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Metal roofing examples (C) Daniel Friedman Comparing Metals Used in Metal Roof Systems

Types of metal used in roofing:

This article discusses the different types of metal used in metal roof systems, the properties of each metal, and their application, including galvanized steel roofs, aluminized steel roofs, galvalume roofing, aluminum roofs, copper roofs, and zinc metal roof systems. This article series discusses best practices in the selection and installation of residential roofing.

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- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

A Comparison of Metal Choices for Roofing Systems

Adapted/paraphrased with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , chapter on BEST ROOFING PRACTICES: [Click any table or image to see an enlarged version.]

While some companies offer roofing products in copper, zinc, and stainless steel, the vast majority are coated steel and aluminum. Coated steel products are the most common and least expensive. In its favor, steel moves relatively little with temperature changes, has good structural characteristics, and resists denting. Its high melting point gives it a Class A fire rating.

All coated steel materials, however, are vulnerable to corrosion at field-cut edges— although Galvalume® is the least affected Table 2-10 below [Click to enlarge any image] lists the advantages, drawbacks, compatibility, and life expectancy of galvanized steel roofing, aluminized steel roofing, galvalume, aluminum, copper, and zinc metal roofing products.

Table 2-10: Metal Roofing Characteristics (C) J Wiley, S Bliss

[Click to enlarge any image]

Also see COMPLETE LIST OF TYPES & PROPERTIES of METALS USED in ROOF SYSTEMS

Aluminized Steel Metal Roofing System Properties

Developed in the 1950s, this is similar to galvanized steel, but it uses aluminum as the coating instead of zinc. The aluminum provides a physical barrier against corrosion and creates a reflective surface that helps reduce heat transfer to attics. However, aluminum does not have the self-healing properties of zinc, so exposed edges and scratches are more susceptible to rust.

Aluminized steel generally outlasts galvanized steel but has largely been replaced in the market by Galvalume® Metal Roofing

Aluminum Metal Roofing System Properties

Details about aluminum roofing materials are at ALUMINUM ROOFING, excerpts are below.

Aluminum roofing (C) Daniel FriedmanAluminum that is anodized or painted is highly resistant to corrosion, making it well-suited to coastal environments provided it is factory-coated with a suitable finish.

Although lightweight aluminum flashings tend to pit and oxidize in salty air.

Aluminum roofing's light weight is an advantage in reroofing.

Aluminum roofing also reflects heat and may thus reduce building cooling costs in hot sunny climates.

Because of its high coefficient of expansion, however, attachment systems must be designed to accommodate the movement of long aluminum roof panels.

Also

see ROOF NOISE TRANSMISSION where we describe the noise transmission properties of metal roofing.

 

Copper Roof Metal Roofing System Properties

Vassar College copper roofing (C) Daniel FriedmanThis high-end material is highly resistant to corrosion and easily formed into panels. Copper roofs have been known to last for over a century and are a common sight on churches and historic buildings.

Left unfinished, the material will oxidize to the familiar green patina (photo) that protects the underlying metal. In arid areas, the color may be more reddish-purple. Special clear acrylic coatings can be applied that will help copper retain its original color.

One concern is that runoff from a copper roof can stain building components below if not managed with gutters. Also, premature failure of copper flashing and roofing has been linked to acid rain and runoff from cedar shingles

See FLASHING WOOD ROOF DETAILS

under

WOOD ROOF SHINGLE PROPERTIES

Clients interested in copper should consider a newly developed proprietary sheet metal called Suscop™, which has copper plating over a stainless-steel core. The material combines the strength and durability of steel with the natural patina of real copper. Because of its greater strength, a lighter-weight sheet (0.4mm) can be used in place of 16-ounce copper, significantly reducing material costs.

Details about copper roofing are found

at COPPER ROOFING.

Galvanized Steel Metal Roofing System Properties

Galvanized metal roof, San Miguel de Allende, Mexico (C) Daniel FriedmanTo protect against corrosion, the steel is bonded to a layer of zinc, which works as a sacrificial coating on the surface and also offers some protection to cut edges and nicks by flowing to these areas. The heavier the zinc coating, the longer the protection. The Metal Roofing Alliance recommends G-90 galvanized steel for roofing, which has 90 ounces of zinc per square foot.

Details about galvanized metal roofing are at GALVANIZED STEEL ROOFS.

Also see COMPLETE LIST OF TYPES & PROPERTIES of METALS USED in ROOF SYSTEMSfor more about galvanized steel roofing.

Galvalume® Metal Roofing System Properties

Also sold under the trade names Zincalume ® and Galval®, Galvalume® was developed in the early 1970s. The underlying steel is coated with a zinc aluminum alloy that combines the long-lasting protection of aluminum with the self-healing properties of zinc.

Details about Galvalume® are

at GALVALUME METAL ROOFING

Also see A Complete List of Types & Properties of Metal Used in Roof Systems for more about Galvalume.

