Modified bitumen is a modern-day built-up roof that consists of the same asphalt plies, but instead of building the plies on the roof, they are produced in rolls in a factory. Modified bitumen roofs offer the same durable protection of built-up, but with a faster and more consistent installation since the rolls are pre-manufactured offsite. Modified bitumen roofs consist of multiple layers including a base ply and a granulated cap sheet (called a two-ply system), where the cap sheet provides an excellent wearing surface. Modified bitumen roofs have a high tolerance for chemicals. While durable, over time, the granules in the cap sheet may become loose and shed exposing the asphalt below, which degrades over time with UV exposure and ponded water. The seams of modified bitumen rolls are often heat welded with a torch that melts the asphalt on the back of the rolls together, which forms a monolithic membrane. Due to the redundancy of the multiple layers in this type of system, modified bitumen roofing is the preferred of many school districts across the country.

Liquid applied roofs, not to be confused with liquid coatings, are roofs that consist of multiple coats of a liquid roofing membrane with an embedded reinforcement material between the layers. Liquid applied roofs are manufactured in many formulations including silicone and acrylic, and the selection depends on the underlayment as well as exposure to chemicals or other natural elements. They can be installed with sprayers or rollers and form a monolithic membrane across the roof surface. Liquid applied roofs are a common choice for school roof replacements due to the compressed replacement period that often occurs during summers. Similarly, liquid coatings, where a coating is installed on top of an existing roof system, are a common choice to extend the life of an existing roof system. Often, minimal preparatory work is required and since the original roof system is left in-place, liquid coatings are an excellent choice for hospitals and schools where minimal disruption is advantageous.

Hybrid assemblies, which is where two types of roof membranes are utilized, is a common choice for schools and hospitals due to the redundancy, ability to manage ponding water, and the cool reflective membrane to reduce energy consumption. One such assembly is where the base layers consist of asphaltic modified bitumen, and the cap layer is a reflective single-ply membrane such as TPO or PVC. A hybrid system such as this has increased durability, with effectively three plies or more of membrane. The addition of the reflective membrane will decrease roof surface temperatures and the building’s heat island effect, as well as can increase the resistance to certain chemicals if exposure is a concern. A hybrid assembly is also a common choice for recovery scenarios, where there is an existing modified bitumen roof and a single-ply membrane is installed on top. The addition of the single-ply membrane adds reflectivity and increases the service life of the roof assembly 15 years or more without an expensive and disruptive tear-off of the existing assembly. The addition of a single-ply membrane can be installed with low-VOC options that can have minimum odor and noise making activities that are beneficial in school and healthcare settings.

Membrane color

Photo courtesy of GAF

White or reflective roofing colors can have significantly lower surface temperatures, as much as 60 degrees Fahrenheit cooler, on a hot day.

Membrane selections can further be influenced by color, thickness, and attachment. For roof assemblies where the membrane is at the top of the system, the color can have a significant impact on the performance of the system and on the roof surface temperatures. Reflective roof membranes can lower the ambient roof temperature. EPDM membranes are traditionally dark in color, while liquid roofs, TPO and PVC are typically white or light in color. Modified bitumen roofs can have light colored granules which can increase reflectivity of a roof’s surface. Two roof surface temperatures, differing only by color, can vary by as much as 60 degrees Fahrenheit in the summer heat. Dark colored roofs can reach up to 150F, whereas white or reflective roofing colors can have significantly lower surface temperatures. Using a lighter colored roof can decrease the urban heat island effect in cities and may decrease the amount of heat that is able to radiate into a building’s interior. The more heat gain that a roof assembly absorbs, the warmer the interior temperature will be. In the summer, while the heat gain is offset by HVAC systems, the warmer an interior temperature is, the longer the systems have to run, which can increase energy use, and potentially raise energy bills.

