The Francis Crick Institute in London, by HOK
Courtesy HOK The Francis Crick Institute in London, by HOK

Metal makes for a durable, long-lasting roofing material. “When fabricated and installed correctly,” says Steven Danielpour, AIA, principal and director of specifications at New York–based HOK, metal roofs “can outlast and outperform elastomeric roof systems, SBS-modified [styrene butadiene styrene–modified] roof systems, and single-ply roofs.” Though they can be expensive, ranging from about $5 to $12 per square foot, an owner with long-term interests may be able to overlook the high first cost. “Typically [metal roofs] can last 30 to 40 years whereas other systems have a 15- to 20-year life span,” Danielpour says.

When it comes to specifying metal roofing, architects must consider many factors, including the site’s climate zone; the building’s structure, form, and roof pitch; and local code requirements.

Picking a Metal Type
The most common metals used for roofing include steel, aluminum, copper, and zinc. Steel must be galvanized or coated for protection, while aluminum is typically painted or anodized for added resilience. Copper offers durability, a proven long life, and an aesthetic patina, while zinc is popular for its corrosion resistance and low-gloss look.

The thickness (or gauge) of the metal will also factor into the cost of the roofing system as well as the building’s structural loads. Metal roofing can range anywhere from 3 to 30 gauge, which translates into an estimated $1,200 to $350 per square (100 square feet) installed. A 26-gauge metal roofing product will weigh in between 0.9 pound to 1 pound per square foot, while a 29-gauge product will be between 0.60 pound to 0.65 pound per square foot. A lower gauge—and thus greater thickness—is typically preferred for its durability. However their heavier weight can make installation more difficult, and their greater thickness can make the metal harder to bend and work.

Roof Configuration and Connections
Metal roofing comes in multiple forms, including vertical or horizontal sheets and panels—most common in commercial projects—as well as shingles and tiles. Connections between metal panels can be made with different profiles, which can affect the system’s durability and maintenance needs. Vertical options include standing seam, through-fastened, and batten seam metal roofing panels. Standing seams join the edges of two adjacent panels in a single- or double-fold perpendicular to the roof plane. The ribs of through-fastened panels are mechanically attached with threaded fasteners and washers. Batten seam roofs use wood strips at panel joints that are then flashed with a metal cap that interlocks with the panels.

Underlayment and Insulation
Options for underlayments include synthetic sheets, self-adhering membranes, and asphalt-saturated felt—one ply when the roof slope is 18 degrees or greater, and two plies when the slope is less than 18 degrees. A slip sheet between the metal roofing and underlayment may be required to prevent the latter from sticking to the former in high temperatures.

A layer of insulation under the panels is critical for mitigating thermal bridging as well as for providing a sound barrier during rainstorms or hail. Rigid insulation boards, for installation over a solid deck substrate, include extruded polystyrene, expanded polystyrene, or fiberglass insulation panels.

Codes and Standards
For information on load-bearing requirements, panel connection and fastening requirements, thermal insulation values, roof pitch, ventilation, and flashing details, architects should refer to the applicable building code for their projects as well as guidelines offered by professional industry associations and roofing manufacturers.