Here in Washington DC, there are several different types of membranes installed on flat roofs, but one of the most common historic types of roof membranes are standing seam metal roots. Standing seam metal roots are not the only type of metal roof, but they’re the most common here. Outside of Washington DC and on other types of buildings, corrugated metal roofs are also a more common option and once in awhile here in DC will also see an alternative type called a flat seam metal roof.
Standing seam metal roofs are quicker and easier to install than flat seam metal roofs and have a similar if not better level of water prevention and endurance. By comparison, flat seam Roost require each seem to be treated with a soldering or welding or sealant application. Standing seam metal roofs, by comparison can terminate from one panel to the next without soldering.
The picture below shows an example of a standing seam metal roof in a significantly aged and oxidized condition. What are the colors that were commonly historically used for painting metal roofs was a red roof paint. The paint application though looks a little bit different than oxidation. Oxidation, by comparison is less consistent, color is more heterogeneous and less homogeneous overall. And there is a darker color to actual rust.
Metal roofs are much less common today than they were in historic times. Today much stronger and more efficient alternative membranes are available.
Flat seam metal roof systems involve joining individual roof panels through this distinctive raised, vertical seam. The panels were (and still are today) manufactured with specially designed interlocking edges along their sides. During installation, these interlocking edges are aligned and overlapped, which allows the adjacent panels to interlock continuously. A seaming tool is then used to crimp and fold the overlapping panel edges together, creating the raised vertical seam profile. As the seaming tool passes along the panel joints, it applies pressure to bend and fold the interlocked edges upward into a vertical seam that stands above the roof surface, typically about two inches tall.
The vertical orientation of the seam serves multiple purposes. It allows for effective water shedding by preventing any ponding areas at the area of a seam, in cases where ponding is less than about the seam height. Also, the standing configuration of the seam minimizes the risk of moisture infiltration. The crimped and folded seam creates a strong, rigid connection between the panels, enhancing the overall structural integrity and resistance to wind uplift forces. Additionally, the vertical seam design accommodates thermal expansion and contraction movements of the metal panels without compromising the seam integrity or inducing panel stress.
Aesthetically, the distinctive raised seam profile contributes to the architectural appeal of the metal roof, complementing various building styles, where the roof is visible. While metal roofs may complement various building styles where the roof is visible, in the densely populated neighborhoods of Washington, D.C., many of the flat roofs are not readily visible from the ground or street level, diminishing the aesthetic advantage of standing seam metal roofs for these buildings. The architectural appeal of standing seam roofs is more prevalent on structures like historic Victorian-style homes with prominent front porches or buildings situated on higher elevations, where the rooflines are more exposed to public view from various angles. Within the historic districts of D.C., the flat roofs are often obscured by parapets, mansards, or the close proximity of neighboring structures, making the visual impact of the roofing style less of a priority compared to performance and longevity.
During the seaming process, a sealant or butyl tape is often applied between the interlocked panel edges to create a watertight barrier, further help the weathertightness of the roof system. Proper installation and the use of specialized seaming equipment are needed for the long-term performance and durability of flat seam metal roofs.
The picture below shows another metal roof with a flat seam portion in the front area of the building. That area is a low spot in the overall roof and ponds water which means it will hold water after it rains because it does not drain properly as it lacks proper consistent grading to the drain.
The roof, both in the picture above, and in picture below are coated with an aluminum paint which looks a bit silverish in color. These types of roofs have a moderate reflectivity but not nearly as high or efficient as modern single-ply roofs with white or light colors. The particular building in the picture below has a AC condenser which indicates that the space has been converted to interior space and is tempered; this means that a low reflectivity matters here more than most typical alley buildings or garages. In a building like this, where an AC system is installed, the system will have a much tougher time, requiring higher energy consumption, just to combat and balance the intense heat from the Sun on the roof.
The next roof below is also a standing seam roof but unlike other ones shown which are a ferrous metal alloy, the roof below appears to be a copper roof with an oxidized patina. Unlike steel or iron, and other types of ferrous metal, copper will not really rust through. In fact, the oxidation almost works like an anode to help protect the metal from further oxidation.
Copper is a non-ferrous metal, meaning it does not contain iron. Unlike ferrous metals like steel or iron, copper does not rust in the same way. Instead, when exposed to the elements, copper undergoes a natural process of oxidation, forming a protective layer called a patina.
The patina is a thin film of copper oxide that develops on the surface of the copper. This patina starts as a reddish-brown color and gradually turns to a distinctive green or bluish-green over time as the oxidation progresses.
The patina forms a barrier that helps prevent further oxidation and deterioration of the copper beneath it. This self-protecting nature of the patina is often referred to as a “self-healing” or “self-anodizing” process.
The patina’s ability to protect the copper is due to its properties as a semi-conductive oxide layer. It acts as an anode, allowing the copper beneath to remain in a reduced state and preventing further oxidation. In contrast, ferrous metals like steel or iron tend to rust continuously, as the iron oxide (rust) does not provide the same protective barrier as the copper patina.
The oxidized patina on copper roofs helps extend the lifespan of the copper by shielding it from further oxidation and deterioration, allowing the copper roof to maintain its functional integrity and appearance for a longer period compared to unprotected copper surfaces.
However, it’s important to note that while the patina provides protection, it can also be affected by environmental factors such as acid rain or atmospheric pollutants, which can accelerate the degradation of the patina over time. Regular maintenance and occasional cleaning or treatment may be necessary to ensure the longevity of the copper roof and its protective patina.
We recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC. Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at DupontRoofingDC.com, and you can call us at (202) 840-8698 and email us at dupontroofingdc@gmail.com.
