Building to Last With Asphalt-Based Roofing

The property owner of this building opted for a BUR/modified-bitumen hybrid system with reflective white coating. Photos: Johns Manville

The advantages of a built-up roofing (BUR), modified bitumen, or hybrid roofing assembly include long life, a variety of maintenance options, and outstanding puncture resistance. This durability means property owners will spend less time worrying about fixing leaking roofs and the associated hassles — lost productivity, disruption in operations, slips and falls, repair bills, and other liabilities.

Recommending clients install a roof system that gives them the best chance of eliminating unproductive distractions is a good business decision for design/construction professionals. A more durable roof will enable property owners to focus on making profits instead of dealing with the aftermath of a roof leak.

“I have no problem endorsing asphalt-based roofing,” says Luther Mock, RRC, FRCI and founder of building envelope consultants Foursquare Solutions Inc. “The redundancy created by multiple plies of roofing is really what sets systems like BUR and modified bitumen apart.”

One can argue BUR’s closest cousin — the modified bitumen (mod bit) assembly — is actually a built-up roof made on a manufacturing line. The reality is the plies of a BUR create a redundancy that can help mitigate any potential oversights in rooftop workmanship.

BUR systems are offered in a variety of attractive and reflective options with a proven track record of performance. Photos: Johns Manville

“I’ve replaced BURs for clients I worked with 30 years ago,” says Mock. “We recently replaced [a BUR] specified in the early 1980s. And the only reason was because some of the tectum deck panels had fallen out of the assembly. Meanwhile, the roof was still performing well after 30 years.”

According to the Quality Commercial Asphalt Roofing Council of the Asphalt Roofing Manufacturers Association (ARMA), one of the main drivers of the demand for BUR systems is the desire of building owners for long life cycles for their roofs.

“A solid core of building owners and roofing professionals in North America continue to advocate asphalt-based roofing systems because of their long lives,” says Reed Hitchcock, ARMA’s executive director.

Benefits of Asphalt-Based Roofing

Over the years, asphalt-based roofing assemblies have earned a reputation for reliability with building owners, roofing consultants, architects, engineers, and commercial roofing contractors. The original price tag tends to be greater than other low-slope roofing options, but these assemblies offer competitive life-cycle costs. BUR enjoys a track record spanning more than 150 years; it provides a thick, durable roof covering and can be used in a broad range of building waterproofing applications.

An aerial view of a reflective roof membrane. Photos: Johns Manville

Available as part of fire-, wind-, and/or hail-rated systems, BUR and modified bitumen assemblies offer proven waterproofing capabilities, high tensile strength, long-term warranties, and a wide choice of top surfacings (including ‘cool’ options). Their components include the deck, vapor retarder, insulation, membrane, flashings, and surfacing material. The roofing membrane can be made up of a variety of components, including up to four high-strength roofing felts, modified bitumen membranes (hybrid systems) and standard or modified asphalt. Hot-applied asphalt typically serves as the waterproofing agent and adhesive for the system.

The roofing membrane is protected from the elements by a surfacing layer — either a cap sheet, gravel embedded in bitumen, or a coating material. Surfacings can also enhance the roofing system’s fire performance and reflectivity ratings.

Another surfacing option is gravel, commonly used in Canadian applications where the existing roof structure can handle the extra weight. There are also several smooth-surface coating options, the most popular of which are aluminum or clay emulsion products offering greater reflectivity than a smooth, black, non-gravel-surfaced roof. These reflective roof coating options are typically used in warmer regions when required by code. Reflective white roof coatings are also becoming more popular.

Cold-Process BUR

Cold application of BUR has provided an alternative to traditional hot-applied systems for more than 48 years. The term ‘cold-applied’ means the BUR roofing system is assembled using multiple plies of reinforcement applied with a liquid adhesive instead of hot asphalt. These cold adhesives are used between reinforced base/ply sheets to provide a weatherproof membrane.

The owner of this shopping mall chose BUR primarily due to its redundancy. Multiple plies of roofing can provide extra insurance against water intrusion. Photos: Johns Manville

In BUR cold-process roof systems, manufacturers typically require that only fully coated, non-porous felts (such as standard base sheets) are used as base and ply sheets. Generally, an aggregate surfacing or a coating is then applied over the completed membrane to provide surface protection and a fire rating for the roof system.

“In the re-roofing market, we’re definitely seeing more cold-applied systems being specified, particularly with modified bitumen,” says Mock. “It’s a natural alternative when a building may be occupied during the reroofing process and hot asphalt is not an option.”

