral is to the sheet, the better resistance it has against being knocked off during a hail event. Innovations and improvements in the polymer modifiers that are blended with the bitumen, such as more specially-designed weather-stable polyurethanes, tenaciously anchor the minerals to the sheet, forming a protective shield over the more UV-unstable bitumen, leading to a longer service life.
For single-ply roofs, harder cover boards can be installed to resist the impact force of hail. However, most of these polymeric materials still rely on plasticizers to improve flexibility. Initially, single-ply roofs made of TPO or PVC are resistant to hailstones as much as 1¾ inch in diameter. But as the plasticizers or flexibilizers leach out of the membrane during aging, it becomes more brittle and susceptible to hail damage. This is why, when considering a single-ply roof, it is important to obtain as much information on its aged hail resistance as possible, especially if the building is in an area prone to hail events.
Water: Rain, Snow, and Ice
The size of a water molecule is really small, yet water can cause an awful lot of damage to roofs. At its freezing point, water does something that most liquids don’t do – as it turns to ice it expands. In fact, it expands by approximately nine percent.
Anyone who ever put a full water bottle in the freezer can confirm this fact. A nine percent increase may not seem like a lot but consider a roof that has a tiny tear in it, say a centimeter-long crack about 1 mm wide, perhaps formed from an earlier hail event. Water fills the crack quite easily and then begins to expand as the temperature drops to below freezing, pushing the dimensions of the crack outward in proportion by about nine percent. The temperature warms, the ice melts, and the water evaporates, but the crack is now larger. This cycle of freezing and thawing continues over and over for several years and one begins to see how a tiny crack can grow to a size that can turn into a real problem.
Like ice formation, prolonged exposure to water, frozen or liquid, is an unavoidable problem on the roof. Regular maintenance performed by roofing professionals, which may include a simple walk over to check for premature cracking, can be the best defense against the damage caused by water.
The sun is an inescapable constant in roofing. The sun will shine, although not always in consecutive days in some climate zones. With the sun comes its two byproducts: UV radiation and heat. UV radiation penetrates into the polymer binder and bitumen of most modern roofs, targeting specific molecular sites to chemically break down and stiffen the membrane. This eventually leads to cracking that presents a problem during a freeze event like the one mentioned above. The mineral granules on modified bitumen membranes act in a similar fashion as sunscreen lotion on skin, protecting the polymer modified blend from the damages of UV radiation. As the minerals gradually fall off with age, the polymer and bitumen are exposed and at the mercy of UV degradation. Single-ply surfaces fare better because the chemistry itself is its best defense. TPO roofs are regarded as being highly UV resistant, while PVC and KEE-PVC roofing products are more geared for high-temperature surfaces.
Heat exacerbates the UV degradation process and contributes to the formation of surface blistering. When moisture is inadvertently trapped in mopping voids or on the inter-ply sheet during storage, evaporating water has no place to go as it tries to penetrate through an impermeable top sheet.
Pressure and volume are directly related to the temperature on the roof, and if temperature increases, either the pressure or volume must increase proportionally. Water vapor trapped in a void (fixed volume and pressure) beneath a membrane on a cool morning (low temperature) must either gain pressure or increase its volume, or a blend of both, as the heat of the day increases.
The heat from the sun softens the asphalt somewhat and allows the increased pressure to expand the void sideways or toward the top cap sheet. This cycle continues, sometimes gathering more trapped water as voids link with other voids until the blister is quite large. The result is a blister that can range from the size of a blueberry to a few feet in diameter. When surface heating is followed by a rapid cooling process, such as a sudden rain, dimensional changes can occur in the membrane that can cause stress cracking or other failure. Additionally, heat on a roof, especially a wet one, provides the perfect conditions for microbial growth. If there are no safeguards in the formulation to account for microbial growth, it will not take long for a continually wet roof to show signs of algal growth or fungal attack.
Advances in polymer modification have improved the longevity of membranes by retaining the minerals longer, purposely engineering the polymers with more heat-resistant or UV-stable components. Furthermore, the use of cool roof coatings such as white or aluminum acrylics, polyurethanes, or silicones can lower the surface temperature by as much as 50 to 60°F. Proper drainage at time of installation still remains the best way to remove standing water and the risk of microbial growth in those areas.
TPO single-ply surfaces are formulated for excellent heat and UV resistance because there is nothing in its formula that UV radiation can further break down. In spite of this superior weather resistance, TPO sheets have their drawbacks, including issues with weld popping, crazing, and cracking. They do wear eventually, and once the scrim reinforcement is exposed, the sheet is doomed. PVC single-ply roofs are more flexible than TPO roofs, but during continual heating, the PVC plasticizers are leached out. The use of KEE as a solid flexibilizer eliminates the need for a plasticizer and does not leach out upon heat exposure. Both are usually white, which cools down the surface and slows the degradation process.
Just as the sun is an unavoidable factor in roof science, so too is the direct and indirect effect of wind; so much so that a sizable portion of roof research goes into how much punishment a roof can undergo strictly because of wind.
The roofing surface is present on the horizontal, or field, where the insulation is mechanically fastened or adhered and the membrane sheets are adhered. This is continued up the wall edges, or flashing, where it is terminated and met with a metal flashing or fascia to protect the materials from wind and rain. Unfortunately, especially in more windy climates or hurricane zones, physics in the form of wind uplift is continually working against this assembly. This uplift is the net upward suction force resulting from two simultaneous sources: wind flowing across the building above the roof surface and increasing internal air pressure caused by cracks, openings, windows, etc.
The uplift pressure is proportional to the density of the air and the square of its velocity. This squared value is significant because when the wind speed doubles, the pressure quadruples. Considering a typical building, as the wind meets that structure, the wind must alter its path and either flow up, down, or around the building. Altering the wind path accelerates the velocity at the roof top, which causes a suction force (uplift) to act against the roof membrane.
Roof systems are tested to withstand the greatest wind speed possible while taking other variables into consideration: building height, type of roof deck, construction, and even building use. Based on this testing, the roof is given a wind uplift rating. When the suction force exceeds this rating, there is a high risk of roof destruction ranging from light damage in the form of bent or dislodged sections of flashing to severe damage where entire areas of the roof are blown free from the roof deck.
Most roofing adhesives are specifically designed to help keep the membranes (modified bitumen and single-ply) in place not only during peaceful wind-free days but also in the face of hurricane force wind. A myriad of mechanical fasteners are also available to prevent catastrophic failure due to wind.
While cost considerations are important for the building owner, it is vital to take into account the weather conditions in the area when deciding which roof will best fit the needs of the building and the owner. Knowing that the owner’s investment will be continually punished by the environment should make one take a closer look – not only at the technology that is available, but also the testing that goes into it to fight against this continual attack.
Roofing professionals can be of valuable service when choosing a roof system. Not only are they are educated on the products being applied to the roof, but they are also keenly aware that the surface will be under continual attack by its environment long after the roof project is finished. Longevity of the roof will determine its true value long after the purchase order has been received and the check has been mailed.
About the Author: Jason Smith is the Sr. Research & Development Chemist for The Garland Company. He has multiple U.S. and foreign patents directly related to roofing and has written several articles related to coatings applications and solvent regulations. Jason serves on the Board of Directors for the Roof Coatings Manufacturers Association and serves as the co-chair for its Technical Committee. Smith received his undergraduate degree in Chemistry from the University of Pittsburgh and his Masters Degree in Polymer Chemistry and Coatings from DePaul University in Chicago.