According to a publication in the National Library of Medicine, “the patient is identified as being of prime importance for comfort standards in hospital ward areas.¹ The steady-state condition preferred by patients was an air temperature of 71F.” Comfort regarding hospital accommodations can be closely tied with room temperature, and unsolicited heat gain or loss through the building enclosure, such as through the roof, can have a profound impact on overall patient satisfaction. Similarly, according to the Institute for Educational Sciences, classroom temperatures should be maintained between 68F and 75F during the winter months and between 73F and 79F during the summer months.”² Inadequate building enclosures, including roof assemblies, where the temperatures cannot be properly maintained, can impede classroom learning. Selecting cool roofing assemblies, where solar heat gain can be controlled, can result in more than an efficient building assembly, they can impact student learning and patient satisfaction.

Reflective roof membranes, otherwise known as cool roofs, account for greater than 50 percent of roof surfaces installed on to low-slope commercial buildings each year, and an even greater percentage are installed in the southern half of the United States each year. Such reflective roofs have the potential for decreasing cooling energy consumption by lowering roof temperatures (Freund et al. 2006; Ennis and Desjarlais 2009; Gaffin et al. 2010; Graveline 2013). Research work strongly suggests that, regardless of location and local climate, selecting “cool roofing” will result in building space heating and cooling energy savings providing that buildings are heated with natural gas and cooled by electric air conditioning. While a building designer should model energy costs versus roof reflectivity on a case-by-case basis, some level of savings can be expected for reflective roof membranes. Even when aged reflectance and emittance values for cool roofs are factored in, energy savings remain at 85 percent of the initial installed values. This would suggest that energy cost savings achieved with cool roofs will remain, albeit slightly reduced, throughout the life expectancy of the roof.

Membrane thickness

The risk of installing a less robust system, such as that provided by a thinner single-ply membrane, could mean that the roof assembly would require replacement sooner than projected. Membrane thickness will increase resistance to puncture and foot traffic and extend the service life of the overall assembly. Note that hybrid assemblies, which combine two membrane types, most often modified bitumen and single-ply TPO, will increase system robustness and add additional protection from failure.

For single-ply membranes, adding additional membrane thickness can provide protection against punctures, which is especially important if the roof experiences foot traffic due to service and maintenance activities. According to a leading roof manufacturer, jumping from a 45 mil to an 80 mil single-ply membrane thickness significantly improves impact resistance by almost 80 percent. A thicker single-ply membrane also provides additional protection to both UV and high surface temperatures. For roofs using single-ply membranes, this is important, since a thicker overall membrane means more thickness over the scrim, or the reinforcing layer. It is this portion of the membrane that provides the weather resistant properties, including UV resistance. For modified bitumen membranes, two plies are considered typical, and the addition of a third-ply is generally not required, however, the addition of a single-ply membrane in a hybrid system will greatly increase the robustness and reflectivity of the system. Modified bitumen roofs, particularly those in a hybrid configuration, are generally considered more durable as total thickness may be 300 mils or more.

Liquid-applied roofs can be installed in varied thicknesses, with thicker systems being more durable over time. Liquid roofs are typically applied in several coats, with a fabric reinforcing layer in the middle to provide enhanced durability. Liquid roofs use liquid flashings at penetrations which is advantageous on roofs where there are a high number of or oddly shaped penetrations, such as roofs over science labs.

Membrane attachment

There are two broad categories of roof attachment: mechanically attached systems, via the use of fasteners, and adhered systems. The attachment method will vary depending on the membrane type, and project specific requirements such as energy efficiency, fume tolerance, and fire hazards.

Selection of attachment methods should be reviewed for ease of installation in the short-term and energy efficiency over the long-term. Energy efficiency from the roof assembly can be directly related to thermal bridging, which occurs when components allow heat transfer through the roof assembly. Loss of internal temperatures means that the mechanical equipment will have to work harder to maintain the desired set points. Thermal bridging has the potential to occur at gaps or discontinuities between materials, such as at fasteners in a mechanically attached system. Particularly where the fasteners penetrate the entire assembly from the membrane through the insulation and into the deck, the fasteners provide a direct thermal path from the exterior to the interior.

