Considerations for Copper Roof Design

ABSTRACT In terms of durability, few offerings in the construction market even approach the service life expectancy of a properly installed copper roof. Much of this durability is corroborated by the preference for copper as a through-wall flashing for various wall assemblies. Indeed, copper roofing is often the benchmark for life-cycle analysis. Copper enjoys obvious architectural appeal; however, it is unique among the metals, and its use on any project merits consideration of these attributes and limitations. This article focuses on such aspects, with discussion on avoidance of opper is the oldest metal low strength, heavier gauge material was stronger and is by far the most popular known to man.1 Few con required. Accordingly, soft-temper copper is variety of copper now used in construction. sumers understand the not recommended for most building appli The tempering windows are quite wide, chemistry and physics of cations. and they overlap. That is, if a particular mill copper cladding, but they do With the development many years ago of is making half hard at the high end, the seem eager to express opin cold-rolled copper, the gauge of the materi product may resemble three-quarter hard ionsC regarding how it looks. Copper has low al could be lightened without compromising in terms of its final engineering properties. “columnar rigidity.” There is nothing partic its function. Cold-rolled temper is less mal Dead-soft tempering is easy to work with, ularly wrong with this property, but cau leable than soft temper but is much but it literally droops when held. tions must be observed to avoid unappealing aesthetics and substan dard performance. Like several other metals, copper is tem pered at the mill to increase its hardness and toughness. If the sheet form complies with ASTM B 370, it consists of 99.9 percent copper and is available in six tempers. These are 060 (soft), H00 (cold-rolled, one-eighth hard), H01 (cold-rolled, high yield), H02 (half hard), H03 (three-quarter hard), and H04 (hard). Soft-temper copper is extremely mal leable and best suited for applications such as in tricate ornamental work. It was historically used in building construction; Figure 1 – Introducing flat pans onto a curved surface can and will produce surface undulations. On curves however, because of its having a long radius, the susceptibility is lower. F EBRUARY 2009 I NTERFACE • 5 Figure 3 – The author (years ago) atop a curved copper roof having no ridge interruption (the nature of safety gear shown can probably date the image). This roof continues to serve satisfactorily on a corporate headquarters facility. Figure 2 – Almost any metal roof will exhibit some oil canning in direct angles of sunlight. The displeased party should exercise patience, as the eventual patina will obscure most of the oil canning. In general, cold-rolled, one-eighth hard oversimplification, since weathering copper is recommended for roofing and may exhibit a wide spectrum of flash ing applications. Soft copper may be unusual colors for a number of rea used where extreme forming is required, sons as set out by Hunt.5 He postu such as in complex through-wall flashings. lates that, due to microclimates and lative dimensional change experienced by However, the cold-rolled version offers far other variables, a metal roof may not be long individual pans. more resistance than does soft to the weathering as intended, but it is probably Curiously, this movement behavior stresses induced by expansion and contrac weathering normally for its environment. takes quite a departure on curved surfaces tion. Copper sheet of higher temper should Copper moves – it is inherent in the (Figure 3). From the previous discussion be specified only if indicated for specific chemistry and cannot be dismissed. With about columnar rigidity, thermal movement engineering applications requiring such long panel runs (40 ft and beyond), the (which cannot be stopped) is accommodated properties.2 magnitude of movement becomes consider on curved surfaces by slight upward flexure Panel width, fluting, swaging, tension- able. Haddock postulates: of panels between clips. Accordingly, long leveling, seam height, and sheet gauge all panel runs can be used without a ridge have bearing on the tendency toward sur A dark-colored panel with low gloss detail (appealing to architects who desire face distortions, also called “oil canning.” at right angle to the summer sun the uninterrupted contours of barrel or Introducing flat pans onto a curved surface can approach temperatures of 200 curved roofs).7 can and will produce surface undulations degrees F. In cold winter nighttime The importance of soldering cannot be (Figure 1). On curves having a long radius, scenarios, the low extremes of sur overstated. Especially for low-slope, flat- the susceptibility is lower, but almost any face temperature can