Adhesives can be manually applied with a squeegee, brush, or spray application equipment. When numerous roof penetrations or rooftop access become issues, manual application of adhesives is usually the best option. Proper coverage rates are vital to a successful, long-term, cold-applied roof system. Both spray and manual application methods require the proper amount of adhesive material be installed. If too little adhesive is applied, there is a potential for an improper bond to be formed between the felts. If too much is applied, then the potential for longer setup times and membrane slippage is increased. Additionally, ambient temperatures must be 40 degrees Fahrenheit (5°C) and rising before installation. This limits, but does not preclude, use of cold-process BUR in much of the northern United States and Canada.

“I’m also comfortable specifying BUR, because I’m confident I will have a seasoned contractor on the job,” says Mock. “The commitment in terms of skilled labor and equipment is simply too great for these contractors to be first-timers.”

Flashings are another critical component of every roofing system, particularly in cold-weather applications. Four-ply BURs use modified bitumen flashings almost exclusively. These membranes are predominantly styrene butadiene styrene (SBS)-modified and offer greater elongation in frigid climates where it counts most — at the interface of the roof system with other building components.

Use of a modified-bitumen base ply is one way of handling general flashing requirements, although modified bitumen cap sheets are more common.

BUR Repair and Maintenance

Like all roof systems to some extent, the life expectancy of a BUR system depends on the property owner’s commitment to routine maintenance. All roof systems can benefit from an owner willing to undertake a proactive management plan. BUR installed over an insulation package lends itself well to non-destructive testing in the future (e.g., infrared) as a means to maximize service life.

“Asphalt roofing systems have the potential for a very long life, and preventive maintenance is the key to realizing that potential,” says Hitchcock.

Non-gravel BUR surfacing options include aggregate, a mineral surface cap sheet, or a smooth, surface-coated membrane. Photos: Johns Manville

The goal is for problem areas to be detected and fixed before they develop into leaks. Inspections can reveal potentially troublesome situations, such as a loss of gravel surfacing, which could lead to felt erosion or brittleness. Less commonly, punctures and cuts to the membrane can occur, so it is wise to remove sharp objects and debris from the roof. Clogged drains or poorly sealed flashings also present problems that are repaired easily. The effects of chemical exhausts on roofing materials should also be monitored.

Preventive maintenance actions can help catch problems before they damage larger areas of the roof system. Inspections should be performed not only on aging roofs, but also on newly-installed roofs to guard against errors in installation, design, or specifications.

BUR and modified bitumen also have a long history of proven performance in the northern United States and Canada, where snow and ice buildup are common. Perhaps more than any other roof membrane, the BUR system shrugs off minor abuse.

BUR has proven to be a low-maintenance roofing system, and it can also be effectively repaired when needed. This means property owners can usually get more life out of a BUR. The ability to enhance the performance of existing BUR membranes with coatings, mod bit cap sheets, or flood coats of asphalt explain the long service lives of these systems in demanding applications.

“Property owners rarely have to replace a four-ply BUR until it is absolutely, positively worn out,” says one roofing contractor who asked to remain anonymous. “Based on experience, these asphalt-based systems ‘hang in there’ longer than less-robust roof options.”

When BUR Is Not the Best Option

There is no roofing product solution that will fit every building specification, and that certainly holds true for BUR. Probably more than any other roofing system (except spray polyurethane foam), the built-up roofing application is more of a skill than a science. As alternative systems have been introduced into the market, the job of finding experienced BUR contractors has become more difficult. This is especially true for the hot mopping of multi-ply BUR systems.

BURs are labor intensive and their installed cost will fluctuate with crude oil prices. However, as oil prices have continued to fall, BUR manufacturers have enjoyed the lowest asphalt pricing since the 2008-09 recession. (The price of oil peaked at about $117 a barrel in September 2012 and is $50 a barrel at this writing.) Typically BUR manufacturers will pass on a portion of these savings to their customers.

BUR has always held up well in life-cycle cost analyses. However, if a roof is not expected to last 20 years or more, it usually does not make sense to specify a premium four-ply BUR.

On larger projects, gravel-surfaced BURs are typically not practical from a cost standpoint unless a source of gravel is available locally. Projects where roof access is difficult often present challenges when roofing kettles are used. And despite the preponderance of low-fuming asphalts and kettles, re-roofing occupied buildings is often unacceptable to neighbors and/or the property owner.

Built-up roofing systems have sufficient strength to resist normal expansion and contraction forces that are exerted on a roof; however, they typically have a low ability to accommodate excessive building or substrate movement. Rephrased, if the roof must be used to “hold the walls” together or if the use of “loose-laid insulation” has a benefit, then a traditional three- or four-ply built-up roofing system is not a good choice.

