Architectural sheet metal roofs speaks to what are architectural sheet metal roofs and have been in use for a very long time. In fact, thin sheets of metal have been used for roofing since ancient times and remain a popular roofing material to this day.
These roofs are installed using ingenious methods of joining and fastening to maintain a water tight fitting. Seams between sheets must be spaced close enough to secure the sheets adequately against wind uplift and to absorb expansion and contraction between sheets due to changes in temperature.
These seams also create strong visual patterns that can be manipulated by the designer to emphasize the qualities of the roof shape. Sheet metal roofs are relatively high in first cost but when properly installed may last for many decades.
Many types of metal may be used in the production of architectural sheet metal roofs:
Lead is a soft, easily formed, very long lasting metal that oxidizes over time to a dull white colour. It is also a toxic material that requires special health precautions during its handling.
Copper is a relatively soft metal that turns a beautiful blue green in clean air and a dignified black in an industrial atmosphere. Various chemical treatments can be usd to obtain and preserve the desired colour.
Lead coated copper sheet is sometimes used to combine the greater strength of copper with the gray white colour of lead and to avoid the staining of wall materials by oxides of copper that can occur with rainwater runoff from uncoated copper roofs.
Zinc metal roofing is a long lasting roofing metal made of zinc alloyed with small amounts of copper and titanium to improve its workability. It normally ages to a dark gray colour and can also be treated in various ways to alter or preserve its appearance.
Stainless steel and titanium are strong and long lasting but less easily worked than other roofing metals and both are silvery white in colour.
Zinc tin alloy coated stainless steel has a darker, duller appearance than uncoated stainless steel. This material is very similar in appearance to and is sometimes confused with a lead tin alloy coated material called terne coated stainless steel that is no longer manufactured but may still be found on older buildings.
The metals listed above all form self protecting oxide coatings known as patinas, that provide long lasting resistance to corrosion. They are usually installed uncoated and allowed to patinate naturally. Other, less expensive metals which are not as long lasting in an uncoated condition are commonly factory coated with high performance organic, paint like coatings that extend their life expectancy and provide a wide range of colour choice.
These include aluminum, metallic coated steel which are steel coated with alloys of zinc or zinc aluminum or with zinc tin alloy coated steel and even on occasion ferrous syeel without any protective, metallic coating.
Thickness of Architectural Sheet Metal Roofs
The thickness of steel sheet has in tradition been specified by gauge or gage which is a system of whole numbers in which lower numbers correspond to greater metal thickness. However, due to the absence of a uniform standard for the translation between gauge and actual metal thickness, ASTM standards discourage the use of gauge numbers to specify sheet metal thickness and instead recommend reliance on decimal or fractional inches to indicate thickness.
The thickness of copper sheet is normally specified by weight by weight, expressed in ounces per square foot (0.092 square meters). Aluminum is specified simply in decimal inches.
In general, thicker metal sheets are longer lasting, less prone to waviness or “oil canning” often more difficult to form into shapes, and more expensive than thinner sheets.
Manufacture of Architectural Sheet Metal Roofs
Metal roofing may be fabricated and supplied in two distinct ways. When a roofing installer purchases unformed sheet metal stock and custom forms the panels to the required shapes, the roofing is specified as sheet metal roofing.
Alternatively roofing panels may be factory formed into a variety of shapes that can be selected from a manufacturers catalog. In this case the roofing is specified as metal roof panels. Metal roof panels are most commonly made from aluminum or metallic coated steel with factory applied coatings.
They may rely on interlocking seams with concealed fastener systems that imitate the appearance of traditional site fabricated standing seam or batten seam sheet metal roofing, or they may consist of simpler corrugated or folded shapes fastened with exposed screws and rubber washers. Metal roof panels are generally less expensive than traditional, custom formed sheet metal. They may also be used in low slope roofing applications.
Minimum recommended slopes for sheet metal roofing vary with the type of seam, the manner in which the seams are sealed and the type of roofing underlayment. For premanufactured metal roof panels, consult the manufacturers recommendations. For custom fabricated sheet metal roofing consult the appropriate references which are readily available.
Protection from Dissimilar Metal Corrosion
The ideal way to avoid the corrosion that can occur between dissimilar metals is to use the same metal for all components of a sheet metal roof system, including its fasteners, anchor clips, roofing sheets, flashing and gutters and downspouts.
Alternatively, where this is not practical metals must be mixed with an understanding of the reactions that can occur between them. For example, sheet metal roofs of copper, lead or zinc may be generally be safely anchored with fasteners and anchor clips made of stronger hardr stainless steel, because the stainless steel is electrochemically noble or passive in relation to these other metals.