SBS-Modified Bitumen Roofing

A look at product composition, properties and code-related attributes
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Sponsored by SOPREMA, Inc.
By Matt Davis

Application Methods

There are a number of application options for SBS-modified bitumen membranes giving this product a great range of flexibility to meet almost any application circumstance.

Hot Asphalt

Let’s start with the oldest method of adhering SBS-modified bitumen membranes: by way of hot asphalt. This method is an offshoot of BUR application, as a majority of U.S. roofing contractors were familiar with the practice of adhering roofing with hot asphalt when SBS-modified bitumen was introduced in the U.S. in the late 1970s. Despite its longevity as an application method, oxidized asphalt mopping is not an optimum adhesive for SBS-modified sheets.

While oxidized asphalt may rarely be given a second thought as an interply component in an overall membrane, the differences between various ASTM D312 mopping asphalt options can affect long-term performance of the membrane. These laboratory findings have been supported by what we see in the field—some asphalts perform significantly better than others. Laboratory testing and field experience have also demonstrated that other interply bonding choices offer higher performance than asphalt mopping, regardless of hot asphalt quality.

Also, asphalt flux sources can change frequently at oxidizing plants, making it unlikely that a key ingredient in modified- bitumen roofing membranes will remain consistent from job to job. Monitoring asphalt quality is not easy, and the only way to ensure total compatibility is to purchase asphalt from the same plant that supplies the asphalt used to produce modified-bitumen sheets—not always an easy undertaking. Finally, all oxidized asphalts become brittle over time, and testing according to standards such as ASTM D5849 may be necessary to understand just how much bond strength and blistering resistance will be lost over time.

While hot asphalt has been used for decades on rooftops and may present a “comfortable option,” at this point, the method is archaic and better options exist.

Heat Welding

Heat welding has been in the U.S. since the early 1970s. It is usually done with a roofing torch, but sophisticated welding equipment including flame and flameless hot air welding is also available.

Certain SBS-modified bitumen products have been specifically engineered for torching and are often paired with patented surface treatments that enhance application efficiency and performance. While torches sometimes cannot be deployed due to job-specific limitations, this roofing application method is quite popular because it is theoretically impossible to separate the two membranes as they are fused together and become one; they become monolithic.

That said, proper training—see the NRCA/MRCA Certified Roof Torch Applicator (CERTA) program—and care during installation are crucial to attaining long-lasting performance. Under-heating can cause sheets not to fuse together, creating the possibility of interply blistering, and overheating can lead not only to poor aesthetics, but potential performance problems as well. The type of modified-bitumen membrane material, installer familiarity with the product, installation technique and ambient conditions can all affect the quality of watertight bonds formed during heat welding. When conditions are ideal, however, this method can yield the ultimate interply weld.

Cold Adhesive

The bonding characteristics of cold adhesives are comparable to heat welding in that they can create a virtually homogenous membrane. Most cold adhesives are primarily composed of asphalt, solvent and a mixture of fillers, fibers and stabilizers. Cold adhesives have been used in roofing since the 1970s but the technology has advanced substantially. Cutback adhesives— basically asphalt mixed with solvents, fibers and stabilizers to create an adhesive—can be used to create a solvent weld that has been shown to hold fast for decades. Solvents bond to the sheet materials and then they evaporate, leaving a thin layer of adhesive to form the membrane.

The choice of solvent is important to consider when preparing for your application, as they can vary by flash point, boiling point, aromaticity, levels of VOCs (which may be regulated), and so on. You may also see variance in base asphalt quality, as well as the amount and types of fillers, fibers and stabilizers that affect performance. Overall, however, cold adhesives have the advantage of creating a chemical bond with a strength similar to that achieved with a heat weld, minimizing the likelihood of blister formation or disbanding over time.

The most significant advances have come in cold adhesive technology with regard to the low-VOC bonding options. New ultra-low VOC cold adhesives are moisture-cured and contain no solvents or isocyanates. These adhesives are also advantageous in that they can be used almost anywhere at any time—from hospitals to schools to offices—all without disturbing the occupants. In addition to the application versatility of theses cold adhesives, they also give the roofing system an additional layer of elastomeric membrane.

Self-Adhesive

Self-adhesive roofing membranes were introduced in Europe in the mid-1970s and were introduced into the U.S. soon thereafter. As with most technologies, their performance has improved notably over the past several decades. Self-adhesive membranes are great when application options are limited:

  • No open flame; no asphalt allowed.
  • VOC limitations for cold adhesives.
  • Odor or other fume concerns.

These challenges, which can result when working with historical structures, healthcare or education facilities, or combustible environments, can be overcome by using self-adhered membranes. These options facilitate speedy installation, leave behind little mess, require few tools and are generally environmentally-friendly options. Most manufacturers have specific primers for self-adhered applications that have been formulated to significantly increase the adhesion of the membrane to the substrate, making it important to ensure you always read the specific instructions for the self-adhesive membrane you are applying. One other thing to keep in mind about self-adhered membranes is that there are some low-temperature limitations, as thermal cycles/solar heating help bond membranes.

Mechanical Fastening

A final application option is mechanical fastening of membranes to structural elements. In high-wind zones (with hurricanes, tornados or coastal areas), mechanically fastened SBS-modified membranes attain some of the highest wind-uplift resistance values.

The membranes are fastened in the side lap, through the insulation directly into the deck below, anchoring the sheet directly into the structural deck. Additional fasteners may be placed in the center of the sheet to further increase the wind-uplift resistance. By fastening directly into the roof deck, you are tapping into all of the strength of the physical structure instead of only relying on the adhesive method used to attach the roofing membrane.

This graph shows a conventional roof system versus a mechanically fastened system.

This graph shows a conventional roof system versus a mechanically fastened system. You will notice that with the mechanically fastened system, cover board is not required.

Mechanical fastening also has the benefit of allowing for the removal of cover board from the system, saving time and money. Mechanical fastening is also a very fast, labor-saving method of application. For this reason, the method is sometimes chosen even when high wind-uplift resistance is not a concern.

 

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Originally published in Building Enclosure

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