10 Design Tips for Low-Slope Roofs

September 01, 2004 |

Low-slope roofs may be the most overlooked part of a building. Our experts offer a checklist of do's and don'ts to plug the holes in your low-slope roof project. They are: Tom O'Connor, FAIA, VP and director of building technology for Detroit's SmithGroup; Richard Koziol, AIA, senior consultant with Wiss, Janney Elstner Associates Inc., Northbrook, Ill.; and Greg Doelp, associate, Simpson Gumpertz & Heger, Waltham, Mass. Here is their advice:

  1. Client expectations. Know what the client wants, besides making sure the roof doesn't leak: Must it be LEED certifiable? Green? What life expectancy — 10 or 20 years? Any design issues like a roof-applied logo or trademark?

  2. Warranties. Even with a warranty, there's no substitute for proper design, appropriate materials, and quality workmanship. Make sure the design complies with any special requirements of the manufacturer's warranty.

  3. Constructability. Don't design a system that can't be built, a system that's inappropriate for its intended use, or one that requires workmen to install flashings or other components in spaces with poor access or other constrictions.

  4. Inspection. Provide full-time field inspection whenever possible. Hold a preliminary roofing conference and a preinstallation conference for every project. Look into leak history and prior repair work done. Make inspection openings to identify type of materials and number of roof layers. Identify if there is entrapped moisture or corrosion in the assembly. Check the condition of the structural deck and the bearing of framing members. Check the condition of parapet walls and other wall materials that interface with the roof. Identify any hazardous materials (e.g., asbestos). Review the impact of any rooftop mechanical equipment additions or alterations.


    Close inspection, including roof openings, should be made on existing roofs before starting.
  5. Design considerations. Consider the structural deck type and how the roofing materials will be attached to the roof structure (fasteners, adhesives, or ballast). Perform a dew point analysis to check if vapor retarder is needed. Review the need for an air barrier within the assembly. Check the level of durability desired. How are adjacent walls, parapets, and mechanical equipment to be integrated into the roof?

  6. Roof slope and flashing design. Roof slope for drainage and locating drains at low points must be properly designed to eliminate standing water. On a re-roofing project, the designer needs to know the slope of the existing roof deck to plan any changes.

    Flashing is the source of most leaks. Design flashings for future maintenance and replacement of the roofing system. Two-piece flashings are required.

  7. Material compatibility. Roofing materials must be compatible. That includes galvanic action with dissimilar metals and sealant compatibility between different types of sealants or with the roofing materials.

  8. Drawings. Shop drawings must be prepared by the contractor and must be complete, project specific, and submitted in enough time for review by the architect or consultant.

  9. Vapor retarders. High-humidity spaces (above 30%) may require installation of vapor retarders. A retarder with a 1-perm rating usually will not do the job. Use numerical analysis to determine the required vapor-resisting characteristics.

  10. Barriers and insulation. Continuity of roof air barriers and insulation with walls and other systems needs to be adequately detailed to eliminate thermal short circuits.

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