roofing is primarily made of steel or aluminum. Copper, zinc and titanium
are also used in specialized architectural applications for homes. Steel
roofing for residential applications is very lightweight with the heaviest
product weighing about 1.5 pounds per square foot when installed. The
products are initially produced in giant rolls at the steel mills.
They are then coated
with a metallic coating to prevent rust at the steel plant. The rolls are
then painted at a coil coating plant using high-speed paint lines that bake
on the coating. The painted coil is then shipped to a roofing manufacturer
where it is formed into long panels by a process called rollforming or they
are stamped into individual shingles using large presses. Aluminum roofing
follows the same process except that it does not require the metallic
roofing is made in thicknesses designated by gauge and is generally 24 to 26
gauge, with the higher gauge being thinner than the lower. These gauges are
appropriate since most residential metal roofing applications are over a
solid substrate. Aluminum residential metal roofing is designated by decimal
thickness and ranges from .023 to .040 thickness. Granular coated steel
shingles, shakes, tile are a special category and are produced by stamping
galvanized steel (26 gauge) then sprayed with an acrylic coating embedding
granular stone then baked in an oven at the factory.
Roofing has a protective barrier on both sides of the sheet called a
metallic coating which protects against rusting. This is underneath
and separate from any paint which imparts color to the product.
There are two
types of metallic coatings used: Galvanized: This is 100% Zinc in
various thicknesses depending upon the product usage. Galvalume or
Zincalume: A mixture of aluminum and zinc. (55 percent by volume
aluminum) Metallic coatings "sacrifice" themselves to protect the
iron (Fe) in steel from oxidation when exposed to the air and
moisture is present.
Zinc is a more
"active" metal than iron so it oxidizes first and forms a protective
barrier — zinc oxide, before the iron (Fe) in the steel can become
Ferric Oxide (rust). When zinc is combined with aluminum to form
Galvalume there is even more protection in most circumstances.
Different Levels of
There are differing
amounts of metallic coating used on sheet steel depending upon its final
product application. For agricultural applications, galvanizing levels
called G-40 or G-60 may be used quite successfully. For houses, the MRA
recommends a minimum of G-90 be used. This is in accordance with the
Guidelines for Residential Metal Roofing published by the Metal Construction
Association. The greater the number, the longer the protection against rust
will last. The numbers G-40; G-60; and G-90 refer to the ounces of zinc per
100 square feet of sheet steel coated (top and bottom). G-90 will have 90
ounces of Zinc- 45 ounces on each side per 100 square feet of roofing sheet
steel. Galvalume has a designation AZ-50 or AZ-55. These are equivalent
levels to G-90 galvanized product. However, in many years of exposure
testing Galvalume has proven to be up to three times more effective in
preventing rust from appearing on the sheet steel. Aluminum sheet does not
require a separate metallic coating barrier since aluminum oxide, when it
forms is not noticeable in most cases.
Aluminum is often
preferred as a roofing material substrate in heavy salt spray environments.
This will occur where there is a lot of wave action in salt water near the
ocean. In many inland salt water situations steel with metallic coatings
performs very well if there is not a lot of wave action — intercoastal area
of Florida and the Southeastern U.S., Puget Sound in Washington, etc.
Another factor to consider is how often the roof is rinsed with fresh water
through rainfall. In shoreline, non-saltwater environments both galvanized
and Galvalume steel will provide a long lifetime of protection.
Most residential metal
roofing has a paint coating applied to the outside of either the aluminum or
steel plus metallic coating. The paint finishes provide the aesthetic
qualities that consumers want to see on their roofs. They can also provide
specific energy saving qualities. Metal roofs that are painted use very
sophisticated paint technology. The roofing material is painted when it is
in flat sheet form in giant coils. Yet, the paint is flexible enough to be
later stamped or rollformed into the final roof shape-long panels, shingles,
shakes, tiles or slate forms. There are differing levels of paint quality
for metal roofing-all affecting the price of the finished product. The
principal attribute that consumers are concerned about is fading of the
original color. When exposed to sunlight (ultra-violet light) paints fade
over time, some more than others. Generally, in this situation you get what
you pay for.