Commercial Roofing Systems
Commercial low sloped roofing systems have a variety of components that are used together to protect buildings against moisture, wind, fire, and heat transfer. There are different types of roof decks, vapor retarders, roof insulation, cover boards, attachment methods, and roofing membranes. There are so many different types of roofing systems that insurer Factory Mutual has nearly one million approved systems in its database. Its important to choose the right commercial roof system for your building so we will describe some of the components for your consideration.
The Roof Deck
A good low sloped roofing system begins with a dry, safe, and secure roof deck. The most common types found within commercial roof systems are wood, steel, and concrete.
- Wood roof decks consist of plywood, OSB, or wood planks. If any roof leaks have occurred for significant periods of time, deteriorated or damaged wood must be replaced when a new membrane roof system is installed.
- Steel decks are more common than wood decks and are also very durable. Steel decks can be attached to the framing with fasteners or it can be welded to the purlins.
- Structural concrete roof decks are also common on commercial and industrial buildings. It is important that the concrete is cured and dry before a new roof system is installed.
Vapor retarders are a good idea in cold climates such as Louisville and Southern Indiana, especially when the interior of a building has relatively high levels of humidity. The laws of thermodynamics tell us that air pressure, heat, and moisture within our buildings attempt to equalize themselves. This means that air movement occurs in the following scenarios: hot air moves towards cold air and moist air moves to dry air. Air movement can be problematic within a roof assembly. If warm and moist air from the inside of a building is able to escape into a roof system, it will likely condense when it migrates all the way to the roof membrane during cold days.
Since the roof membrane itself is an excellent air retarder and it prevents moisture from entering (and exiting) the building, condensation will have a negative impact on the rest on the components within the roof system. Condensation will damage roof insulation such as polyisocyanurate as well as roof decks such as wood or steel. A vapor retarder placed beneath the warm side of the insulation will prevent moist air from entering the roofing system and is an excellent roof system component to prevent damage caused by condensation. Common types on vapor retarders include polyethylene and polymer modified bitumen.
Roof insulation is typically installed above the roof deck on most membrane systems and the most common products used are rigid foam insulation boards. Most building codes in the Louisville area require the roof system to have a minimum R-value of 25. Common types rigid foam insulation are expanded polystyrene or EPS, extruded polystyrene or XPS, and polyisocyanurate.
The most widely used commercial and industrial roof insulation board today is polyisocyanurate. According to the several polyisocyanurate manufacturers the long term thermal resistance (LTTR) of polyisocyanurate is R-5.7 per inch. When a new roof is installed and polyisocyanurate in used, approximately 4.5 inches of insulation is required to reach R-25.
To avoid thermal voids or gaps in the rigid insulation boards, a minimum of 2 layers of insulation should be installed with the seams staggered. If the roof deck is structurally sloped towards the roof drains or gutters, insulation with consistent board thickness can be installed. However, when the roof deck is flat or water needs to be diverted to roof drains or gutters, a tapered insulation system can be designed to ensure the roof has enough slope to provide positive drainage.
Cover boards are beneficial to a roof system in a number of different ways. They can provide fire protection, additional R-value, add compression strength, and provide impact resistance. Cover boards protect a roof system from foot traffic, other trades, and hail. They can also deliver greater wind uplift resistance and hide imperfections in the rigid insulation boards. Common types of cover boards that can be found on a roof system include perlite, gypsum, fiberglass, HD polyiso, and wood fiber.
The products in roof assembly can be attached in a number of different ways. The four most common attachment methods in commercial roof systems are adhered, ballasted, mechanically attached, and electromagnetic induction welded.
- Adhered systems perform better than any other roof system. The membrane can be adhered directly to the roof insulation or a cover board.
- Mechanically attached membranes are loose laid and then fastened through the roof insulation to the roof deck. The fastening pattern is determined by the manufacturer and occurs in a linear fashion.
- Ballasted roofs contain a loose laid membrane with at least 10 pounds per square foot of ballast (depending on the wind load) to hold the system in place. Ballast can consist of aggregate or paver systems.
- Electromagnetic induction welding is common for TPO and PVC roof membranes. The single ply membrane is welded to special plates that are attached to the insulation with an induction tool. This system is commonly referred to as RhinoBond or InvsiWeld.
The most common roof coverings for low sloped commercial and industrial roofs are BUR, modified bitumen, and single ply membranes. The three most popular types of single ply membranes are TPO, EPDM, and PVC.
- BUR or Built-up Roofs are typically applied with several plies of felt and asphalt. The asphalt is coated with an aggregate, cap sheet, or coating to protect the asphalt from UV degradation.
- Modified bitumen roof systems are also asphalt based but polymers are added to enhance performance and make the roof easier to install.
- TPO roof systems (thermoplastic polyolefin) make up the largest market share (by square footage under warranty) in the U.S. The membrane seams and flashings remain watertight as they are hot air welded.
- EPDM is a rubber membrane that has been a high performing roof system since the 1970’s. The membrane is an excellent choice for cold weather climates such as Louisville.
- PVC roof systems (polyvinylchloride) have hot air welded seams similar to TPO roofs. Unlike EPDM, PVC is resistant to oils and grease which makes it an excellent choice for restaurants and other industrial applications.
For more information about commercial roofing systems, please contact a professional at Deer Park Roofing for further assistance.