Metal Roofing Systems: The Preferred Choice

For years now, metal has been proclaimed throughout the industry as the most versatile roofing material available. Proponents sing the praises of metal: it’s strong, long lasting, low-maintenance, recyclable, predictable and reliable nature. Metal roofing also lends itself well to many design elements currently used in today’s architecture. Hips, valleys, slope changes, transitions and dormers are all available to the designer.
With metal roofing gaining ground as the roof of choice for many commercial and industrial buildings, it is important that owner’s know the benefits and downfalls of metal roofing systems. Before making the choice to go with a new metal roof system, many factors that will aid in helping one to choose the right type of metal roofing for building must be considered.
Types of Metal Roofing
Metal roofing can be broken down into two main classifications
(A)    Hydro-kinetic    (B) Hydro-static
(A) Hydro-kinetic metal roofing:
Such roof systems are typically known as architectural style metal panel systems. These systems are water shedding, similar to the function of asphalt shingles, and do not provide any structural integrity to the building. Hydro-kinetic metal panel roof systems require moderate to steep slope applications, and typically have a solid substrate, such as a wood deck to provide stability to the system. A waterproofing underlayment is installed beneath the panels to deter water infiltration into the facility. The metal panels for these types of systems can be purchased in many shapes and sizes, from many different seam profiles, to metal roofing that resembles shingles, tiles, and/or slate.
(B) Hydro-static metal roofing:
Hydro-static roof panels are a completely sealed panel system that can add structural integrity to the building structure. In many applications, these types of metal roofing panels act as both the waterproofing and roof deck structure, spanning from joist to joist with no underlaying support. Due to the water-tight integrity of this type of metal panel installation, these systems can be installed on lower sloping structures. These panel systems can also be exposed fastener, or concealed fastener systems, with a variety of different seam profiles.
Both the above variety of metal roofing comes in any of the Copper /  Aluminium/  Zinc/ Steel / Titanium / Tin / Stainless Steel / Galvanised Steel / Alloys metal.
 
Coating on Metal Roof
The Need for Coatings- Metal roofing systems with original factory-applied coatings will experience coatings degradation due to weathering and are recoated with an appropriate maintenance coating to restore and extend the life of the roof. Uncoated metal roofing systems experience galvanic corrosion and are also candidates for roof lifecycle extension by being coated with a corrosion-inhibiting coating system. White reflective coatings have the added benefit of a “cool roof” for energy savings.
Factors Influencing Current Demand for Metal Roof Coatings : Energy shortages are driving the need for energy savings, and the building industry is also experiencing a greater sensitivity to environmental concerns. These two issues are influencing demand for field-applied roof coatings that deliver cool roofs for energy savings as well as sustainability. Three industry programs are leading this trend – Leadership in Energy and Environmental Design (LEED) green building rating system, Energy Star, and California Title 24
 
Newly Designed Coatings for Metal Roofs
One of the latest coatings designed for metal roofs are zinc-rich metal roof primers that improve adhesion of finish coatings while encapsulating rust and inhibiting the development of new rust.
Polyurea is a relatively new two-component coating technology that is well suited to metal roofing. It also has good elongation and a high tensile strength, i.e., about 1,500 pounds per square inch (psi) compared to 200 psi to 400 psi for other coatings. Polyurea also results in a harder surface than other types of coatings, so rain will wash dirt off the hard surface relatively easily; it is easy to keep clean. Another new technology is based on fluoropolymer coatings, such as Kynar, which promise high reflectivity, durability, and excellent cleanability
Type of Substrate for Roof Coating & Type of Coatings:

Both factory-applied coated metal roofing systems and uncoated metal roofing systems are ideal candidates for roof coating restoration, including steel, aluminum, and galvanized metal systems. An adhesion test is recommended prior to coating. Some factory-applied metal finishes, such as Kynar-500 and some coatings such as silicone, are surfaces to which acrylic products may not adhere well, and a different solution may be more appropriate. While acrylic coatings are growing in popularity due to their environmentally friendly attributes, polyurethane, silicone, and polyurea are used for certain applications. Typically four products are used in a metal roof coating system — a cleaner, a metal roof primer, a flashing grade sealant, and a finish coating.

