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Corrosive Resistant Sheet Metal
by Curt Liscum, RRC
For years, aluminum, copper and stainless steel have been used with varying degrees of success as architectural metal work in corrosive atmospheres. In this article, we will explore the uses and limitations of metal types when roofing in a corrosive environment.
Architectural or roofing metal work is used as metal edging, gravel stops, copings, counterflashings, flanges, rain collars and pitch pans. The function of this metal work is to terminate and seal off the roofing and flashing membrane at walls, curbs and penetrations. Although generally not a part of the material supplied by a roofing material manufacturer, metal work is an integral part of the roofing system and must be properly designed, fabricated and installed in order to obtain the desired watertightness from the roof.
A corrosive environment is any area where particulate matter causes corrosion and accelerated deterioration of standard G-60 and G-90 galvanized sheet metal. The particulate matter can be either man-made or natural. Two such corrosive environments are manufacturing plants (paper and pulp mills, petrochemical plants), and coastal areas. Not all areas of any plant are corrosive, generally they are associated with specific processes. For example, most warehouse and office buildings at a corrosive plant can be treated as any normal non-corrosive roofing project. In coastal areas, the corrosive nature of the salt spray depends greatly on how long it remains on the metal. For example, metal work that is frequently washed by rain requires less corrosion protection than sofit or fascia metal that seldom comes in contact with rainwater.
Aluminum has been used with limited success asroofing metal work in corrosive environments. In addition to having a relatively low resistance to corrosion, aluminum has a high coefficient of expansion and contraction. This makes aluminum difficult and somewhat undesirable to use in corrosive environments. We have observed mild to severe corrosion of aluminum in both plant and coastal locations. Aluminum's advantages are its malleability (easily fabricated), aesthetic appeal and low cost in relationship to other corrosive resistant metals. A common aluminum thickness for roof metal work is .032". At this thickness, the aluminum material is approximately $0.75 per square foot.
Back in the good old days, copper was used almostexclusively as roofing metal work in corrosive environments. This was probably due to its widespread acceptance as the roofing metal of choice on non-corrosive projects, and its ability to withstand the corrosive environments. We have seen several linear feet of copper metal edging and counterflashings that are performing quite acceptably in mill, plant and coastal areas.
One area of major concern when using copper is the attachment fasteners used. These fasteners must not only be corrosive resistant, they must be compatible with the copper so as not to create galvanic action and accelerated corrosion of the fastener or copper metal work. We have seen numerous pieces of copper metal work that look good and are not corroded, but are not adequately attached because their fasteners have corroded away. Advantages of copper are its malleability and aesthetic appeal. Disadvantages include its cost, and the difficulty in finding trained sheet metal mechanics to fabricate and install the metal. Copper material is approximately $2.60 per square foot for 16-oz copper, a common roofing metal thickness.
Today, stainless steel is the metal work of choice for corrosive environments. For years, stainless steel has been used at plants and mills that were prone to corrosive process. It appears that only recently the aesthetic of stainless steel has become acceptable as architectural metal work in coastal locations. Originally Type 304 stainless was used. The industry found out that Type 304 was okay, but not great in severely corrosive environments of some paper mills and coastal areas. Enter Type 316, a more resistant grade of stainless for these severely corrosive environments.Although Type 316 stainless appears to stand up to most corrosive environments, we have observed at both manufacturing plants and coastal areas significant corrosion of even this grade of stainless. This corrosion appears to be on metal work that is not periodically rinsed off with rainwater.
The advantage of 22-gauge stainless steel (common roofing thickness) is its malleability andavailability. Stainless steel can be manufactured in a variety of finishes for aesthetic appeal. The approximate cost for 22-gauge stainless steel material is $1.35 per square foot for Type 304, $1.70 per square foot for Type 316, and $5.20 per square foot for Type 317.
Sheet metal used in corrosive environments must provide service for at least the same design life expectancy as the roofing system. Failure to accomplish this goal will require removal and replacement of the metal work prior to the roofing.
The process of removing and replacing metal can be expensive and detrimental to properly functioning roofing and flashing membranes. This process can in fact cause stress to the membrane and reduce serviceable life. Selecting and installing the wrong metal in a corrosive environment can not only increase the life cycle cost of a roof but also cause unanticipated and unexplained leakage when the metal work corrodes beyond the point of being able to properly function.