A built-up roof typically provides high tensile strength with low elongation. Guidelines about where expansion joints should be installed in the roofing system should not be ignored by the designer. These guidelines include installing expansion joints where the deck changes direction, approximately every 200 feet (61 meters), although many consider that this dimension can be expanded for single-ply roofing membranes; where there is a change in deck material; and, anywhere there is a structural expansion joint, etc. Based on these requirements, on some projects it simply isn’t practical to use a BUR.

BUR materials must be kept dry before and during installation to prevent blistering in the roof system. Proper storage is the key: Do not overstock the roof; use breathable tarps to cover material on the roof; store material on pallets to minimize the possibility of material sitting in water; and store rolls on-end to prevent crushing. In general, polymeric single-ply membranes like TPO (thermoplastic polyolefin) are less susceptible to storage issues.

Many roof consultants and product manufacturers clearly state that there should be no phased construction of a built-up roof. If phasing is required, then a BUR should not be specified. This is a clean and simple rule to understand; if the roof being constructed is a four-ply BUR, then only as much insulation should be installed as can be covered the same day with all four of the plies in the built-up roofing membrane. Phased construction of a built-up roof greatly increases the potential for blistering of the membrane and does not allow for the total number of plies to be installed in a shingled fashion. Phased application contains other perils, such as roofing over a small amount of overnight precipitation or dew that, even with the best of intentions, can cause harm.

As stated above, costlier modified bitumen materials should be specified for flashings and to strip in metal. Stripping in two plies of felt will most likely result in splitting at the joints in a gravel stop because the two-ply application cannot accommodate the movement in the edge metal. On new or existing buildings where significant expansion/contraction is expected, a TPO, PVC or EPDM roof membrane can save the property owners money and eliminate premature roof failure due to roof splitting.

Conclusion

Manufacturers across North America are making asphalt roofing systems like BUR better and more versatile for architects, builders, contractors, roofing consultants, and building owner/managers. Thanks especially to the addition of polymers that add stretch and strength, architects can now specify a commercial, low-slope roof as part of a multi-ply BUR system any way they want it — hot, cold, torch, or self-adhered (hybrid BUR) — to meet the individual low-slope roofing project’s needs.

Most importantly, asphalt-based roofing products offer exceptional life-cycle cost performance. They have proven to be reliable, easy to maintain, and are trusted to perform exceptionally well in extreme weather conditions.

Reflective Granulated Cap Sheet Is for BUR Systems

The reflective granulated cap sheet may be used as a cap or flashing sheet in built-up roof systems.

The reflective granulated cap sheet may be used as a cap or flashing sheet in built-up roof systems.

Johns Manville has introduced a highly reflective granulated cap sheet called GlasKap CR G, which may be used as a cap or flashing sheet in built-up roof systems. The CR G membranes are surfaced with white reflective minerals and can be installed using standard granulated cap sheet installation methods. GlasKap CR G joins the larger offering of JM cool roof membrane solutions. These products are approved by FM Global and UL. They are also listed with CRRC and meet current Title 24 requirements. GlasKap CR G provides an additional option for customers who want a reflective cap sheet as part of a reliable BUR system.

The Roof Cover: The Cap on the Roof System

For nearly two years in this magazine, I have been discussing the various components that make up a roof system: roof deck, substrate boards, vapor/air retarder, insulation and cover boards (see “More from Hutch”, page 3). Although each component delivers its own unique benefit to the system, they are intended to work together. When designing a roofing system, components cannot be evaluated solely on their own and consideration must be taken for a holistic view of the system; all components must work together synergistically for sustainable performance. Unfortunately, I often have seen that when components are not designed to work within the system unintended consequences occur, such as a premature roof system failure. A roof system’s strength is only as good as its weakest link. The roof cover is the last component in the design of a durable, sustainable roof system—defined previously as being of long-term performance, which is the essence of sustainability.

This ballasted 90-mil EPDM roof was designed for 50 years of service life. All the roof-system components were designed to complement each other. The author has designed numerous ballasted EPDM roofs that are still in place providing service.

PHOTO 1: This ballasted 90-mil EPDM roof was designed for 50 years of service life. All the roof-system components
were designed to complement each other. The author has designed numerous ballasted EPDM roofs that are still in place providing service.

The roof cover for this article is defined as the waterproofing membrane outboard of the roof deck and all other roof-system components. It protects the system components from the effects of climate, rooftop use, foot traffic, bird and insect infestation, and animal husbandry. Without it, there is no roof, no protection and no safety. When mankind moved from cave dwellings to the open, the first thing early humans learned to construct was basic roof-cover protection. Thus, roof covers have been in existence since man’s earliest built environment.

