This past week, we started looking at a roof which was consumed by overgrown vegetation, at the rear of the building, with wild and uncontrolled vines and trees. These plants and wild growth are causing serious damage to the building but the building has also been neglected for decades. If a building is properly maintained and managed with good upkeep and monitoring, damage will never be allowed to go unchecked to this level of disrepair.
Today we’re looking at some of the details of the original built-up roof (BUR) and showing both the types of damage that occurs with neglect, but also looking at the different elements of the original BUR roof.
The rooftop access hatch, which allows access from the interior of the building, is covered with hey flat seam metal roof membrane. Flat seam metal roofs are very similar to the more common standing seam metal roofs except the flat seam metal varieties are generally soldered at each seam area where is standing seemed are crimped and bent in on top of each other to prevent direct leakage from falling precipitation. One of the differences is though that the standing seam metal roofs do not necessarily do a good job preventing water entry when there is head pressure. Essentially, in other words, that means that when standing seam metal roofs are underwater they can leak. Loose lap roofs or most roofs for that matter, generally are not underwater often and or for much time, but during particular winter weather conditions where there is ice damming, parts of the roof can be submerged under a little bit of water for limited yet significant amounts of time. The level of significance, in terms of time underwater can be just a few hours, or a few days. Seams in a roof, when they’re not sealed with either a gun applied sealant or soldering, cannot really handle being submerged, even if just with a tiny bit of water within which they are submerged. In addition to being underwater, there are often slight capillary action type hydromechanics happening on roofs with pressure differentials. Pressure differentials at all different parts of the exterior of buildings, often driven from air pressure differentials on the interior of the building can cause leakage and or capillary action in places that typically will not be affected by normal or common precipitation events.
This particular hatch cover, and likely the entire roof, was originally probably built with this flat seam metal material and then later covered with what is now a defunct layer of built up roofing membrane.
Built-up roofing (BUR), was historically a common tar and gravel roofing, and is one of the oldest methods for covering flat roofs. BUR roofing basically made it possible construction configurations or construction building options to change significantly in the 19th century to allow the construction of more flat roofs throughout the urban areas of the eastern part of the United States.
This roofing system is made of alternating layers of bitumen, such as asphalt or coal tar, and reinforcing fabrics, creating a membrane system. The origins of BUR date back to the 19th century when flat roofs started becoming popular in urban areas, particularly for CBD areas with densely built flat roof buildings, and commercial and industrial buildings. Initially, coal tar pitch was the primary waterproofing agent, but advancements in materials and application techniques over the years refined the BUR process, and it should be considered a predecessor which led to the evolution of the contenporary single-ply roofing which is significantly more efficient and effective.
The typical materials used in BUR include bitumen, which can be asphalt or coal tar, and reinforcing fabrics such as ply sheets. These fabrics can be made from organic materials like cellulose or inorganic materials such as fiberglass, providing additional strength and stability to the roofing system. However, the fiberglass varieties but have only been available in more recent decades.
The top layer of a BUR system is typically composed of gravel, slag, or mineral granules, often referred to as a ballast which helps to protect the bitumen layers from some of the UV radiation and mechanical damage. Reflective coatings are sometimes applied to improve energy efficiency.
The application process of BUR involves several steps. A base sheet was often installed to provide a smoother and more consistent surface for the successive BUR layers. Bitumen tar was then heated in a kettle or tanker and applied hot. Ply sheets were embedded in the hot bitumen while it was still tacky, with this process repeated in alternating layers of bitumen and ply sheets, typically three to five layers depending on the desired thickness and durability. Finally, a top layer of bitumen was applied, and surfacing materials like gravel were often embedded into the hot bitumen. Reflective coatings or mineral granules can also be used for the top layer, in some cases.
One of the primary advantages of BUR is its durability, as these systems can last 20 to 30 years or more with proper maintenance. The multiple layers provide a relatively high resistance to water infiltration, and the gravel surfacing offers good fire resistance. Yet, they can be extremely difficult to troubleshoot precisely because of the multilayered assembly. Additionally, BUR required relatively higher maintenance compared to modern roofing systems, but it was often considered better than a metal roof.
In urban settings like Washington, DC, flat roofs are common due to building aesthetics and densely built use of space. The climate in DC, characterized by hot summers and cold winters, today, alternative systems like TPO, and other single-ply thermoset membranes are more common and normally a much better choice.
The installation of BUR can present some challenges. Weather conditions are a significant planning factor, as the installation process is weather-dependent, requiring dry and moderate temperatures for proper adhesion and curing of bitumen. The labor-intensive process necessitates workers trained in the antiquated methodology. Safety precautions are also a factor affecting usage, as hot bitumen poses fire hazards, requiring appropriate safety measures during installation.
To extend the life of a BUR roof, regular checkups and maintenance are needed. This includes checking for signs of wear, ensuring that the gravel or surfacing material is evenly distributed, and making necessary repairs. Although built-up roofing was once a common option for flat roofs, it s no longer widely considered a optimal type of roofing membrane.
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.
