We’re continuing our five-part introduction to low-slope roofing systems. Part I defined low-slope roofing, described how it differs from steep-slope roofing and shared a brief history of low-slope roofing. In this post, we’ll review the components of a low-slope roofing system and the vital role each plays in a roof’s performance, paying particular attention to the roof deck and insulation/cover boards.
Structurally, a low-slope roofing system can be thought of as an open-faced sandwich. The roof deck is the hearty slice of bread upon which the sandwich is built. The middle layer of insulation and/or cover board is the meat. Finally, the roof covering – a single-ply membrane, mod bit sheets, BUR layers or a fluid-applied system – is the cheese. A description of a particular roofing system will include each of these components. For example, “The specification calls for a 60-mil fully adhered EPDM system over two layers of poly ISO insulation mechanically attached to a steel deck.”
Let’s dig in, starting with the bread and working our way up.
Roof Deck
The roof deck is the structural component of the building to which the roofing system is applied. It provides a flat surface upon which to install the roofing system and supports the weight of the roofing system, along with any equipment or other items that will be placed on the roof and the expected foot traffic.
Steel is the most-used material for roof decks. If you’re in a building and look up and see metal that looks like a repeating “W”-shaped pattern, it is a steel deck, likely 22-gauge.
Wood comes in second and is a well-performing, cost-effective choice. Plywood is used most often. Planks and oriented strand board (OSB) are other options.
Less commonly used materials are:
- Structural (Poured) Concrete: Structural concrete is found on older buildings but is not used as often today due to its higher cost.
Cementitious Wood Fiber: Better known by its brand name, TECTUM, cementitious wood fiber is a composite of wood fiber and cement. It has great acoustic insulation properties, making it a solid choice for schools, gyms, restaurants, indoor pools and other facilities where noise dampening is a priority.
- Gypsum: Gypsum, when used for roof decking, is like drywall but much thicker and denser. It comes in wide planks that are placed on steel beams, or it can be poured into place.
- Lightweight Insulating Concrete: Lightweight insulating concrete roof decks were primarily used before the introduction of polyisocyanurate (poly ISO or simply IOS) insulation boards. Expanded polystyrene (EPS) insulation is embedded between layers of lightweight insulating concrete.
Insulation and Cover Boards
Cover boards and insulation work together to create long-lasting, energy-efficient low-slope roofing systems.
Insulation reduces heat transfer between a building’s interior and the environment, keeping conditioned air inside and unconditioned air outside to reduce energy use and costs while keeping occupants comfortable. In low-slope roofs, it is placed on top of the building, rather than inside as is done in homes. This cuts in half the amount of insulation required to achieve the same R-Value, a measure of how well insulation prevents heat transfer. Most insulation boards also provide a surface for attaching the roof covering.
There are four types of low-slope roofing insulation to know:
- Poly ISO: Poly ISO boards deliver the highest R-Value and are the most-used option. A polyisocyanurate foam core is sandwiched between non-asphaltic fiberglass-reinforced felt facers, providing both insulation and a nailable or glueable surface for attaching the roof covering. Poly ISO comes in sheets 4 feet by 4 feet or 4 feet by 8 feet.
- EPS (expanded polystyrene): The consistency of EPS resembles that of the plastic foam coffee cups once common in hospitals, offices and other public places. It frequently comes without facers, in which case a cover board often must be installed on top of it to provide a surface for attaching the roof covering.
- Tapered Insulation: Boards (either poly ISO or EPS) are thicker on one side than the other and are used to create the additional slope needed to provide positive drainage. Boards are 4 feet by 4 feet. The tapered take-off can be designed to direct water to drains in the center of the roof or toward gutters at the roof’s edges. Our distributor, ABC Supply Co. Inc., has a dedicated Tapered Solutions team that provides accurate take-offs.
- Flute Filler: Flute filler is used when recovering a metal roof with a single-ply membrane. EPS or poly ISO insulation is pre-cut in a factory to fit into the flutes of the metal roof panels, creating a smooth, even surface which is then covered with additional insulation or a cover board and, finally, the membrane.
Cover boards are thinner substrates to which the roof covering is attached. They are mandatory when the roof covering cannot be directly attached to the insulation layer. And even when a cover board is not required, adding one between the roof covering and insulation can significantly enhance the system’s durability and performance.
Cover boards – and gypsum cover boards in particular – are exceptional fire barriers. If the roof covering ignites, the cover board will help keep the flames from spreading to the highly flammable insulation or wood roof deck and then into the building’s interior.
Cover boards deliver excellent wind uplift performance by providing a strong, stable surface for attaching the roof covering. In high winds, they distribute the wind more evenly throughout the roof, reducing the risk of the covering tearing or becoming detached.
Finally, cover boards can make a roofing system far more resistant to hail and foot traffic, which can pierce roof insulation boards and create holes that allow heat transfer. Cover boards absorb the impact, protecting the insulation to maintain the building’s energy efficiency.
There are four primary types of roof cover boards:
- Fiberboard – made of wood or cane fibers – is typically the lowest-cost option. They have an R-Value of 1.2 for a ½-inch-thick board.
- Gypsum-fiber roof boards, such as USG Securock®, are made of gypsum and cellulose fiber. The dense concentration of those fibers gives the boards exceptional strength and resistance to water and mold. They have an R-Value of approximately .5 for a ½-inch-thick board.
- Glass-mat faced gypsum roof boards, such as Dens Deck® by Georgia-Pacific Gypsum, have a specially treated gypsum core with an embedded glass mat facer on the top and bottom. They have an R-Value of approximately .5 for a ½-inch-thick board. They are ideally suited to fully adhered systems.
- Poly ISO HD cover boards are high-density poly ISO insulation boards specifically designed for use as cover boards. They have a much higher compressive strength than standard poly ISO boards – between 80 psi and 110 psi vs. 20 psi or 25 psi – which protects the insulation layer below from impact and compression. They are lightweight and easy to move around the job site and deliver the highest insulation value of the cover board options, with an R-Value of 2.5 for a ½-inch-thick board.
Roof Covering
The final layer of a low-slope roofing system is the roof covering. It is the protective, weatherproof layer that shields the roofing system from damage by sun, rain, hail, snow, extreme temperatures, chemicals, foot traffic, equipment and other potentially destructive forces. There are four types of low-slope roof covering – built-up roofing (BUR), modified bitumen (mod bit), single-ply membranes (EPDM, TPO or PVC) and fluid-applied systems (coatings).
In Part III of this series, we’ll describe each of these roof covering types and explain how they work.
By: Rob Keating, Regional Director – Northeast
Tim McDermott, Territory Manager – Western New England