Technical Elements

If you use The NRCA Roofing Manual: Mem brane Roof Systems—2007, you should note that NRCA has reformatted its construction details pertaining to curb design. The manual's previous edition, The NRCA Roofing and Waterproofing Manual, Fifth Edition, provided details for the following curb conditions:

• Rooftop air-handling unit with a prefabricated metal curb (Details BUR-12, MB-12, TP-12 and TS-12)
• Rooftop air-handling unit with a job site-constructed wood curb (Details BUR-13, MB-13, TP-13 and TS-13)
• Skylight, scuttle and smoke vent (Details BUR-14, MB-14, TP-14 and TS-14)

In the 2007 edition, NRCA addresses these details by curb type rather than penetration type. The curb construction details now are referred to as:

• Insulated prefabricated metal curb (Details BUR-13, BUR-13A, MB-13, MB-13A, EPDM-13 and SP-13)
• Prefabricated metal curb (Details BUR-14, BUR-14A, MB-14, MB-14A, EPDM-14 and SP-14)
• Wood curb (Details BUR-15, BUR-15A, MB-15, MB-15A, EPDM-15 and SP-15)

Detail MB-14A from The NRCA Roofing Manual: Membrane Roof Systems—2007

Although the curb construction details' organization has changed, the detailing and notes relating to curb design remain largely the same. Using MB-14A, "Prefabricated Metal Curb," as an example, the following design issues haven't changed:

• Minimum recommended flashing height is 8 inches
• Removable sheet-metal counterflashing is optional
• Sealing material is recommended between the curb and frame

However, there are two new notes in the footnote sections of the curb construction details that address safety concerns. In the built-up and modified bitumen torch-applied details, NRCA has added: "NRCA does not recommend direct torching of torch-applied membrane flashing sheets over combustible substrates. NRCA suggests the back-torching method—sometimes referred to as the torch-and-flop method—of application."

In all the curb details, NRCA has added, "NRCA recommends that designers consider permanent internal or external fall-protection devices at all skylights."

Joan P. Crowe, AIA, is an NRCA director of technical services.