Understanding Surface Cracking at BUR Flat Roofs – Causes & Solutions (Part 1)
The photo below shows an example of an older, low-scope roof system on a row home, just like many found throughout Washington, DC. This city is full of historic buildings, thousands of which were originally constructed with double wythe brick exterior walls on at least three sides. While the architectural styles may vary, most of these structures followed similar construction principles. At the time of their original construction, the majority of these buildings were originally either covered with built-up (BUR) systems or standing seam metal roofs. Since then many have been overlaid or replaced as the decades have passed.
You can see the distinct lines of the underlying standing sesm metal roof showing signs of pronounced wear at the surface of the newer (yet currently very old) BUR roof on top.
As seen in the next photo, this particular roof system was installed directly on top of a previous roofing substrate. The current building code allows a new roof to be installed over an existing one, but only once. If a single layer of membrane is already present a second can be added. However, more than two layers are not permitted.
In essence, this means that when there’s an existing membrane, a new one can be added. However, if there are multiple layers or roofing systems from different generations, the rules of the code generally require full removal of all layers down to the wood deck substrate. You cannot strip off one or two and leave other old layers behind. Once you begin a tear-off, you must take it all the way down to the base, by these particular constraints or rules.
In this particular case, the current roof was applied over a previous one. Possibly there are additional hidden layers even further below that, although that hasn’t been confirmed. The only sure way to know is to cut a core sample all the way down to the deck surface, or remove the roof.
It’s not uncommon to discover roofs that have been layered / overlaid repeatedly over time, With each iteration, new materials were installed on top of the old, leaving a, today / now, unknown number of buried systems beneath. This situation often leads to surprise cost once the tear- off (removal) begins, especially when multiple hidden layers are discovered.
The photo above shows the rear part of the same built-up roofing system coated with aluminum paint, from the pictures above. Here though, you may notice, there are no visual queues that there there is a standing seam metal roof below. We don’t know, for sure, here, and whether more layers exist below that remains uncertain.
The next photo shows an example of a poor quality application of asphaltic mastic over a BUR surface. This repair has deteriorated significantly. Chunks of the asphalt have delaminated, and widespread cracking is clearly visible. In Washington, DC, it’s common practice to use drip edges at the rear of flat roofs. These flashing terminations direct water into a gutter, which then flows to a downspout.
The particular cornice shown below includes a decorative sheet metal feature attached to the front façade and set into the low-slope roof system. The BUR wraps around this feature. Originally, the decorative element may have been installed independently or soldered to a standing seam roof system. The rear termination of the roof, normally a drip edge, at this location leads into a gutter, but in this case, both the drip edge and gutter are missing.
Also without a proper drip edge, water tends to flow directly down the building’s façade rather than being directed away. Even when a drip edge is present, wind-driven rain can push water back toward the masonry. The close-up in the next photo shows the surface of the BUR in more detail. Compared to modified bitumen systems, BURs are equally—or sometimes more—vulnerable to long-term damage from ultraviolet (UV) exposure. UV rays gradually break down roofing materials, especially those based on bitumen. (If BUR are sufficiently covered in aggregate , they may have better UV resistance.
Yet, as we explain in other blogs, that difference generally isn’t enough to make BUR systems a better choice.) Nearly all types of flat roofing materials—except some metals and other less common materials—suffer considerable deterioration over time when exposed to the sun.
We provide this information here on our blog, and our website, to help our customers and future clients, and 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.
Our company specializes in flat roofing here in Washington DC and we’re happy to help building owners of almost all types. 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. We are happy to help and at least talk through options.
