Building Envelope Vulnerability

04/08/2009 | By Richard L. Fricklas

Building managers and owners need to be aware of the many hazards that must be addressed to keep their buildings operational. The building envelope deserves attention for its vulnerability, as it receives the full brunt of the weather.

In the case of roofing systems, building stresses become magnified at walls, transitions, and other closures. To add to these expected challenges, we have the human factor: abuse, misuse, vandalism, and neglect.

When conventional asphaltic built-up roofs were dominant, the flashings at walls and curbs were always beefed up. The system generally started with a cant strip to break the angle between the horizontal and vertical elements, softening the angle of change and to provide support for all the plies that would follow. Ply felts, adhered using hot asphalt, were mopped to the cant strip and to each other, followed by a “backer-ply” that ran up the wall or curb for extra kick-through resistance. Finally a fabric-reinforced asbestos-flashing sheet was bonded to the backer sheet.

When reinforced asbestos base flashings were withdrawn from the marketplace in the 1980s, it was found that the substituted glass fiber mat systems did not provide the same conformability, durability, and toughness as their predecessors. Fortunately, polymer-modified bitumens (also referred to as mod bits or MBs) were being introduced first in Europe, and then here, which were robust enough to fill this gap in requirements.

Another trend brought to us from Europe during this period was substitution of the heat of a propane torch flame for the complicated process of mopping hot asphalt. The backing bitumen on the MB sheets was factory applied, and the roofing contractor needed to simply reheat this material to achieve a durable, waterproof assembly. (See Fig. 2.) The logistical necessities were thus reduced to a bottle of propane, a hose, and a torch head compared to the previously required cartons of asphalt on the ground (or a tanker), a kettle-man, pumps, hot luggers, mop carts, and the like. The needed heat was delivered instantly by pulling on the propane torch trigger rather than relying on the long-distance chain involved in getting hot material from the kettle to the point of application.

The simplicity of the torch-melted systems offers other benefits as well. Repairs could more be more feasibly made to weathered built-up roofing, as well as to MB systems. Rather than lug 5-gallon pails of mastic to sites of broken blisters, puncture damage, or slipped flashings, a handheld propane torch could soften damaged materials; evaporate snow, water, or ice; permit embedment of roofing granules at tie-ins and laps; and compensate for ambient conditions, such as freezing weather. Once the back of the MB patch was heated, and the patch pressed in, we were done.

The Fire Concern
With all the virtues of torched-applied MB systems, there came a new downside – the fire hazard. Without the proper precautions, a fire could occur during application, or even several hours after the roofing crew had left the jobsite. These hotspot fires could result from smoldering combustible underlayment materials, such as fiberboard cant strips, lint-coated ducts, unprotected wood curbs, and the wood deck itself. European constructions tended to be monolithic concrete, limiting potential fire danger, whereas, in the United States, there are many air-permeable, wood, OSB, and steel decks overlaid with combustible wood fiber, glass fiber, or perlite thermal insulations.

FM Documentation of Fire Occurrences
FM Global published a Loss Prevention Data Technical Advisory Bulletin in October 1988 to address some of these concerns:

Select excerpts follow:

Fire Started by Roofer’s Torch
Repair activity to MB-recovered BUR consisted of 2- by 6-foot patch being applied with a propane torch. Fire watch was 10 feet away, and a 25mph wind was blowing. About 2 squares of glass fiber insulation and new MB roof covering were fire damaged, as well as five roof-mounted air-handling units.

This loss illustrates the need for an adequate fire watch, the use of charged hose lines, and the use of a base sheet over certain insulations.

Roofer’s Torch Ignites HVAC Unit Support Structure
HVAC units were mounted on the roof of an electronic assembly and machining area. The roof was constructed of plywood on wood glulam beams. The support structure for the HVAC units was also wood. Roofers were installing base flashing with a torch along the perimeter of an HVAC unit located over a clean room area. The open flame from the roofer’s torch ignited the wood support structure for the HVAC unit. Smoke was noted exiting the perimeter of the HVAC unit. A dry chemical extinguisher was used unsuccessfully. Three sprinklers and hose streams controlled the fire about 25 minutes later.

The HVAC unit was fire damaged, and water and soot damage occurred in the clean room area. This loss illustrates the need to exercise caution when working around roof penetrations.


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