By Justin Koscher

When planning for a roof on a new building or reroofing an existing structure, finding the right products to balance cost and performance is a tall order. Whether it is replacement on an older building or new construction, the right roof system can reduce installation costs and improve building performance.

Many architects, specifiers, and contractors include cover boards in conventional low-slope roof assemblies (with a roof membrane as the outermost component) to enhance overall system durability and lower long-term maintenance costs. A cover board is a thin substrate to which a roof membrane is secured. It can help extend the life of a roof assembly by providing a tough, resilient layer for improved wind uplift resistance, increased impact resistance from construction/service traffic, and high compressive strength.

In a conventional roof system, the cover board is installed on top of insulation (below the membrane). Many product types are available, ranging from traditional gypsum board, cement board, perlite board, asphalt/glass board, glass-based board, mineral wool, oriented strand board (OSB), polyurethane-based board, and wood fibre board to highly engineered, high-density (HD) polyisocyanurate (polyiso) cover boards.

Specifying polyiso cover boards

All HD polyiso cover boards are developed and tested to be part of a roofing system. Most HD polyiso cover boards are compatible with mechanically attached and fully adhered single-ply roof systems (e.g. thermoplastic polyolefin [TPO] or ethylene propylene diene monomer [EPDM]) and cold-applied modified bitumen (mod-bit) and built-up roof (BUR) applications.

When specifying cover boards, several factors should be considered, including roof-covering type and installation method, project/building location (which may govern wind uplift requirements), as well as anticipated service conditions (i.e. traffic or additional rooftop equipment such as photovoltaic [PV] panels). The designer, specifier, or contractor is encouraged to consult roof system manufacturers for guidance and assistance in specifying the system that best meets the needs of the project, design team, and/or building owner.

In Canada, HD polyiso cover boards may be designated as Type 4, 5, or 6 (depending on physical properties, including compressive, tensile, and flexural strength) in accordance with the Underwriters Laboratories of Canada (CAN/ULC) S704.1:2017, Thermal Insulation, Polyurethane and Polyisocyanurate, Boards, Faced. Type 4, 5, and 6 products have significantly higher requirements for physical properties as indicated in Figure 1. Type 1, 2, and 3 products are typically designated for roof insulation (installed above the roof deck and below the cover board).

The use of cover boards, although not required by the National Building Code (NBC), is a best building science practice as it offers added resiliency and durability to the roof system. An ongoing research consortium at the National Research Council Canada (NRC) is focusing on evaluating the available cover board product types. The outcome of this research will pave the way for developing a Canadian standard for cover board products.