Which Roofing Selection Is Right?

In the past, selecting a roofing system was relatively easy, since there were only a limited number of membrane types available. Today, however, the person specifying the roofing system has many choices, which certainly makes the decision-making process more challenging. Any roofing system (built-up, modified bitumen, EPDM, TPO, PVC, metal, etc.) can be the right choice, depending on a variety of factors.

Cost: Cost is often considered the most important factor in selecting a roofing system. However, initial cost of the membrane is only one component in determining the total cost of the system. For example, the material cost for a built-up roof is only 20 to 35 percent of the installed cost. With modified bitumen and single ply membranes (rubber and thermoplastics), materials account for approximately 30 to 50 percent of the installed cost.

The cost of removing the old roofing system needs to be factored into the total. Therefore, the cost of membrane alone may not be the best way to gauge the cost of the system. Sometimes more expensive membranes may actually have lower installed cost than less-expensive membranes.

Other factors to be considered in determining the cost are the number of units/curbs to flash, wall flashing details, insulation system, fastening and roofing accessories.

Longevity: Consider how long the roofing system is likely to last. Traditionally, education facilities are built for the indefinite future, in which case it is advantageous to install a higher-cost, 20-year type roofing system.

However, if the building is likely to be vacated, sold or even demolished in a few years, it would be a financial error to install an expensive roofing system. In this case, rather than replacing the entire system, maintenance or partial replacement may serve to extend the life of the existing roof.

Value of Operations: The value of the materials contained in the building plays a role in roofing system selection. For example, high-tech labs with expensive computer equipment may warrant a high-quality system.

Warranties: Roof system warranties are usually only a fraction of the cost of the total system cost. They come in terms from five to 20 years and occasionally longer. One thing to remember is that roofing warranties usually exclude consequential damages. Therefore, it is extremely important to consider the cost of a leaking roof. Replacing damaged ceiling tiles, dry wall and disrupted class time can easily exceed the cost of the repair covered under the warranty. As a result, consideration should be given to installing the best available system that fits within budget parameters.

Building Structure and Design: The structure and design of the building often affect the system chosen. Every building needs a solid foundation and, for the roofing system, that foundation is the roof deck. Various types of decks are used in the building industry. Each deck type has advantages and disadvantages, and the roofing membrane, insulation and deck must be carefully matched to ensure a quality application.

The deck must have adequate strength to support the live- and dead-load weights anticipated. A structural engineer should be consulted when any weight change results from the installation of a new roofing system. Some ballasted systems can weigh more than 20 pounds per installed square foot of roofing. The cost to analyze load factors is small compared to the potential consequences of an error.

The size and access to the roof can affect the choice of the roofing system. Small roofs, for instance, can become costly if the membrane system requires a great deal of equipment staging. Tall buildings make it difficult to pump asphalt materials to the roof, and small kettles may have to be lifted to the roof in this case. A cold applied or torched applied membrane may be the better choice in this case.

Old vs. New: If a new membrane is to be placed in direct contact with an existing system, care must be taken to ensure the new system is compatible with the membrane left in place. Some thermoplastic membranes may be adversely affected when placed in direct contact with bituminous material, for instance.

It is a mistake to install a new roofing system over an existing system that is currently saturated with moisture. The moisture left in the system may degrade the R-value of any new insulation added, cause potential blistering problems, and degradation of concrete decks due to rot, rust or spalling.

Other Factors: A number of other factors should be taken into consideration in selecting the most appropriate roofing system.

-Interior conditions, roof traffic, chemical resistance/exposure and interior operations should be considered. For example, more complicated roofing systems are required where there is high interior humidity, pressure or temperature such as in cafeterias with cold storage equipment.

-Buildings that are subject to high roof-top traffic due to equipment servicing, or where equipment is routinely added or removed from the building, require systems that are more tear- and puncture-resistant.

-In addition, if chemicals or foreign materials are vented or discharged onto the roof, consideration should be given to selecting a membrane that is resistant to the material or one that is easily cleaned. Manufacturers generally know their membranes’ resistance to various contaminants and can assist in system selection.

A roof requires a substantial financial investment; therefore, it is critical that time and attention be given to making sure the best roofing system is selected. There is no single best choice for all situations, so all factors need to be considered. A good source of information is the roofing system manufacturer, who generally is willing to assist you in developing a specification for your situation. Spending the time selecting the right system in the beginning saves time and money across the life of the roof.

David Carl is western region technical manager for Johns Manville Roofing Systems Group, which is based in Denver.