actually dip seamed copper (Figure 4), the time-honored metal roof will exhibit some oil canning in 25 or 30 degrees below ambient air. skill of soldering is absolutely central to sat direct angles of sunlight. Particularly when As a result, the “delta T” figure used isfactory performance. Even where auto pans are new and reflective, these undula in movement calculations can ap matic seaming is the predominant method tions have often prompted disapproval and proach 250 degrees in cold north of installation, soldering may be necessary debate regarding cause.3 The displeased ern climates.6 at features such as concealed gutters, party should be patient, as the eventual which are often problematic in the best of patina will obscure most of the oil canning Structural metal roofs use large, sturdy circumstances. (Figure 2). clips with considerable freedom of move Solder follows heat, and pretinning is The initial bright salmon-pink color is ment. Pitched copper roofing, however, usu crucial for laps and seams that are to per highly reflective but will mature into various ally has smaller, sheet-metal clips. These form in a watertight manner.8 There are four shades of brown as oxides and (later) sul may be seamed too tightly, restricting panel distinct steps to be followed when soldering. fides form on the surface. The very desir movement. At a minimum, oil canning is They are: able blue-green patina is actually copper likely. In extreme instances, fracturing of 1. Edges that are to be seamed should sulfate, which is the sole corrosion-resis seams can result. Breaking the work up be cleaned with an acid agent (flux). tant agent of copper roofing.4 This is an into shorter segments “manages” the cumu 2. Using an iron, edges should be 6• INTERFACE F EBRUARY 2009 tinned with a thin coat of solder. be provided when This should be done immediately, using stainless prior to folding or seaming, as the steel clips to solder oxidizes rapidly. secure copper 3. Once nested onto the adjoining piece panels. (and fastened as appropriate), solder Over its ser is then “sweated” into the seam to vice life, copper completely fill it. will eventually be 4. A “third” pass is made with addi subjected to some tional solder, perpendicular to the type of concentrat seam. This final pass is known as ed loading. Minor “stitching,” and the finished work indentations (such should barely (if at all) telegraph the as from hail) do edge being closed. (Some highly not compromise skilled mechanics maintain that water-shedding stitching is not needed. Indeed, they capability or ser argue that any solder flowing into vice-life expectan the joint during this step signals cy (Figure 5). A Figure 4 – Especially for low-slope, flat-seamed copper, the time- deviant work during step number 3. number of note honored skill of soldering is absolutely central to satisfactory This argument is compelling, but worthy and elabo performance. (Photo courtesy of Christopher Waites, ADC the author has not always enjoyed rate copper roofs Engineering, Inc.) association with highly skilled serve admirably in mechanics.) zones having rou tine hail exposure. If eventual hail dents will meaning in recent months. The author has Again, the great preference for this work be considered analogous to dents in the been a direct party to dump truck loads of is by iron, not by open-flame torch, which fender of an automobile, an alternative cov copper being taken to a landfill. At that time can destroy the flux; it can also destroy ering should be considered.11 (1982), it was not profitable to clean sal anything under the metal that happens to Pricing of copper has taken on new vaged metal (remove bituminous contami- be flammable.9 When soldering is rejected (for whatever reason), it may be wise to replace the work rather than to attempt repairs. Adding more heat and solder is not a viable remedy – it will seldom, if ever, res cue substandard workmanship. Other aspects are worthy of considera tion. Though sometimes omitted, underlay ments should be used in most (if not all) copper roofs, especially if panels are installed over wood decks. If self-adhering ice-and-water membrane is to be used, soft ening point of the bitumen must be consid ered (remember, copper panels can become quite hot during full solar gain). Condensation can sometimes occur, even below open mansard roofs. Some wood treatments (especially fire retardants) can induce corrosion of any metal roof, includ ing copper.10 This underscores the need for underlayment. Ventilation of the deck un derside is an appealing attribute of virtual ly all pitched roofing. Potential for galvanic action will eventu ally be encountered in copper construction, so precautions are advisable. For instance, steel can drain onto copper, but the reverse should be avoided – copper gutters should be used to drain copper roofs.

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