Roofing System 14

Roofing System 14Reviewed by Roco on Aug 6Rating: 4.0

COST

Many select a roof system primarily on initial cost. Although cost is an important element of a project, when cost is a governing factor in system selection, typically there are ramifications. If a less expensive system is selected, invariably something suffers in comparison with the system(s) that fell from consideration because of the greater cost. The cheaper system generally will not have the reliability or durability of other systems; it may be more maintenance intensive or it may not be as energy efficient. Over the life of the roof, the system with the lowest initial cost often is more expensive than other options that were discarded because of their higher initial cost.

In evaluating cost, it is important to look at the life-cycle cost (LCC). In addition to the initial construction cost, LCC includes energy consumption (for building heating and cooling), maintenance, repairs, length of service life, and disposal at the end of the roof’s life. Of these factors, the most difficult to assess is the design service life. A common service life to be expected is 20 years.

The service life can have a dramatic impact on the LCC analysis. For example, if a 40-year service life is assumed, but the roof fails after 15 years, the true roofing costs will be much higher than calculated. Lack of good data on design service life is often a significant limitation to developing a reliable LCC. It is difficult to have confidence in a manufacturer’s claim of, for example, a 30-year life for products that have been in the marketplace for only a few years. Accelerated aging testing is of limited help, as it has not progressed to the point where credible estimates of service life prediction can be made. The selection of a predicted service life should be conservative. For most low-slope systems, use of a service life in excess of 20 years should only be done with caution, evaluation, and justification.

For most projects, the costs associated with eventual tear-off and disposal are seldom considered. Because some systems are inherently more difficult to tear-off than others, LCC analysis should consider this issue. Also, it may be possible to salvage or reuse some of the system components. For example, with a PMR, it would be reasonable to assume that much of the ballast and insulation could be reused on the replacement roof.

Where the cost to access and/or repair a roof are very high, or the risk of failure results in an excessively expensive repair due to landscaping or difficult access, it is important to make the investment in the highest quality, most durable, long-term roof system available. The later costs of repair are usually enormous relative to, and in comparison to, first-cost investment difference for the highest quality roof.

Although there are difficulties and limitations with the LCC approach, economic decisions based on LCC are preferable to those that only consider initial system cost. ASTM E 917, Standard Practice for Measuring Life-Cycle Costs of Buildings and Building Systems provides further information on LCC.

IMPLICATIONS OF SUSTAINABLE ROOF DESIGN

Sustainable roof design should be considered on most roofing projects today. If an emphasis on sustainable roof design is desired, sustainable design criteria can become major factors in the selection process, depending upon the degree to which sustainability is pursued. At the very least, the selected system should be thermally efficient, with consideration given to R-value, reflectivity and emissivity. And for those buildings that are intended to have a service life in excess of 20 years, a system with enhanced durability should be selected to reasonably maximize the life of the roof to the extent that the budget allows.

Sustainability goals (among others):

  1. Minimize the burden on the environment.
  2. Conserve energy.
  3. Extend roof lifespan.
  4. Reduce carbon footprint.
  5. Reduce heat-island effect.

See Sustainability of the Building Envelope, Cool Metal Roofing, and Extensive Vegetative Roofs for further information on sustainable design considerations.

SPECIFIC ROOF SYSTEM SELECTION RECOMMENDATIONS

Roof system selection can be complex and challenging. Selection should start with if the project is a new or existing roof. This step is necessary to proceed with proper system selection as it determines roof system type. New construction allows better choice in the roof system selection and should begin with designing steep slope systems >3 in./ft. when possible, followed by low sloped <3 in./ft. where required. Generally, when a building least dimension is 75′ or less, a steep-sloped system should be selected over a low-sloped system.