Building to last with BURs

April 21, 2017

Photos courtesy John Manville

By Michael Russo
The advantages of a built-up roofing (BUR) 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 with endorsing built-up roofing,” says Luther Mock, RRC, FRCI and founder of building envelope consultants, Foursquare Solutions Inc. “The redundancy created by multiple plies of roofing felt is really what sets BUR apart.”

One can argue BUR’s closest cousin—which is 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 exceed any potential oversights in rooftop workmanship.

Both attractive and reflective, this built-up roofing (BUR) system has a proven track record of performance unmatched by other low-slope roofing systems.

“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 hot-applied asphalt systems because of their long lives,” says Reed Hitchcock, the group’s executive director.

Benefits with BUR
Over the years, BUR 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 roofcovering and can be used in a broad range of building waterproofing applications.

Available as part of fire-, wind-, and/or hail-rated systems, BUR assemblies offer waterproofing, high tensile strength, long-term warranties, and a wide choice of top surfacing (including ‘cool’ options). Their components include the deck, vapour retarder, insulation, membrane, flashings, and surfacing material. The roofing membrane is made up of two components—felts and bitumen. The former strengthens and stabilizes the latter, which 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 rating.

Another potential surfacing option is gravel, commonly used in Canadian applications where the additional weight on the roof system would not be a problem. 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 of Canada when required by code.

An aerial view of a reflective roof membrane.

Cold-process BUR
Cold application of BUR has provided an alternative to traditional hot-applied systems for more than 30 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.

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 protection and a fire rating for the roof system.

“In the reroofing 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.”

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. If too much is applied, then the potential for longer setup times and membrane displacement is increased. Additionally, ambient temperatures must be 5 C (40 F) and rising before installation. This limits, but does not preclude, use of cold-process BUR in much of Canada.

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

“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 labour 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 cap sheets are more common. Roofing manufacturers have also developed liquid-applied flashing systems offering numerous benefits. First, they are typically included in manufacturers’ guarantees. For property owners, this means fewer guarantee exclusions and less maintenance, with an increased return on investment (ROI) due to lower life-cycle costs. Liquid-applied flashings also offer esthetic benefits by matching the finish of surrounding roof areas.

Fluid-applied flashings are especially welcome when roofing contractors are faced with irregular roof penetrations. One roof system manufacturer uses a flexible, strong, and puncture-resistant polyester scrim as the reinforcement for its flashing system. The scrim is sandwiched between layers of solvent-free elastomeric, cold-applied adhesive. The two-component product consists of an asphalt base material chemically cured with an ‘activator.’ Reinforced with the polyester scrim, the resulting low-permeability system forms a durable elastomeric seal, which bonds strongly with a range of substrates. Again, it is important to check with the manufacturer’s instructions regarding correct use for cold-weather applications.

Non-gravel BUR surfacing options include aggregate, mineral surface cap sheet, or a smooth, surface-coated membrane. After many years of exposure to the elements, all these surfacing materials may need some attention due to possible aging and weathering of the BUR surface.

BUR repair and maintenance
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 (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.

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.

On the positive side, BUR has a long history of proven performance in Canada, where snow and ice buildup are common. Perhaps more than any other roof membrane, the system shrugs off minor abuse. When facilities managers feel the need to shovel snow off the rooftop (though this method is rarely recommended), BUR’s puncture resistance rises to the forefront.

Nevertheless, 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 new roofs to guard against errors in installation, design, or specifications.

BUR has proven to be a low-maintenance roofing system, and 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. These BUR maintenance and repair solutions are typically more involved than peel-and-stick patching on some single-ply systems. However, like BUR, these repair options have proven to work well over time.

Non-gravel BUR surfacing options include aggregate, mineral surface cap sheet, or a smooth, surface-coated membrane. After many years of exposure to the elements, these surfacing materials may need some attention due to possible aging and weathering of the BUR surface. The loss of protective surfacing or coating from weathering may give the BUR a cracked appearance known as ‘alligatoring.’ This form of BUR aging is much less common in Canada than in warmer climates.

