Metal Roofing Underlayments Protect Structures in Hawaii

The newly constructed Safeway Shopping Center, Honolulu, happens to be the largest Safeway on the Hawaiian Islands. It contains a parking garage below—in part, because of its location in a densely populated neighborhood.

The underlayment manufacturer worked on and approved a design in which the underlayment could be installed directly on the metal deck.

The underlayment manufacturer worked on and approved a design in which the underlayment could be installed directly on the metal deck.

ITS METAL ROOF WAS INSTALLED by Kapolei, Hawaii-based Beachside Roofing, which has been doing business in Hawaii for more than 25 years. The company, which installs all kinds of roofing and waterproofing systems, specializes in high-rise buildings, resorts and complex projects.

The 20,000-square-foot metal roof on the Safeway store had to meet strict color requirements in keeping with the Safeway brand. The color of the roof is Gargoyle, which is a greenish-brown.

The metal roofing was designed to be installed over corrugated 20-gauge steel decks. The underlayment manufacturer worked on and approved a design in which the underlayment could be installed directly on the metal deck.

The metal deck (HSB-36SS type) was installed with the wider corrugations facing up and parallel to the eaves (horizontally). The self-adhering underlayment also was installed horizontally, and the metal panels were then attached to the horizontal corrugations of the deck using panel clips and self-drilling fasteners penetrating through the underlayment into the flattop of the corrugations of the steel deck.

The self-adhering underlayment also was installed horizontally, and the metal panels were then attached to the horizontal corrugations of the deck using panel clips and self-drilling fasteners penetrating through the underlayment into the flattop of the corrugations of the steel deck.

The self-adhering underlayment also was installed horizontally, and the metal panels were then attached to the horizontal corrugations of the deck using panel clips and self-drilling fasteners penetrating through the underlayment into the flattop of the corrugations of the steel deck.

The walkability of the underlayment was an important factor, considering that the roof slope was 4 inches per 12 feet in some places. Also, the 120-day exposure allowance for the underlayment was reassuring, though not necessary for this project.

The metal roofing system included many architectural elements, such as canopies, penthouses and mansards. It covers not just the Safeway supermarket, but also other shops in the Safeway Shopping Center. The way the metal was used architecturally really dressed up the exterior of the project.

Secondary Water Barrier

A self-adhering metal roofing underlayment, like the one on the Safeway Shopping Center, perfectly complements metal roofing panels. The underlayment provides a watertight secondary membrane while the metal panels serve as the primary roof to protect against wind-blown objects and UV radiation. If the primary roof is damaged, the secondary roof acts as the water barrier.

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Substrate Boards

The third installment in my series on the roof system is about the substrate board. (To read my first two articles, “Roofs Are Systems” and “Roof Decks”, see the January/February issue, page 52, and the March/April issue, page 54, respectively.) For the purpose of this article, we will define the substrate board as the material that is placed upon the roof deck prior to the placement of thermal insulation. It often is used in part to support vapor retarders and air barriers (which will be discussed in my next article in the September/October issue).

The type of substrate board should be chosen based on the roof-deck type, interior building use, installation time of year and the cover material to be placed upon it.

The type of substrate board should be chosen based on the roof-deck type, interior building
use, installation time of year and the cover material to be placed upon it.

Substrate boards come in many differing material compositions:
• Gypsum Board
• Modified Fiber Reinforced Gypsum
• Plywood
• High-density Wood Fiber
• Mineral Fiber
• Perlite

Substrate boards come in varying thicknesses, as well: 1/4 inch, 1/2 inch, 5/8 inch and 1 inch. The thickness is often chosen based on the need for the board to provide integrity over the roof deck, such as at flute spans on steel roof decks.

TOUGHNESS

The type of substrate board should be chosen based on the roof-deck type, interior building use, installation time of year and the cover material to be placed upon it. For example, vapor retarder versus thermal insulation and the method of attachment. Vapor retarders can be adhered with asphalt, spray foam, bonding adhesive, etc. The substrate board must be compatible with these. You wouldn’t want to place a self-adhering vapor retarder on perlite or hardboard because the surface particulate is easily parted from the board. Meanwhile, hot asphalt would impregnate the board and tie the vapor-retarder felts in better. The substrate board must have structural integrity over the flutes when installed on steel roof decks. The modified gypsum boards at 1/2 inch can do this; fiberboards cannot. If the insulation is to be mechanically fastened, a substrate board may not be required.

It should be more common to increase the number of fasteners to prevent deformation of the board, which will affect the roof system’s performance.

It should be more common to increase the number of fasteners to prevent deformation of the board, which will affect the roof system’s performance.

The substrate board should be able to withstand construction-generated moisture that may/can be driven into the board. Note: In northern climates, a dew-point analysis is required to determine the correct amount of insulation above the substrate board and vapor retarder, so condensation does not occur below the vapor retarder and in the substrate board.

Substrate boards are often placed on the roof deck and a vapor retarder installed upon them. This condition is often used to temporarily get the building “in the dry”. This temporary roof then is often used as a work platform for other trades, such as masonry, carpentry, glazers and ironworkers, to name a few. The temporary roof also is asked to support material storage. Consequently, the substrate board must be tough enough to resist these activities.

The most common use of a substrate board is on steel and wood decks. On steel roof decks, the substrate board provides a continuous smooth surface to place an air or vapor retarder onto. It also can provide a surface to which the insulation above can be adhered. Substrate boards on wood decks (plywood, OSB, planking) are used to increase fire resistance, prevent adhesive from dripping into the interior, provide a clean and acceptable surface onto which an air or vapor retarder can be adhered, or as a surface onto which the insulation can be adhered.

PHOTOS: HUTCHINSON DESIGN GROUP LTD.

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