Slope

Positive drainage is key to the durability of a membrane; while some membranes can tolerate standing water better than others, it is a best practice to drain water from the roof and not promote ponding. Standing water can collect organic material, which over time can degrade the roofing membrane. Vegetation is also likely to grow in moist areas on the roof, and the roots of the plants can puncture the roof membrane.

The taper design of the insulation becomes incredibly important, particularly if there is no slope present in the deck. A proper taper design that considers Code requirements, including slope minimums, and drainage around rooftop units will minimize the risk of ponding or standing water accumulating on the top of the roof. Incorporation of slopes that may exceed code minimums may be necessary to adequately drain water as well as crickets to assist in diverting water to drains are critical to ensure that water will drain from the roof.

A proper taper design that considers Code requirements, including slope minimums, and drainage around rooftop units will minimize the risk of ponding or standing water accumulating on the top of the roof.

Membrane Seams

Discontinuities in the membrane, such as at seams and penetrations, represent a potential location for water infiltration into the building below.

As part of membrane selection, consideration of the type of seam between the sheets of membrane can be as important as the membrane selection itself. Different membrane types have variations in seam attachment and strength; these seam attachments also vary by ease of construction in the field. TPO and PVC thermoplastic membranes are installed with seams that are hot-air welded that fuse together to create a monolithic membrane. In thermoplastic membranes, the seam becomes the strongest part of the membrane. Thermoset membranes, such as EPDM seams, are attached with adhesives or tape.

Since membrane seams are common failure points, it is critical to specify adequate field quality control testing during installation. While thermoplastic seams are typically welded with a robotic heat welder, the seam integrity is sensitive to surface temperature and air temperature changes as the sun, clouds, and air influence the temperature and speed requirements on the robotic welder. Seam integrity should be tested throughout the day, particularly in the morning and in the afternoon when there are air temperature changes, as well as after lunch when there are breaks from the installation. For thermoset seams, the seams require cleaning prior to gluing or taping, and it is also important to continually test adhesion since inadequate seam preparation can prematurely fail seam connections.

Performance Durability

Durability over the life of the roof presents a final challenge to roofs, not only from accumulating moisture but from storm damage. The key to preserving a roof’s resistance to water during storms is designing the roof system for resilience. Damage prevention and impact resistance is a system discussion, covering not only the single-ply membrane but also the overall roof attachment method, coverboard use, and insulation selection. Depending on the building location, designing for the impacts of hail can arguably be one of the most important factors to implement durability into roof design. Thicker membranes, including fleece- back membranes offer increased impact resistance for both hail and traffic on the roof. Attachment method becomes crucial, as impact over a fastener can result in a laceration of the membrane. Adhered systems where fasteners are buried in the assembly offer better impact resistance, as well as reduction of the thermal bridging impact. Finally, coverboards can bolster the roof system’s performance as they provide a rigid substrate beneath the membrane. Higher density coverboards should be installed where there is greater risk of hail and increased frequency of foot traffic.

Redundancy becomes key for roofs in areas facing high storm damage risk. A hybrid roof approach blends the best attributes of an asphaltic roof and single-ply membrane together. The redundancy and toughness of an asphaltic membrane overlaid with the lighter colored and ponding water resistance of a single-ply membrane utilizes the strengths of each roof technology. Asphaltic membranes, used as the first layer, provide redundancy and protection against punctures as it adds overall thickness to the system. However, asphaltic systems, such as modified bitumen and built-up roofs, require some type of surfacing to protect them from UV degradation, such as granules, coatings, or ballast. For a hybrid roof, the addition of a single-ply white reflective membrane provides that protection. The single-ply membrane can also be reflective, which decreases the roof surface temperatures and potentially reduces the building's heat island effect as they are commonly white or light in color.

Water damage from slope, seams, and storms can be prevented with proper design and planning. However, the roof also faces challenges from air leakage and vapor diffusion.

Continuous Control Layers

While the concept of continuous control layers is not new, the addition of materials added into the building envelope to increase energy efficiency may inadvertently decrease the drying potential of roof assemblies. In other words, older assemblies utilized less insulation to control interior temperatures, and may have included discontinuous control layers that allowed for uncontrolled air flow that allowed moisture to make its way into roof assemblies. These designs relied on darker membrane colors that allowed for heat gain to dry out the assembly so as not to accumulate moisture which can lead to premature failure of structural elements. As assemblies have increased in efficiency higher R-values and reflectivity, and ultimately air tightness, moisture laden air that does make it into the assembly no longer has the ability to dry out. When not properly detailed, this moisture can accumulate within the assembly causing degradation of materials. Proper design, which includes appropriate insulation, consideration of an air or vapor retarder, and continuity of all of the control layer materials, is the foundation of a high-performing roof.

Water, air, vapor, and thermal control layers, while they may not be separate individual layers, all play an important role in the performance of a roof. Ensuring that the water control layer prevents water from entering into a building is the fundamental reason for a roof. Controlling air and vapor becomes more important as the ability for the roof assembly to dry decreases. The thermal control layer, which is influenced both by codes and energy efficiency, is crucial to maintain interior temperatures. Identifying each of these control layers is the first part of the design process; detailing them to ensure that they are continuous across the field of the roof and properly tied into the exterior walls is what establishes a high performing roof.

Air and Vapor Concerns

Costs due to heating and cooling are often the largest utility bill for building owners, so mitigating loss of conditioned air can be top of mind. According to Environmental Building News, air infiltration and exfiltration make up 25 percent to 40 percent of total heat loss in a cold climate and 10 percent to 15 percent of total heat gain in a hot climate. Losing one-third or more of conditioned air has a significant impact on the operational costs of a building.