Mechanically attached single-ply systems are also subject to billowing in high wind events. Billowing, or fluttering, of a membrane is when wind causes a negative pressure by pulling interior air into the roof assembly creating uplift force on the roof assembly. Although this is an acceptable behavior of single-ply membranes, over time, it can cause stress and fatigue on the mechanical attachments and membrane. Interior air that is pulled into the roof assembly equates to energy loss, since often the temperature-controlled air may warm or cool based on the temperature of the membrane. Additionally, in healthcare settings where negative pressure rooms are required, uncontrolled air loss through fastener penetrations will likely impact the pressure in the room as well as the ability to effectively control contagions.

Image courtesy of GAF

Mechanically attached single-ply system, fasteners through the system act as thermal bridges.

Systems that do not use fasteners, such as those which use adhesives, greatly reduce thermal bridging by eliminating the path from the interior of the roofing assembly to the exterior. Adhering also prevents billowing of the membrane, by mitigating the interior air that can be brought into the roof assembly.

Asphaltic based membranes use asphalt to adhere the roof assembly. Modified bitumen roofs can be installed by several different ways, including using a torch to melt the asphalt plies or by using a cold applied adhesive. The multiple asphalt plies form a robust roofing system that is not penetrable to air or the effects of billowing. Asphalt, and particularly hot (torch-applied) asphalt, can have strong fumes. On an occupied school or healthcare building, on a building in close proximity to other buildings, or on a building expansion, the use of asphalt may not be preferable since HVAC intakes may transport asphalt fumes into the building. Furthermore, some insurance companies and jurisdictions do not allow the use of torches on the roof due to the fire hazard of open flames.

Cold applied modified bitumen applications are an alternative that use an asphaltic based adhesive that is rolled onto the substrate prior to installation of the membrane. Cold applied applications offer the same modified bitumen wearing surface, but with fewer fumes than a traditional torch applied application. Modified bitumen roofs are excellent for mitigating air movement, as the multiple layers are impermeable to air, and therefore are not subject to billowing like mechanically fastened single-ply systems. Liquid applied roof membranes form a continuous membrane and are excellent for mitigating air movement, and are not subject to billowing, as they are installed directly on the insulation. However, the fumes of liquid applied membranes should be considered, as different formulations may exceed desirable VOC levels in occupied buildings.

Single-ply membranes tolerate a wide variety of attachment methods and are able to be both adhered and mechanically attached. The types of adhesives can vary from melted asphalt (with fleece-backed membranes) to various types of commercial adhesives manufactured for specific single-ply membrane types. Although product specific, single-ply adhesives release fewer hazardous fumes during installation, in contrast to those emitted by asphalt-based products. These adhesives do not require heating or torching for application, and generally come in pails or canisters and can be installed with a roller or a spray attachment. Single-ply adhesives are generally less messy and can be quicker to install than traditional asphaltic-based products. Mechanical attachment of single-ply systems uses fasteners to install both the insulation layers and the membrane.

The fasteners must be continuous through each layer and into the structural roof deck for securement. The number of fasteners will depend on project specific requirements, but typically for a mechanically attached system, there is a minimum of six fasteners per 4-foot-by-8-foot insulation board, and additional fasteners for membrane attachment, which can lead to a substantial number of fasteners penetrating and creating thermal bridges within the roof assembly. It is important to note that most systems require mechanical attachment of the first layer of insulation, even if the specified system is to be adhered. However, by burying the fasteners within the system, and adding adhered layers of insulation and membrane on top of the mechanically attached insulation layer, the interior air loss and thermal bridging is significantly reduced. Location of fasteners in the insulation below a liquid applied membrane should be a consideration, as thermal bridging can still be a concern.

Insulation

Insulation is a critical part of any roofing assembly as it contributes to the overall energy efficiency of the building. According to a study completed by ICF International in 2021, roofing assemblies installed to current energy code requirements as compared to those installed prior to 2015, result in whole building energy savings of 2-11 percent.³ The study found that savings increase for colder climates and are highest for building types with large roof-to-floor ratios such as schools and hospitals. The effectiveness of roof insulation is determined by its R-value, which is a measure of thermal resistance. The higher the R-value, expressed per inch, the better the thermal performance of the insulation and its effectiveness at maintaining interior temperatures.