26 September 2006
Metal generally has greater initial cost than other roofing alternatives, but studies have
found that life cycle costs are among the lowest of any roofing material. In some
specific re-roofing situations, metal may have the lowest initial cost, particularly when
low-slope to steep-sloped conversions are undertaken. This is due to the avoidance of
tear-off, the cost effectiveness of light gauge subframing for slope build-up, the
avoidance of asbestos abatement, the ease and cost efficiency of re-insulation.
Prior to the specification of metal roofing systems designers should analyze the life-
cycle costs associated with each system. Compliance with the appropriate UFGS is
Design-build contracts for metal roofing systems must name the specific type of metal
roofing system desired and identify all critical performance requirements. It is not
sufficient to simply require a metal roofing system per the MBMA Manual. Compliance
with the appropriate UFGS is mandatory.
Material must be either sheet aluminum or GalvalumeTM coated sheet steel. Both
materials may be factory painted, and in either case substrate life will exceed paint life.
In non-industrial, inland, moderate climates, the service life of unpainted GalvalumeTM
steel can be expected to be in the range of 40 years. In desert climates, the service life
may exceed 60 years. In humid or industrial environments, the service life will be under
40 years depending upon severity of environment. GalvalumeTM steel does not perform
as well when in direct contact with salt spray. It is commonly used in coastal
applications, but will not render a 40-year service life. In some of the aforementioned
environments, aluminum may be the better choice, albeit at increased cost and lesser
availability. Contact of either metals with strong acids and alkalis should be avoided.
Avoid the use of dissimilar metals. Jacket any copper lines to isolate and prevent
copper ion water runoff.
Protective coating for carbon steel must be 55% (by weight) aluminum-zinc alloy,
GalvalumeTM in application rate of 15.6 g minimum per 0.1 m2 (.55 oz. minimum per ft.2)
for unpainted material (AZ55) and 14.2 g minimum per 0.1 m2 (.50 oz. minimum per ft.2)
for pre-painted material (AZ50). When unpainted GalvalumeTM material is used it must
have an additional protective coating of acrylic at a nominal thickness of .0076 mm (0.3
mil) to protect against handling stains.