However, uncontrolled air movement presents additional hazards to a building besides just increased utility bills. Uncontrolled air is a cause for concern because of the moisture that air carries. Moisture in buildings can be damaging; moisture accumulation can lead to degradation of building materials including mold growth which ultimately impacts indoor air quality. Designing to minimize air flow not only allows a building to save energy but also to mitigate moisture accumulation. Consideration to air barriers and vapor retarders during design are important to mitigate both conditioned air loss and moisture accumulation.

Air Barriers and Vapor Retarders

In 2012, the International Energy Conservation Code (IECC) published air barrier requirements, stating that a “Continuous air barrier shall be provided throughout the building envelope…” The purpose of an air barrier is straightforward: first, to minimize the loss of conditioned air from within a building, and second, to reduce energy loss and increase building energy efficiency.

Uncontrolled air movement in and out of a building is a key contributor to heat loss and gain. It is important to distinguish that the air barrier is often a system of materials that controls air leakage and convective heat flow through the building enclosure from the interior to the exterior. The air barrier is, critically, not one material, but instead is an integrated system of many different materials and components. As such, the connection of air barriers at interfaces, such as the roof to exterior wall interface, is critical.

Detailing to ensure continuity of the air barrier helps guard the building’s energy efficiency. It is the responsibility of the registered design professional to determine the need for an air barrier, verify an air barrier’s compatibility with other materials, clearly identify all air barrier components within the envelope, and provide details on joints, penetrations, and transition areas. While most often the roof membrane is considered the air barrier, other times, a dedicated vapor retarder at the deck level serves as both the vapor retarder and the air barrier. The building designer should specify that the roof air barrier not just overlap the wall air barrier, but also be flashed or sealed to it so that air cannot move through the interface.

It is important to distinguish that a vapor retarder is always an air barrier if detailed as such, but an air barrier may not control vapor movement. Additionally, while air barriers are a code requirement, vapor retarders are currently not. Vapor retarders are typically used in roofing assemblies when there is increased moisture content in interior air, such as over an indoor natatorium. Similar to air barriers, vapor retarders should be continuous at any penetrations and roof to wall interfaces in order to adequately prevent moisture vapor from entering into the roof system. It is important to remember that air leakage can cause more moisture-related problems than vapor diffusion.

Condensation

Contrary to popular belief, air-transported moisture is much more critical to control than water vapor that enters a building by diffusion; condensation is likely to occur when moisture vapor in uncontrolled air is allowed to reach dew point. For interiors with high humidity, such as natatoriums, it is often more obvious to consider the inclusion of a vapor retarder at the deck level to limit condensation potential, but controlling air leakage, and the associated moisture that air contains, can be of added difficulty when there are sources of moisture from the exterior to the interior of the building.

Control of moisture ingress from air leakage at assembly joints, including those at the roof to wall interface, are where the vapor control layer can have the greatest impact on condensation potential in roof assemblies. Proper design includes ensuring that the transitions and gaps between materials are sealed. Inadequate design, where there are discontinuous control layers, allows for warm, moist air to enter into the roof assembly and mix with cooler air. When the warm, moist air meets a surface below the dewpoint, condensation forms. Often, there is little to no drying potential within roof assemblies, and the moisture is trapped. Trapped moisture can lead to degradation of roofing materials, including decreased R-value of insulation and rotten wood decks.

Traditionally, darker colored roofs with the roof membrane performing as the air control layer have allowed for roof assemblies to ‘dry’ when condensation is introduced into an assembly. The darker membranes allow for the roof to ‘heat up’ and effectively evaporate condensation that may have occurred over the course of the day. While a darker colored roof may allow for the moisture to dry, there is a fundamental flaw with the theory that this is an acceptable way to design for a roof. Introduction of moisture into a roof assembly may degrade roofing materials over time, albeit slower than if the moisture was trapped. There is further misconception that by installing white or reflective roofs, the roofs no longer have the capability to dry. The theory is that since white roofs will reflect the sun’s rays and are not allowing for as much heat gain into the roof assembly, trapped moisture will not dry out. While this may or may not be true for a specific building in a specific location, roofs should be designed to prevent moisture infiltration and reduce condensation potential by including continuous control layers that prevent uncontrolled air and vapor movement.

Thermal Concerns

Managing thermal concerns at the roof centers around appropriate insulation, system attachment, and continuity at details. Attention to detail, including thermal bridges at critical intersections and perimeters, is important for energy efficiency over the life of the roof.

While insulation selection is important, the overall R-value of the system and its installation should meet code requirements and the desired energy efficiency of the roof. As referenced in the IBC, the IECC 2018: C402.2 denotes minimum R-values based on the climate zone where the project is located. The IECC indicates:

• “Continuous insulation … shall be installed in not less than two layers ...”
• “…and the edge joints between each layer of insulation shall be staggered.”

Thermal bridging, which is when heat loss occurs from a discontinuity in R-value between materials, occurs when there are gaps in the insulation such as between board joints. Adding a second layer of insulation and staggering the joints on the boards below mitigates thermal bridging between board joints. Another common place for thermal bridging to occur is at transitions, particularly at the roof to wall interface, and the code requirement for continuous insulation addresses this by requiring the insulation be continuous from the roof to the exterior wall. Another common location for thermal bridging to occur is at fasteners attaching the roof assembly to the structure.