Choosing Between Metals
By Mike Huber, PE
Architects,
engineers, and
specifiers have never had such
a wide array of metal options,
and aesthetic concerns are the driving factor when choosing one
substrate
over another. The other major factor to consider is longevity, which
tends
to proportionally increase with a material’s cost.
Steel roofs, which are almost always treated with a metallic coating to
protect against oxidation and corrosion, can generally be expected to
last
30 years or longer with traditional metallic coatings (e.g.
galvanized).
Manufacturers sometimes offer proprietary metallic coatings for steel
systems that can further extend their anticipated life span.
Aluminum roofs are more corrosion-resistant than their steel
counterparts,
and can be expected to last from 50 to 100 years, depending on
environmental
considerations, such as weather and corrosives. Other premium metal
substrates, such as copper, stainless steel, zinc, and titanium, can be
expected to last centuries.
Within the wide realm of metal roofing solutions, there are three basic
subcategories:
-
Structural
Systems
The most capable of withstanding
wind uplift pressures, structural systems are
designed for watertight rooftop performance
-
Architectural
Systems
The least capable of sustaining wind uplift pressures, architectural
systems are primarily a decorative covering (they typically require an
underlying waterproofing membrane)
-
Hybrid systems
Share
characteristics of both structural and architectural assemblies
Systems are also classified by the way in which they
are anchored to the
substrate. They fall into one of two categories, as described below.
-
Through-fastened Systems
These assemblies use screw fasteners drilled through the metal roof
panels
into the deck. They are more economical and fairly strong, but
typically
considered less aesthetically pleasing and more susceptible to leaks
since
the fasteners are exposed to the elements.
-
Standing Seam Systems
These systems employ clips located within the metal seam, concealed
from the
roof’s exterior surface. The clips that hold the adjoining roof panels
together are then fastened to the substrate. (Such clips are not
exposed to
the elements.) It is generally considered to be easier to make a
standing
seam system weathertight, due to the configuration of the panel that
places
the seam above the roof surface and, therefore, above the path water
takes
when flowing off the roof.
When a specifier is choosing among standing seam systems, another
important
consideration is the height of the seam leg. Generally speaking, the
higher
the leg height, the stronger the system. To understand this concept, it
might be helpful to imagine the seams as I-shaped beams: the taller the
I-beam, the stronger the member.
Clip designs
for standing seam metal panels also distinguish metal roofing
systems from each other. In evaluating clip designs, one must
prioritize the
building owner’s performance requirements. The material thickness of
the
clip helps determine its inherent strength, which in turn helps
determine
the minimum wind uplift resistance requirements for a particular roof.
For help determining the right kind of system for your project,
contact the Engineering department at IMETCO.
For more information about symmetrical panels,
click here.