WHAT CONSTITUTES AN APPROPRIATE ROOF COVER?

There is no one roof cover that is appropriate for all conditions and climates. It cannot be codified or prescribed, as many are trying to do, and cannot be randomly selected. I, and numerous other consultants, earn a good living investigating roof failures that result from inappropriate roof-cover and system component selection.

There are several criteria for roof-cover selection, such as:

  • Compatibility with selected adhesives and the substrate below.
  • Climate and geographic factors: seacoast, open plains, hills, mountains, snow, ice, hail, rainfall intensity, as well as micro-climates.
  • Compatibility with the effluent coming out of rooftop exhausts.
  • Local building-code requirements, such as R-value, fire and wind requirements.
  • Local contractors knowledgeable and experienced in its installation.
  • Roof use: Will it be just a roof or have some other use, such as supporting daily foot traffic to examine ammonia lines or have fork lifts driven over it?
  • Building geometry: Can the selected roof cover be installed with success or does the building’s configuration work against you?
  • Building occupancy, relative humidity, interior temperature management, building envelope system, interior building pressure management.
  • Building structural systems that support the enclosure.
  • Interfaces with the adjacent building systems.
  • Environmental, energy conservation and related local code/jurisdictional factors.
  • Delivering on the expectations of the building owner: Is it a LEED building? Does he/she want to go above and beyond roof insulation thermal-value requirements to achieve even better energy savings? Is he/she going to sell the building in the near future?

ROOF-COVER TYPES

There are many types of roof-cover options for the designer. Wood, stone, asphalt, tile, metal, reed, thatch, skins, mud and concrete are all roof covers used around the world in steep-slope applications. This article will examine the low-slope materials.

The dominant roof covers in the low-slope roof market are:

    Thermoset: EPDM

  • Roof sheets joined via tape and adhesive
  • Installed: mechanically fastened, fully adhered or ballasted
  • Thermoplastic: TPO or PVC

  • Roof sheets joined via heat welding
  • Installed: mechanically fastened, fully adhered or plate-bonded (often referred to as the “RhinoBond System”)
  • Asphaltic: modified bitumen

  • Installed in hot asphalt, cold adhesive or torch application
  • EPDM (ETHYLENE PROPYLENE DIENE MONOMER)

    Fully adhered EPDM on this high school in the Chicago suburbs is placed over a cover board, which provides a high degree of protection from hail and foot traffic.

    PHOTO 2: Fully adhered EPDM on this high school in the Chicago suburbs is placed over a cover board, which provides a high degree of protection from hail and foot traffic.


    EPDM is produced in three thicknesses— 45, 60 and 90 mil—with and without reinforcing. It can be procured with a fleece backing in traditional black or with a white laminate on top. The lap seams are typically bonded with seam tape and primer.

    EPDM has a 40-year history of performance; I have 30-year-old EPDM roof systems that I have designed that are still in place and still performing. Available in large sheets—up to 50-feet wide and 200-feet long—with factory-applied seam tape, installation can be very efficient. Fleece-back membrane and 90-mil product have superior hail and puncture resistance. Historical concerns with EPDM lap-seam failure revolved around liquid- applied splice adhesive; with seam tape technology this concern is virtually moot. Non-reinforced ballasted and mechanically fastened EPDM roof membrane can be recycled.

    EPDM can be installed as a ballasted, mechanically fastened or fully adhered system (see photos 1, 2 and 3). In my opinion, ballasted systems offer the greatest sustainability and energy-conservation potential. The majority of systems being installed today are fully adhered. Ballast lost its popularity when wind codes raised the concern of ballast coming off the roof in high-wind events. However, Clinton, Ohio-based RICOWI has observed through inspection that ballasted roofs performed well even in hurricane-prone locations when properly designed (see ANSI-SPRI RP4).

    PHOTOS: HUTCHINSON DESIGN GROUP LTD

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Asphalt-based Low-slope Roof Systems Provide Long-term Service Life

Asphalt-based roof systems have a long-standing track record of success in the roofing industry. In fact, asphalt-based roof systems have more than a century of use in the U.S. Building owners, roofing specifiers and contractors should not lose sight of this fact. It is important to understand why asphalt roofing has been successful for so long. Asphalt roofs demonstrate characteristics, such as durability and longevity of materials and components, redundancy of waterproofing, ease and understanding of installation, excellent tensile strength and impact resistance. Each of these characteristics helps ensure long-term performance.

