ggregate Ballasts for Low-Slope Roofs
Recently, we got a chance to take a look at a aggregate ballast covered roof. Historically, here in Washington, DC, there were many aggregate ballast roofs like this, even as recehtly as just a few decades ago. In recent times, most of these aggregate ballast roofs have now been replaced with modern fully adhered single-ply membranes. Our company would generally install a new modern single-ply roof membrane where the single-ply membrane is fully adhered to the substrate. In modern times these types of single-ply roof systems are much more common, but just a few decades ago, these historic ballasted roofs were more common here in Washington DC.
As time has went on, many of these older roof systems have begun to leak and fail because they’ve just outlived their intended useful lifespan. It’s rare that these ballasted roofs are replaced with new ballasted roofs today because the weight of the ballast is a significant burden to deal with during installation and modern single-ply fully adhered systems perform very well.
Nonetheless, these older roof systems are very interesting for a handful of particular characteristics that we’ll discuss here today. As well, it’s helpful to learn about this type of roof system because there are some still out there, even in modern cities like here in Washington DC, and it’s good to understand the characteristics of all different roof system types.
A picture of a traditional aggregate ballast covered roof system follows below. The layout is simple, overall, it’s a rectangular shape, almost a square, but there’s a side roof at a lower elevation with what appears to be abandoned steel I-beams discarded on the rooftop.
Often these ballasted roof systems are referred to as loose laid roof membranes because unlike the fully adhered systems that are so common today, the historic loose laid systems did not necessarily have complete attachment to the substrate decking or roof underlayment. In many cases, including today, that roof underlayment may actually be a cellulose or polyisocyanurate type of insulation.
Insulations like this often act as recovery boards which are used to cover over old roofs where the decking may have splinters or nail heads that protrude. The recovery board can protect the membrane from most types of imperfections in the substrate and also provide a degree of thermal resistance to keep the building a tiny bit warmer but even more importantly deter condensation between the cold roof system and the warmer substrate building interior during the winter months. The Washington DC winter can be punishingly cold.
At this particular rooftop, a system of cementitious pavers has been installed around the perimeter of the roof membrane. These cementitious pavers work both as a walking path and also as a type of ballast in themselves. They can weigh down the roof to prevent uplift during wind storms and or gale force type winds, and they also create a walking pathvsurface that protects and prevents damage to the substrate membrane below. In this particular case, the substrate membrane is a EPDM or similar type polymer membrane which is susceptible to damage if the aggregate is stepped on repeatedly and / or with the full weight of heavy individuals.
One of the main purposes of the aggregate is to prevent uplift during wind, particularly where the membrane is not fully adhered and or loose laid. The weight of the aggregate stone can help hold the membrane in place during high winds, preventing it from lifting or “billowing”. Lifting or billowing like this, once it starts, can cause the entirety of the membrane to lift up like a sale in the wind and rip right off the building and blow away. Although the membrane might be light enough for the wind to carry it away, it is still so heavy that it can cause damage or death to any individuals below and or building or property.
The next picture shows a different angle of the same roof. You can see a large exhaust duct that runs through the roof, set on a curb with an aluminum termination that is self rimming around the elevated curb. If you look very closely towards the bottom left-hand corner of this image, you also may notice an internal drain. For a large loose laid roofs, particularly commercial layouts where the perimeter of the roof may be a significant distance from the center, at all angles, there are handful of different ways to drain the water from the roof.
In residential historic row homes here in Washington, DC, one of the most common methodologies for drainage is to direct the roof water towards a rear termination. In this case, a perimeter termination would require guttering around the entirety of the building and would also require long travel distances for rain water to flow across the roof surface. Internal drains, in this case, based on the geometry and configuration, may work better from a perspective of capacity and water flow.
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.
