Technical | Low-slope | Report on blistering of SBS-modified bitumen sheet products

(Adapted from a two-part article that appeared in Professional Roofing, April 2000, Page 55, and May 2000, Page 71)

SBS-modified bitumen sheet products have been used successfully in the United States since 1975. However, during the past several years, there has been a noticeable increase in the number of reports of blistering associated with mop-applied SBS-modified bitumen roof systems.

In addition, an increase in the number of telephone calls to NRCA's Technical Services Department reporting blistering associated with mop-applied SBS-modified bitumen sheet products was noted as early as 1994. Callers typically characterized the problems as random blistering throughout large portions of roof systems. (See Figure 1)

Data contained in NRCA's Project Pinpoint-a database of problem roofing jobs reported by NRCA members-reveal increases in the frequency of blistering reports for mop-applied SBS-modified bitumen sheet products from 1992 through 1998.



The ages of roof systems at the time blistering was reported range from one to 14 years old; six years old is the average age.

Manufacturers typically claimed blistering probably resulted from phased application. Other possible causes of blisters, according to manufacturers, include the use of improper mopping asphalt, voids in mopping, moisture in substrates (e.g., insulation) and excessive internal building moisture.

Field Investigation

An NRCA task force has evaluated a number of completed roofing projects where blistering has been reported. For each of the roof systems investigated, thorough visual inspections were conducted and numerous test cuts were taken.

From NRCA's field evaluations, it was determined that blistering typically occurred between base plies and SBS-modified bitumen cap sheets. For a majority of blisters, solid asphalt moppings were apparent and no visible foreign materials (e.g., debris) or moisture was observed.

For most blisters, evidence of contact and some adhesion between the substrates and bottom sides of cap sheets at the time of installation were noted.

Manufactures' instructions

To determine manufacturers' installation instructions, NRCA surveyed 25 manufacturers of SBS-modified bitumen cap-sheet products. It was discovered that manufacturers recommend specific temperatures that range from 390° F to 450° F (199° C to 232° C). Seventeen manufacturers recommend application at the equiviscous temperature (EVT) of the mopping asphalt; several manufacturers recommend EVT "at the point of contact." Five manufacturers provide specific recommendations for mop lead, ranging from 4 feet to 10 feet (1.2 m to 3 m).

Research review

Previous technical research provides useful information about mop-applied SBS-modified bitumen sheet products. "Cooling of Bitumen During Construction of Built-Up Roofing Systems-A Mathematical Mode," published in 1981 by the National Bureau of Standards (currently the National Institute of Standards and Technology), reveals that mop-applied asphalt cools relatively quickly.

For example, during ambient conditions of 70° F (21° C) and 10 mph (4.5 m/sec) winds, asphalt applied over an insulated substrate cools from 500° F (260° C) to 400° F (204° C) in six seconds and 500° F (260° C) to 300° F (149° C) in 16 seconds. This illustrates the importance of an SBS-modified bitumen sheet contacting hot asphalt as soon as possible after application to a substrate.

Also, a technical bulletin published by the Canadian Modified Bitumen Manufacturers in 1994 provides specific recommendations for installing mop-applied SBS-modified bitumen sheet products in cold weather (i.e., 40° F [4° C] or colder). The bulletin suggests applying "heat simultaneously to the underside of the sheet and the asphalt while unrolling using a torch with a rapid sweep motion." The bulletin also recommends a maximum 36 inch (914 mm) mop lead during cold-weather applications.

Laboratory analysis

Several samples obtained during NRCA's field investigation were submitted to the National Research Council Canada Institutes for Research in Construction for detailed laboratory analysis.

The analysis revealed that physical properties of blistered membrane samples did not differ significantly from samples obtained from nonblistered areas. Gas samples obtained from within membrane blisters contained air and water vapor with negligible amounts of volatile inorganic compounds. Also, there appeared to be no physical or chemical incompatibility among SBS-modified bitumen sheets and mopping asphalt samples obtained during the field investigation.

To determine the relative softening points of bitumen on the bottom sides of SBS-modified bitumen sheet products, NRCA had 10 samples of new SBS-modified bitumen sheet products analyzed. The softening points of the bitumen on the bottom sides of the sheet products ranged from 233° F (112° C) to 256° F (124° C).

Conclusions

From NRCA's field investigation and manufacturer survey, it is apparent the findings do not support manufacturers' claims that blistering typically is the result of phased application, mopping voids or substrate moisture. It also is clear there is little agreement among manufacturers regarding recommended asphalt mopping temperature, mop lead and the need to use mopping asphalt that is compatible with SBS-modified bitumen sheet products.

Recommendations

Based on the findings of NRCA's limited research, it appears blistering associated with mop-applied SBS-modified bitumen sheet products mostly is caused by inconsistent and sometimes inadequate adhesion between cap sheets and underlying base plies. Cap sheets that contact asphalt that is allowed to cool somewhat are a particular concern and likely cause of this type of inadequate adhesion.

NRCA recommends manufacturers provide more detailed instructions for SBS-modified bitumen sheet product installation. Instructions should include specific suggestions for asphalt heating and transportation; cap-sheet handling techniques; desired asphalt mopping temperature at application; a mop's handle length, head type and size; and mop lead for specific ambient application conditions.

Until manufacturers provide such detailed installation instructions, NRCA suggests the following:

• Materials should be kept dry, and surfaces covered with SBS-modified bitumen sheet products should be kept as free of debris as possible.
  
  
• Asphalt used to apply mop-applied SBS-modified bitumen sheet products should be heated as hot as possible but without damaging the asphalt or creating hazardous conditions.
  
  
• Asphalt should be applied with a maximum mop lead of about 8 feet (2.4 m), and an SBS-modified bitumen sheet should be placed in the asphalt as soon as possible. Consideration should be given to using mops with relatively short handles and lightweight head sizes; this will minimize the potential for over-extending mop lead.
  
  
• For applications in cold weather (i.e., ambient temperatures colder than 40° F [4° C]), manufacturers should be consulted for specific written installation instructions.