Using a composite built-up/ modified bitumen roof system provides redundancy helping ensure durability and longevity. Surface reflectivity and a multilayer insulation layer provide excellent thermal resistance. Quality details and regular maintenance will provide long-term performance. PHOTO: Advanced Roofing

Using a composite built-up/
modified bitumen roof system provides redundancy helping ensure durability and longevity. Surface reflectivity and a multilayer insulation layer provide excellent thermal resistance. Quality details
and regular maintenance will provide long-term performance. PHOTO: Advanced Roofing

There are two types of asphalt-based low-slope roof systems: modified bitumen (MB) roof systems and builtup roof (BUR) systems. MB sheets are composed primarily of polymer-modified bitumen reinforced with one or more plies of fabric, such as polyester, glass fiber or a combination of both. Assembled in factories using optimal quality-control standards, modified bitumen sheets are manufactured to have uniform thickness and consistent physical properties throughout the sheet. Modified bitumen roof systems are further divided into atactic polypropylene (APP) and styrene butadiene styrene (SBS) modified systems. APP and SBS modifiers create a uniform matrix that enhances the physical properties of the asphalt. APP is a thermoplastic polymer that forms a uniform matrix within the bitumen. This matrix increases the bitumen’s resistance to ultraviolet light, its flexibility at high and low temperatures, and its ability to resist water penetration. SBS membranes resist water penetration while exhibiting excellent elongation and recovery properties over a wide range of temperature extremes. This high-performance benefit makes SBS membranes durable and particularly appropriate where there may be movement or deflection of the underlying deck.

BUR systems consist of multiple layers of bitumen alternated with ply sheets (felts) applied over the roof deck, vapor retarder, and most often insulation or coverboard. BUR systems are particularly advantageous for lowslope applications. The strength of the system comes from the membrane, which includes the layers of hot-applied bitumen and the reinforcing plies of roofing felt.

FACTORS FOR LONG-TERM PERFORMANCE AND SERVICE LIFE

It is important for building owners and roof system designers to recognize the principles of long-lasting, high-performance roof systems. Roof longevity and performance are determined by factors that include building and roof system design, job specifications, materials quality and suitability, application procedures and maintenance. The level of quality in the workmanship during the application process is critical.

Longevity and performance start with proper design of the asphalt-based roof system. Proper roof system design includes several components: the roof deck, a base layer supporting a vapor retarder or air barrier when necessary, multi-layer insulation and a coverboard, the asphaltic membrane, appropriate surfacing material or coating, and the attachment methods for all layers. Roof consultants, architects and roof manufacturers understand proper design. Roof design needs to follow applicable code requirements for wind, fire and impact resistance, as well as site-specific issues, such as enhanced wind resistance design, positive drainage and rooftop traffic protection. Roof designers can provide or assist with the development of written specifications and construction details that are specific to a roofing project for new construction or reroofing.

Low-slope asphalt-based roof systems are redundant; they are multi-layered systems. BUR systems include a base sheet, three or four reinforcing ply sheets and a surfacing, either aggregate (rock) or a cap sheet. MB sheets include one and sometimes two reinforcing layers and are commonly installed over a substantial asphaltic base sheet. Modified bitumen roofs can be granule surfaced, finished with reflective options or coated after installation. Aggregate, granules, films and coatings add UV protection, assist with fire resistance, provide durability to the roof system and can improve roof aesthetics.

An asphaltic cap sheet with a factory-applied reflective roof coating is installed over three glass-fiber ply sheets and a venting base sheet. The reflective coating reduces heat gain, and insulating concrete provides a stable substrate and high R-value. PHOTO: Aerial Photography Inc.

An asphaltic cap sheet with a factory-applied reflective roof coating is installed over three glass-fiber ply sheets and a venting base sheet. The reflective coating reduces heat gain, and insulating concrete provides a stable substrate and high R-value. PHOTO: Aerial Photography Inc.

Coverboards provide a durable layer immediately below the membrane, are resistant to foot traffic and separate the membrane from the thermal insulation layer. Protecting the thermal insulation helps maintain the insulation R-value as specified and installed.

Asphalt is a durable and long-lasting material for roof membranes and flashings. Asphalt is stable under significant temperature swings and can be highly impact resistant. Various reinforcements can be used to increase an asphaltic membrane’s durability. All asphaltic membranes are reinforced, during installation (BUR) or the manufacturing process (MB membranes). Polyester reinforcement has excellent elongation, tensile strength and recovery. It provides good puncture resistance and stands up well to foot traffic. Glass fiber reinforcement resists flame penetration and provides excellent tensile strength and dimensional stability.

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