Any moderate alligator cracking can be treated by cleaning the surface of the roof, applying a primer, and resurfacing with an asphalt emulsion. Bare spots resulting from wind scour or water erosion require the removal of dirt and loose aggregate. These areas can then be primed and recoated before new aggregate is re-imbedded into the surface.

Splits or tears in aging BURs can also be readily treated. Any poorly attached elements of the BUR can be re-secured with fasteners into the deck. The fasteners and splits can then be stripped in with a mod-bit membrane. Ridging on aging BUR membranes can form due to internal moisture collecting at the insulation joints, insulation movement, or curling. When the top of these ridges lose their surfacing, they can be cleaned and recoated with a roof coating or mastic and aggregate to prevent further roof deterioration. Crazing, punctures, building movement, damage to base flashings, and membrane curling will expose a BUR to increased weathering. However, these issues can be addressed through proper maintenance.

Before the advent of modified bitumen flashings, metal-edge gravel stops and edging for BUR generally consisted of a double layer of felt stripped in after the finish coat of bitumen and surfacing or cap sheet was applied. After years of thermal or wind-induced stresses, splits can form at the joint between the metal edging and the stripping felts. Today, these areas can be repaired with mod-bit flashing material to add strength and flexibility to the edge metal detail.

Missing or badly deteriorated metal cap flashings can also be replaced. Loose metal sections can be refastened and then resealed at the reglet or top of the counter-flashing with a construction-grade sealant. Generally, this is only an option for reglet- or surface-mounted flashings. All too often, using this sealant repair option on through-wall flashings makes a bad problem worse.

Expansion joint problems are generally associated with failures at the joint connection and deterioration of rubber bellows due to weathering. Deteriorated bellows should be replaced, while loose metal cap expansion joints can be re-secured, along with the use of an elastomeric sealant at joints for added flexibility. On older BUR designs using pitch pockets, the fill material has a tendency to shrink over time. In this case, an elastomeric sealant (not bitumen or roof cement) can restore pitch pocket seals and slope.

“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 fits every building specification. Probably more than any other assembly (except sprayed polyurethane foam [SPF]), the BUR application is much more an art 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 BURs.

Labour-intensive, built-up roofs’ installed cost fluctuates 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. Of course, these providers may or may not be passing on these savings to their customers. BUR has always held up well in life-cycle cost analyses. However, if a roof is not required to last more than 20 years, it usually does not make sense to specify a premium four-ply BUR.

On larger projects, gravel-surfaced BURs are typically impractical from a cost standpoint unless a local source of gravel is available. Projects where roof access is difficult often present challenges when roofing kettles are used. Despite the preponderance of low-fuming asphalts and kettles, reroofing occupied buildings is often unacceptable to neighbours and/or property owners. (Local codes and regulations should also be consulted.)

Built-up roofing systems have sufficient strength to resist normal expansion and contraction forces 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:

In some cases, it simply is not 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, including avoidance of overstocking. Breathable tarps can cover material on the roof. Materials should be stored on pallets to minimize the possibility of material sitting in water, with rolls kept ‘on-end’ to prevent crushing. Generally, polymeric single-ply membranes are less susceptible to storage abuse.

Many roof consultants and product manufacturers clearly state there should be no phased construction of a built-up roof. If phasing is required, than a BUR should not be specified. This is a 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 mentioned, costlier mod-bit 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 1.5-mm (60-mil) thermoplastic polyolefin (TPO) or polyvinyl chloride (PVC) roof membrane can save the property owners money and eliminate premature roof failure due to roof splitting.

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 like BUR offer exceptional life-cycle cost performance. They have proven to be reliable and are trusted to perform exceptionally well in extreme weather conditions.

[6]Michael Russo is a consultant to various manufacturers and industry associations, including SPRI and the Roof Coatings Manufacturers Association. He was editor of Roofing/Siding/Insulation Magazine from 1980 to 2005. Russo has been reporting on the low-slope roofing industry for more than 35 years. He can be reached at[7].

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