Core Sampling in Flat Roofing – Part VII

Washington DC is a bit of a unique type of city because in most parts of America, and the world for that matter, it’s rare to find flat roofs on residential buildings. In fact, most flat roof space in America is found on commercial or industrial buildings.  Most residential roofs are pitched, and the gable and hip roof styles are very common.  Even in modern times, city redevelopment often has tract-style rowhomes, referred to as townhomes, in the city with pitched roofs covered with cheap asphalt shingles.  Functionally, asphalt shingles work ok, but they look like a suburban tract home development type of building. That’s where this particular architectural element is most common. 

Core sampling is a fancy term for cutting a hole through a roof to see how many layers of roof membrane systems are installed on top of the original base roof deck. A roof deck, similar to a floor deck in a house, is different from the type of deck you have on the back of a house, for example, where you have outside, barbecues, and things like that. A roof deck is generally built with plywood almost butted tight at the edge from one board to the next.

For a proper core sample to be done, you almost have to cut all the way down to that substrate roof deck. In some cases, you can cut down to a polyisocyanurate insulation board, with care to verify that insulation isn’t covering an additional membrane further below that, in which case it might be technically called a recovery board.

Chapter 9 of the building code, which governs the rules for the installation of roofing, is very sparse, and there are only a few requirements. Most of the requirements that are imposed by jurisdiction or authority come from the roof manufacturers, driven by their own inherent interest in avoiding failures of their product through misinstallation. The building code however does say that if there’s more than one membrane of existing roofing on top of a roof deck, the membrane must be removed all the way down to the substrate deck. 

It’s acceptable to cover over a single layer of membrane. But if there’s two there, you can’t take off just the top one and put a new membrane on top of the first remaining membrane. Both must be torn off in that case.

The picture below shows an area where a cut has been made through the existing membrane.  A single layer of thin recovery board has been removed to expose a gravel ballast on top of BUR roof below. BUR roofs are built-up roofs and are generally very old and can be particularly onerous and complicated to remove and dispose.

core sampling in flat roofing

One of the problems with core sampling is that it generally cannot be done at the same time as the new roof installation. For example, if you core sample just before installation of a new roof and then find out you also have to do a major demolition or addition to the planned scope of the demolition, it’s a game changer. Incorporating a game changing type additional amount of work into the scope right before your plan to begin progress in one area is potentially ruinous to the plan itself.

cut in membrane roof

For this reason, core sampling is a bit more complicated than it would be if it could be done at the same time or in the flow with other work.  In some cases, the project or particular job will allow for an additional level of coordination between planning, in real time without remobilization, but that is a rare circumstance.

The keys is of jobs or work orders or projects which allow for that normally require multiple different roofs to be repaired to replaced within the same project. But in most cases it is not that simple and the work has to be staged separately so that the findings from the core sampling can be incorporated into the plan.

Upkeep, Care and Maintenance of Washington DC Roofs

This case study is a perfect example of why a professional roofer should be used for all types of roof installation, even things that seem as simple as a balcony.  Consulting with a roofing professional, like Dupont Roofing, familiar with the specific climate conditions and the building science of roofing in Washington DC is advisable to ensure the roof assembly is appropriately designed and constructed to manage the flow of water, ice damming, capillary action and condensation effectively.

The upkeep, maintenance, and general care of flat and mansard roof systems should be driven by an understanding and passion for historical methodologies, waterproofing principles, engineering and building science. Washington DC, a city built with both vintage charm and contemporary modernities, residential and commercial buildings of substantial value. 

The roofs of these buildings are their defensive shield from the harsh elements of nature. To our clients, as well as all readers of this article and our blog, we emphasize the importance of quality construction and active building maintenance.  Our website includes informative resources you can use to understand and learn about best practices on preservation of your building.  If you are in need of further guidance on the roof and its associated systems for your Washington DC property, we are here to assist, where we can. Simply contact us or complete the webform below and drop us a line and we’ll respond if we can help.

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