Metal Roofing Underlayment - Tips on Choosing the Best System

October 30, 2010
Publication: SMACNA 67th Annual Convention, October 2010, Phoenix, AZ

Abstract: Underlayment products for use beneath metal panel roof systems should be carefully selected to provide a roofing system that performs satisfactorily for its intended service life. Several considerations must be addressed in the roofing system design, including ensuring that the four barriers needed in any building enclosure (ie., water barrier, thermal insulation barrier, vapor retarder, and air barrier) are provided and are in the correct location for a given climate. Long term, in-service roof performance can be affected by several important factors including: the metal roofing system and its expected service life, the climate at the building location, roof slope and geometry, and ambient conditions (temperature/relative humidity) within the building. Improper selection of the roof underlayment may allow roof leakage or entrap moisture due to vapor diffusion or air exfiltration, which may accelerate deterioration of concealed components, shortening their service life and necessitating repairs. Further, to provide longevity the underlayment should match the intended service life of the metal roofing system. Although there is no universal design that applies to all buildings, this paper attempts to clarify many of the issues involved in selecting appropriate metal roof underlayment materials, and provides guidelines to builders who are responsible for constructing metal roofing systems. In general, contractors should be wary of changing the system design or recommending an underlayment product without considering all of the parameters that may affect performance. The system designer should be responsible for determining an appropriate design for any given project.