Preferred Method of Applying Coating to Metal Roofs : Typically four steps are needed to properly coat a metal roof. First, all loose coatings, heavy rust, debris, and fresh roof cement must be removed, and the existing roof system must be repaired — such as replacing missing or loose fasteners and metal panels as needed. Then the roof is power-washed at 2000 psi with a cleaner, using an airless spray rig. Step two is to prime the roof with a metal roof primer using an airless spray gun followed by a wet film gauge to determine the proper mil thickness has been applied. Step three involves flashing fasteners, penetrations, seams, and lap joints with a flashing grade sealant that can be brushed, rolled or extruded. The final step is to apply the finish coating, using the airless spray gun, followed by the wet film gauge to ensure proper mil thickness. A typical dry mil thickness for the finish coating is 18 to 25 dry mils, achieved in two to three coats. The finished coating system serves as the top layer of the roofing system. This is quite different from a paint film thickness of approximately three dry mils

Effect of Coating Color on Temperature – White coatings offer the highest reflectivity and emissivity, but other “cool colors” have been introduced that are listed by the Cool Roof Rating Council (CRRC). Reflectivity is the amount of solar energy a roofing material’s surface reflects. Emissivity is the amount of absorbed energy a roofing material radiates from itself because of the material’s own heat and temperature. A top quality roof coating both reflects infrared and UV of the sun and emits absorbed heat. A white or light “cool roof” reduces air-conditioning costs and stress on the roofing system and can extend the life of the roof.
Panel Installation Procedure:- Panel installation method is provided by each manufacturer separately, however for the benefit of the reader method of installation as provided by Valley Rolling metal roofing is briefly mentioned below.


Panel Installation – Panels should be started at the end of the building, opposite from the direction of prevailing wind. Suggested minimum overhang is 2” and minimum pitch is 3/12. For pitches less than 3/12, sealant is suggested. Girt spacing should be no more than 36” for siding application, and purlin spacing should be no more than 24” for roof application.
Cutting and Drilling Steel Panel – Steel panels may be cut with metal snips, electric or pneumatic shears, a portable profile shear, or an electric nibbler. Some installers prefer using a circular saw to cut metal panels. Refer Figure below.

Screw Placement – Generally, 1” screw fasteners are placed in the flat area of the panel at 24” on center, along the length of the panel and next to each major rib approximately 1/2” from the rib. If purlins are placed over 24” apart, stitching screws are recommended on the lapping rib between the purlins. Refer Figure below.
Lap Sealant – If roof pitch is less than 3/12, a side-lap sealant is recommended. Caulk side-laps at the top of the rib. Seal end laps across the width of both the top and bottom anels, below the fasteners, and 1” to 2” above end of the overlap. Refer Figure below.
Closure – 3’ Closure strips are available in all panel profiles. Closures are recommended under the ridge cap, endwalls and panels at the eave, unless ridge venting is utilized. Silicone caulking is applied to the top and bottom of the closure, and will assist in keeping closures in place. Refer Figure below.
Benefits of Metal Roofing

The choice to install a metal roofing systems on your building comes with many benefits. The following is a summary of pros that a building owner can expect from choosing to install a new metal roofing system:
-    Metal roofing is very durable. Manufacturer warranties range from 20 to 50 years for defects, and warranties range on average from 20 to 25 years on the coating finish.
-    Because metal roofing is non-combustible, most roofing applications receive a Class A fire rating. Ratings may be lowered if the metal roofing systems is installed over existing wood and other combustible substrate.
-    Metal roofing can be selected in a variety of colors, shapes, and sizes for pleasing building aesthetics.
-    Minimal maintenance and upkeep is required for the performance and long term integrity of a metal roofing system.

This leads to a low life-cycle cost analysis of expenses pertaining to the systems over a metal roof’s life span.
Additionally, concealed fastener type systems hide the fasteners and clips that secure each panel.
-    Metal roof systems reflect more of the sun’s radiant heat than conventional asphalt based roofing components. This leads to decreased heat loads that get transferred into the building. Annual cost savings on
heating and cooling expenses can be 20 to 40 percent, depending on application. Lighter metal panel colors can reflect up to 20% of additional radiant solar heat.
-    Metal roofing systems are more capable to handle the expansion and contraction caused by daytime heating and evening cooling temperatures.
-    These systems can be installed with ease and in certain applications, can be installed directly over existing roofing components.
-    Concealed fastener type systems hide the fasteners and clips that secure each panel.
-    Metal roof systems are typically lighter in weight compared to conventional asphalt roof components.

Demerits of Metal Roofing:-


Just as metal roofing adds plenty of benefits and positives to the facility on which they are installed , several cons are related to certain metal panel systems:
-    Metal roofing is typically more costly to install that conventional asphalt based roofing components. A complete life cycle cost analysis of the initial investment, and recurring expenditure anticipated over the life of the system, should be calculated prior to investing in a metal roof system.
-    Exposed fastener systems require regular maintenance to protect the seals where the fasteners penetrate the metal roof panels.
-    Increase in noise from rain on certain applications.
-    Possible condensation at metal panels if not insulated properly and/or voids are created in insulation exposing metal panel system to interior conditioned air.
Conclusion
As a building owner, metal roof systems are just a few choices of many for the building. Pros and cons of each system, along with up front and life cycle costs should be analysed, as well as aesthetics and future plans for the building before making decisions on which roof system will be right for the particular application.