Selecting Rigid Cover Boards in Commercial Roofing Systems

Using a roof cover board on every project is the emerging norm—and the type matters
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Sponsored by Georgia-Pacific Gypsum
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Learning Objectives:

  1. Discuss the evolution of roofing cover boards from the 1970s to present day, particularly in light of the use of single-ply membranes.
  2. Explain how the forces of nature (i.e., fire, wind uplift, moisture, hail/puncture, foot traffic, and sound) affect roof function and durability of different types of roof cover boards.
  3. Summarize the advantages related to construction process and cost benefits when including a cover board in all types of commercial roof assembly design.
  4. State the roof system types, specialty systems (i.e., photovoltaic and vegetative roofs), and attachment options that can be utilized specifically with glass mat gypsum cover boards.

Credits:

1 AIA LU/HSW
1 AIBD P-CE
0.1 IACET CEU*
AAA 1 Structured Learning Hour
AANB 1 Hour of Core Learning
AAPEI 1 Structured Learning Hour
SAA 1 Hour of Core Learning
MAA 1 Structured Learning Hour
NSAA 1 Hour of Core Learning
OAA 1 Learning Hour
NLAA 1 Hour of Core Learning
NWTAA 1 Structured Learning Hour
 
This course can be self-reported to the AIBC, as per their CE Guidelines.

On most commercial, institutional, and industrial buildings, roofing is a big investment whether at the time of first construction or when it needs to be replaced. That’s the primary reason durability of the roof is so important: it translates directly into a lower total cost of ownership. It can also make a difference in terms of how the roof performs during routine and severe weather conditions to protect the building from the elements. Recognizing this need for durability, the National Roofing Contractors Association has recommended, for many years, the inclusion of a rigid cover board in all single-ply commercial roofs. However, the design and construction community has been slow in adopting this best practice for all situations and instead only tends to specify or use a roof board in specific situations, such as when a high-performance roof is necessary. Of late, new technologies, new studies, and experiences both with and without cover boards have pointed to the fact that such roof boards are a logical and economical choice. Based on all of these factors, this course looks at the value that rigid cover boards deliver to a commercial roof assembly, how they perform in adverse conditions, and how different types of cover boards compare to each other.

All images courtesy of Georgia-Pacific Gypsum

Roofing system performance can be enhanced by incorporating the NRCA recommendation to use a noncombustible, rigid roof board as part of all low-slope commercial roofing assemblies.

Evolution of Roofing and Rigid Cover Boards

We begin with a brief historic perspective of how cover boards have evolved over time to support new types of roof assemblies and meet the growing needs of building owners. Note that the terms “roof board” and “cover board” are sometimes used interchangeably. Roof board is a generic term describing a rigid board used in a commercial roof assembly for a number of reasons. When it is used directly over the deck, it is being used as a thermal barrier for fire protection (mitigates risk of internal building fire spreading via the roof). When used between the insulation and membrane, it is called a cover board. Cover boards are used to protect different parts of the assembly from a number of concerns, including external fire, wind uplift, puncture, and impact.

From the 1920s through the 1970s, there was relatively little innovation in commercial roofing systems. It’s true that there were changes during this timeframe in the materials used (e.g., coal tar pitch to modified bitumen), but the essential process of mopping tar or asphalt between multiple layers of felt material remained essentially unchanged. When insulation was used in commercial roofing through this time period, it was a soft fiberglass material that was rolled over the structural deck of the roof. Cover boards (which were called overlayment at the time) were primarily used to keep the asphalt from being absorbed into the fiberglass insulation. Popular overlayment types included perlite and wood fiber boards.

An example of a fairly common commercial (low-slope) roof assembly includes: A) single-ply membrane, B) gypsum-based roof board used as a cover board, C) rigid foam insulation (typically more than one layer), D) gypsum-based roof board used as a thermal barrier, and E) metal roof deck.

The 1980s marked the start of over three decades of innovation in commercial roofing systems when the first glass mat gypsum roof boards were introduced into the market place. These boards provided superior fire resistance, wind uplift performance, moisture resistance, and sound isolation compared to traditional overlayment materials. Also introduced in the 1980s were the first rigid insulation boards, including polyisocyanurate (ISO) boards. These boards were easier to stage and install, provided better R-value, and did not absorb asphalt. Throughout the 80s, there was a lot of innovation in these rigid insulation boards, with several companies launching new, enhanced boards. Some of the first single-ply membranes were introduced during the 1980s, but hot-mop assemblies remained very much the norm.

During the 1990s, single-ply systems slowly became more popular, gaining acceptance in the marketplace due in large part to the ease of installation and overall cost. Single-ply options provided other benefits too, including enhanced energy efficiency, easier/quicker installation on larger roofs, and ease of repair. Because of these advantages of single-ply roofing, the use of hot mopping dropped from roughly 80 percent of commercial roof projects in 1990 to about half of that by the end of the decade. As the demand for single-ply systems increased, so did the demand for glass mat gypsum cover boards. The cover boards provided a structurally strong, smooth surface to adhere the membrane and provided an additional sound barrier while, most importantly, protecting the rigid insulation boards from foot traffic, hail impact, wind uplift, and fire.

Throughout the 2000s, single-ply membranes have continued to grow in popularity, currently comprising approximately 80 percent of the overall market. In response, the demand for cover boards also increased. To feed this demand, high-density ISO (HD ISO) and gypsum fiber cover boards were introduced to the market and glass mat gypsum boards were enhanced to improve adhesive coverage. At the same time, the trend for companies to lower their carbon footprints by reducing their reliance on fossil fuels led to an increase in alternative roofing systems, such as vegetative and photovoltaic roofs.

 

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Originally published in Architectural Record

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