Stainless Steel Roofing System Properties

Stainless steel is generally a rust-resistant material but it's corrosion resistance to salt depends on the stainless steel alloy or grade. Also, stainless steel roofing durability depends on installation and ventilation details. Stainless steel grade 316) work well is suitable for salt-spray areas.

Terne coated stainless steel roof panels (Terne coating is a a zinc-tin alloy metal coating process that gives extra corrosion resistance. Other terne coatings using lead can present an environmental contamination worry from lead leachate found in roof runoff.)[1]

Because it is harder than aluminum metal roofing, stainless steel roof panels will be more resistant to impact damage from hail, and the metal also has less thermal expansion/contraction movement than either aluminum or copper roofs.

Watch out: other grades of stainless steel (SS 304) are not recommended for use near areas of salt spray such as close to oceans. And to avoid pitting-corrosion, stainless steel roof panels are installed using methods to provide panel ventilation on both the exposed and down-facing sides.

Details about terne metal roofing are at TERNE METAL ROOFING

Sources of Stainless Steel Roofing Products

Reasons for Stainless Steel in Roofing In architectural applications, stainless steel is often used because of its attractive appearance. Facades, interior cladding, lifts and escalators, handrails and parapets are some of the most typical areas of stainless steel use. The family of stainless steels, however, has more to offer than just good looks. Its technical properties make it an ideal choice for many other building applications, in which additional durability requirements are essential.

Stainless steel is an alloy containing at least 10.5 % chromium 1). This gives the steel an inherent ability to protect itself from corrosion. Chromium in the steel reacts with oxygen in the air and/or water to which the steel surface is exposed, and forms an invisible protective film of a chromium-rich oxide. If this layer is damaged, mechanically or chemically, it is spontaneously rebuilt if oxygen is present.

Corrosion resistance is increased with higher chromium levels and, additionally, by adding molybdenum to the alloy. The presence of nickel improves formability and weldability. Nickel-containing stainless steels work-harden on cold forming and can thus give the fabricated part an additional structural function. The most commonly used stainless steels have a chromium content of around 17 - 18% and a nickel content of 8 – 10.5%.

This is why they are known as "18/8" or "18/10". These chromium-nickel grades are called “austenitic stainless steels” Another family of stainless steels are mainly alloyed with chromium and possibly other elements like Titanium. These are called “ferritic” grades. For roofing purposes, 12 - 17% chromium grades with organic or metallic coatings can be used.

Zinc Metal Roofing System Properties

Zinc roofs are similar to copper in their durability and also similar to copper roofs in their installation procedures. But unlike copper, zinc roofs weather to a bluish-white color rather than green. Zinc has been used in construction for nearly two thousand years and its corrosion-protetive features when used on iron and steel has been familiar for about that long. Zinc roofing material, widely used in Europe, is very malleable and can be formed into intricate patterns for metal shingles.

Details about zinc metal roofing are at ZINC METAL ROOFING

Also see A Complete List of Types & Properties of Metal Used in Roof Systems

-- Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

Resources: Roofing Materials & Equipment Suppliers

Where to Buy Metal Roofing: Manufacturers & Metal Roofing Sources, Associations

Metal Roofing

Venting Underlayments

Benjamin Obdyke www.benjaminobdyke.com Cedar Breather, a 3/8 -in.-thick matrix-type underlayment designed to provide ventilation and drainage space under wood roofing

More Information about Metal Roofing Materials, Methods, Standards

-- Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: is there another term for a corroded metal roof?

(Feb 2, 2014) Anonymous said:
is there another term for a corroded metal roof?

Reply:

Sure: rusted. Or for metal that is very rusty and corroded such that it is flaking apart, we may use the term exfoliating rust.

(Dec 30, 2014) Lisa Anderson said:
Coatings also can be an excellent way to save your roof. Coating will stop the spread of rust. Many coating manufacturers also offer coatings in a variety of colors allowing to select a coating that blends in with other build components.

Question:

(Apr 20, 2015) Paul Balga said:
"Terne coated stainless steel roof panels (Terne coating is a a zinc-tin alloy metal coating process that gives extra corrosion resistance. Other terne coatings using lead can present an environmental contamination worry from lead leachate found in roof runoff.)" Question-- the steel portion of terne coated panels is not stainless, is it? I'm confused by the terminology used here because it implies a zinc coating is being placed on material that is already stainless, as opposed to the terne coating creating the stainless property.

Reply:

Paul, thanks for the question:

Details about terne metal roofing are at TERNE METAL ROOFING

at

inspectapedia.com/roof/Terne_Metal_Roofing.php

there we discuss both standards for terne coatings and the composition of different alloys used for Terne coatings.

Some metal roofing is Terne coated stainless but certainly not all. Other metals are used, as we list in the article above. Keep in mind that depending on its particular alloy mix, some stainless steels will indeed rust and thus could benefit from a corrosion-resistant coating.


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