More than any other single component, the roof is most responsible for maintaining the comfort and safety of a building’s occupants. At its most basic, a roof will serve as a structural, weather protective barrier. A good roof will also be durable, energy efficient and cost effective. When designing a roof, we consider appropriate slope, resistance against freeze/thaw cycles, reflectivity, UV resistance, elasticity, strength, permeability, ease of construction and cost.
The individual roof components and how they are configured will vary depending on site conditions and climate. In an urban setting such as New York City, where our work is focused, flat roofs are used almost exclusively due to economy of design and advances in roofing technology.
Four Main Categories
Types of flat roofs can be broken down into four main categories: bitumen, modified-bitumen single-ply and liquid-applied roofs.
Bitumen roofs: More commonly known as built-up roofs (BUR), they consist of multiple ply sheets of reinforcing felt and bitumen. The layers are held together using hot mopping asphalt or cold-applied adhesives and provide heavy duty protection. Bitumen roofs can last 20 to 30 years with proper maintenance.
Modified-bitumen roofing systems: Close relatives to the built-up, they only require 2 to 3 layers of ply sheets. The most common modified-bitumen roofing system is SBS (Styrene-Butadiene-Styrene). Layers are applied as either self-adhering sheets, or with hot mop asphalt or cold-applied adhesive. The bitumen is combined with rubber to enhance its elastic qualities, making it popular in northern climates where resistance to freeze/thaw cycles is needed. The APP system (Atactic-Polypropylene) is another modified-bitumen system, engineered for strength and weatherability as it combines asphalt and plastic. Because it is applied with an open flame torch, it cannot be used on combustible roofs. Modified-bitumen roofing systems can last 10 to 40 years, depending on the quality of installation and maintenance.
Single-ply rubber roofing: Easy to install and cost-effective but not recommended for residential or large-scale commercial applications. It is most often used in large manufacturing or warehouse type buildings. The most well-known single-ply roofing material is EPDM (Ethylene Propylene Diene Monomer), which has been used since the 60s. Other single-ply roofing materials are TPO (Thermoplastic polyolefin) and PVC (Polyvinyl Chloride). Depending on climate, maintenance and quality of materials used, a single-ply rubber roof will last 15 to 30 years.
Liquid-applied roofing: This is the latest type of roofing system. Because it is both lightweight and cold applied, it can be installed over concrete, brick, wood sheathing and other roofing systems without consideration for added loads or concerns regarding dangerous fumes. The cured surface is a seamless, UV-resistant, chemical-resistant, waterproof membrane that allows for movement. These systems are flammable and cannot be used in high-risk applications such as high-hazard industrial buildings. Given that it can be a topping for other flat roofing systems, it is a popular method for existing roofs. It is also commonly used in inverted roof systems. Notable manufacturers are Sika, GAF, Kemper and Suprema. A liquid-applied roof can last 15 to 25 years, with some high-quality systems lasting 30 years or more.
IRMA or inverted roof membrane assembly is not a roofing system, but an assembly where the typical membrane arrangement is inverted to protect the membrane from roof traffic and excessive weather conditions. For this reason, IRMA systems will typically last longer than traditional roofing systems. In this arrangement, the membrane is fully adhered to the substrate which is covered by rigid insulation boards held in place by ballast such as pavers or vegetation. IRMA systems have long life spans of 40 years or more. Given its durability and the uptrend of recreational and green roofs, IRMA systems have become much more popular in recent years.
Roof Success
The success of a roof can be attributed both to the selection and proper installation of the roofing system. BUR roofs, with modern innovation and time, have started to be phased out. Modified-bitumen roofs are an improved replacement for the BUR, while single-ply roofs are recommended in limited applications. Liquid-applied roofing is the most versatile and can be used for new installations and for re-roofing.
To make the correct assessment for selection, it is necessary to have a full understanding of the site conditions, building needs (climate, budget, lifespan) and physical properties (pitch, substrate, flammability, new vs. retrofit). As all of these questions will inform what roof system is most suitable for a particular project, it is important to be guided by knowledgeable experts and seasoned builders.
ABOUT THE AUTHOR: David Katz is principal and founder of Katz Architecture & Project Management, New York City. For more information, go to www.katzarch.com.
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