Roof System Selection - Criteria

Kent Mattison, PE

When faced with the need to replace a problem roof, determining the "best" replacement system is not always an obvious choice. If corporate standards have not been established to dictate to some extent what materials are acceptable for your specific situation, the choices are almost endless.

For example, the 1993 NRCA commercial Low-Slope Roofing and Materials Guide lists 126 manufacturers of 228 roofing membranes and well over 1,000 different specifications. These membranes can be categorized into five generic categories:

1. Built-Up (asphalt and coal-tar pitch)
2. Modified Bitumens
3. Single-Piles
   1. Vulcanized elastomerics (EPDM and Neoprene)
   2. Non-vulcanized elastometrics (CSPE, CPE, PIB and NBP)
   3. Thermoplastics (PVC and CIP)
4. Polyurethane Foam
5. Metal

A suggested starting point in the selection process is to first evaluate the following design parameters, their importance to your roofing situation, and the generic roof system that best meets the requirements of each factor.

Design Parameters

1. Structural/Deck Considerations
2. Building Usage
3. Slope/Drainage
4. Substrate Components and Conditions
5. Code/Insurance Requirements
6. Geographical Locations
7. Roof Traffic
8. Roof Access
9. Energy Requirements
10. Maintenance Requirements
11. Budget Limitations
12. Safety
13. Aesthetics
14. Warranty Provisions

When evaluating each parameter, some assumptions may be made. For example, in a given situation some factors are usually non-issues or do not effect one product more or less favorably than another. However, there are usually a few factors where a particular roofing system would better meet your needs. The key is to know upfront what your requirements are for each parameter. Otherwise, any product salesperson can make your performance criteria fit their product.

After the generic membrane system(s) is chosen, the products and manufacturers are evaluated to determine the specific system that would perform the best if it is properly designed and installed.

In subsequent newsletters, each of the design parameters and its effect on the generic roof membranes will be discussed in detail.