Reverse Coping Case Study

Reverse Coping: Insights from a Case Study

Today, in our weekly blog article, below, we’re looking at a bunch of pictures from different angles at a relatively common type of roof issue or problem found here in Washington DC. Many roof contractors are not very professional and do not install coping at the side walls and or parapet walls of our low slope roof row buildings. In some cases, years ago, at the time that the most recent roof system was installed, the last contractor to build a roof system on the building actually did install a proper metal coping system. In some cases, the next generation of roofing was installed without setting the new membrane properly in relation to the existing coping. With a new roof, if the existing coping is still in acceptable condition, it might be possible to reuse that existing coping.

The picture below shows a newer modified bitumen roof system on top of a building here in Washington DC. There’s mechanical equipment set above the modified bitumen roof system. At the edge of the roof system, there is an old ferrous metal coping that is deteriorating, significantly. The deterioration is visually observable because of the amount of rust, both on the surface of the coping itself and also visibly identifiable because of the rust stains running down the wall below.

Reverse Coping Case Study

In this case, the coping metal, is exposed on the side, but partially covered on the top by the new roofing membrane

If the coping is not in good condition, it should be replaced. It’s important to understand the sequence though because the new roof membrane should be installed underneath of the coping metal. Not all coping have to be made with metal but metal is one of the most common modern materials used for coping. In most cases the coping metal selected will be a relatively light gauge aluminum material.  Factory finished pre-painted aluminum coil stock is one of the most common types of materials used for coping.

The picture below shows a closer view of the rusting metal at the exposed side of the old ferrous alloy coping.

old ferrous alloy coping.

In most cases, the coping metal is custom built in the field to match the specific dimensions of the parapet wall to be covered. Generally, when measuring for the specific custom sizes of new coping you will measure the thickness of the wall and assembly of the proposed elements to be added to the new roof system.

In the next two pictures, you can see closer views of the rust stains discoloring the wall below.  Rust stains can be surprisingly difficult to remove from many types of building surfaces. Particularly in the case of porous surfaces such as masonry and concrete, rust is difficult to remove because the molecules of the rust enter into the porous surface of the stone and masonry. In this case, the building is made with a masonry facade, but a coat of paint has been applied on top of the exterior surface. In the case of a painted surface though as well, it would be difficult to remove the rust stains from the wall.

rust stains discoloring the wall below

The important distinction here though is that the new coping metal should be laid on top of the new membrane. Even if the old coping is to be reused it should be at least partially removed and then reattach to refas into a completed position on top of the new roof membrane.  In this particular example, and many other similar examples here in Washington dc, the previous roof contractor just took a shortcut and laid the new roof membrane directly on top of the old coping.

roof membrane directly on top of the old coping
There are several problems with this type of improper installation and you can see a picture of a quintessential example of one of the main problems in the picture below. At the edge of the modified bitumen roof membrane, where it lays directly on top of the coping, there is separation between the edge of the roof membrane and the metal coping below. The gap between these two materials can allow water to infiltrate under the modified bltumen membrane and will eventually work its way back towards the area of the main root system, above the roof decking, and then leak into the interior of the building.

leak into the interior of the building
One of the principles of dealing with water management is that any edge above a lower surface or area of building materials should extend, with a declining angle, away from the substrate surface. This is important because water will run backwards into an interstitial area, through a phenomenon called capillary action.

backwards into an interstitial area
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.

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