Why Missing Roof Coping Poses a Serious Threat to Your Building’s Exterior Walls
Some roofs here in Washington DC are just in bad shape. Flat roofs are not easy to see from the ground, in most cases you have to get up on top of the roof to see the condition and it’s not easy for everyone to climb up to the roof of a multi-story building, up a ladder. We are up on all of these roofs every single day and we’ve seen all sorts of different circumstances that vary from relatively good installations to terrible conditions of extreme deterioration.
Here, in today’s blog article, we’re going to show you examples of a roof that has been overlaid with a modified bitumen membrane. However, the last person or contractor to install the overlaid modified bitumen membrane did not address the pre-existing deteriorated coping. Overall, they did a very poor quality job, and they simply did not do things professionally. We will look closely at some of the examples here.
A coping is intended to be an impermeable element that spans over top of a parapet wall to prevent the infiltration of moisture which is particularly problematic at the top of parapet walls. Parapet walls, at the majority of the historic brick buildings, here in DC, have a near flat surface. Technically, in most cases, they have a slight amount of grade, but in most cases, that slight amount of grade might be near the minimal amount required or desired for good building practices.
It’s beneficial to have at least a minimal slope built into all elements of the building assemblies on top of a building roof. That little bit of slope deters ponding or build up of water accumulation, and encourages flow of water. When all of the elements of a rooftop and accessory elements are built properly, water will flow off the roof passively and naturally by gravity alone.
As you can see in the picture below, the original membrane which was installed to span and cover the top of the parapet was largely deteriorated, and now, the outer side or exterior side of the top of that wall is bare.
The past, most recent, roof installers basically took a shortcut, they didn’t care or take responsibility for items that were directly outside of their scope of work and now although a new roof membrane has been installed, it has problems because there are points of water leakage coming directly through the top of the parapet.
If you look in the next picture below, you can see that where the original membrane was installed at the sides, at the top of the wall, the membrane is mostly gone and now missing, but even more problematic, where the original membrane laid on top of the horizontal or near horizontal area of the top of the parapet it is now separated and has delamination which can allow typical precipitation to enter into the space below the newer roof membrane edge.
This is a quintessential real world example of the adage that a chain is only as strong as it’s weakest link. The next picture below shows the newer roof membrane as it spans across the majority of the field of the low slope rooftop. Although the membrane was never coated and it was never installed with good building practices, it should have been installed to at least be sealed at the perimeter, even if the original contractor was only asked to overlay the area of the original roof membrane and nothing else. Leaving or omitting a detail such as terminating the membrane properly at the perimeter is a big oversight.
You can also notice that there are significant voids in the mortar at the brickwork at the top of this exterior facade brick wall. Historic brick needs to be repointed every several decades because the mortar will deteriorate at a rate faster than the deterioration of the brick itself and is intended to be refurbished or revitalized through repointing.
The next picture looks down, at a low angle, over the interior parapet which separates one row home from the next. The problems we’re talking about generally occur at the building on the right side of that dividing parapet, but even where the internal parapet wall meets the external parapet wall, there are omissions that will likely affect the neighbor on the left side who has hired a better contractor to do their most recent roof overlay.
The next picture below shows the same area at the exterior of the parapet wall, but here you can see a thin piece of historic wood that was installed between the brickwork. This wasn’t a shortcut, it was an intentional installation of a nailer board so that the bottom of the original coping sheet metal could be fastened and nailed to something other than just masonry.
Masonry is not a good substrate for most types of historic framing nails because historic framing nails did not have shaft rings or any type of texture to allow the nail to stay embedded into the masonry. Cut nails, which were common at the time of the original construction, would work in certain configurations, in masonry, but the heads of these nails would generally not work well with attaching sheet metal.
The nailer boards that were here did not go directly or entirely through the area of the wall, instead they were installed into a void area where a portion of the exterior most motor was left out, intentionally. With this type of nailer board inserted partially into the mortar joint, it would provide a location to attach the sheet metal coping.
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. We are happy to help and at least talk through options.
