Roofing Over Abandoned Structures: Key Techniques
Today, we’re going to look at a couple different items on existing older modified bitumen roofs that have been abandoned, but instead of just removing the original elements these elements have been left behind and then modified bitumen membrane has been applied around them to cover them. This approach saves a little bit of money because it’s more expensive to remove old abandoned chimneys, skylight curbs, and access hatch curbs, but it might actually be better to just remove them all together to save costs of maintenance in the long run. Today, we will look at some examples.
The picture below shows a curb used to mount a historic skylight. Many of the neighboring buildings were built with the exact same layout and configuration. In some of those cases, the original skylight still remains in place but in varying stages of dilapidation and deterioration. It’s understandable that the skylight has been removed in this particular case, but the skylight could have simply been replaced with a new skylight in good condition. Here though instead, to save a tiny bit of money, they removed the old skylight, patched over the old curb and then installed the roofing over top of the old curb.
When they last replaced the roof membrane, they had a few options. As we discussed above, one of those options was to replace the skylight with a new modern skylight. Another option was to remove the curb entirely, patch the substrate roof deck, and then install new roofing on top of that area. The cost of that option is not much more expensive than the option they chose, but the option they chose is probably the cheapest overall. The option they chose and did in the end, was to remove the old skylight, install decking over top of the skylight opening or curb at the skylight, and then reroof on top of the existing curb.
Technically though, while it may save a little bit of money, leaving older abandoned items, especially when they complicate or cause deviations from a simple planar form of a regular roof field, can increase the likelihood and the cost of future maintenance. In this particular case, you can see a crack that has formed at the outer corner of the old abandoned access hatch curb.
Often, when a modific membrane is intended to bend a steep 90°, it’s better to install a cellulose or similar material type cant strip. Wood can also be used to create a break to reduce the angle to a maximum of 45°, even when compounded.
In the next picture below, we look a little bit closer and you can see that there’s an actual separation of the membrane materials, at least at the upper layers of the membrane.
Here, you can see, the next picture below, an example of a typical joint between the planar field of the roof membrane and the vertical parapet wall. It’s very often that cant strips are not used to transition between these planar elements, but a cant strip can definitely be helpful and useful. In most cases we would recommend a cant strip.
This chimney is far from an example of good quality installation though because the original roof installers here at this location have omitted several types of installation requirements. They have not installed a counter flashing or a termination strip where the modified bitumen membranes terminates against the side of the chimney.
When you look closely at the area where the counter flashing was omitted, you can see that this is a potential point of water entry. Any water that essentially runs down the side of the chimney, will reach the top of the roofing base flashing and then that water will slip behind that part of the membrane. Although it isn’t difficult to install counterflashing, it’s omitted so often that we consider it a basic checklist type item, when doing a typical general roof consultation.
The next picture shows an abandoned chimney at a different rooftop. You can see that this particular chimney is covered with overgrown weeds. Eventually, plants and vegetation will have roots that make their way into the chimney itself which can exacerbate any problems or openings and eventually lead to channels or holes that allow water to enter into the interior of the building from the exterior.
The next picture shows a different abandoned skylight curb, and we don’t want to pretend that this is a perfect example of how work should be done, but it is better than the one we had showed previously. In the previous skylight curb, at the top of the article, you can see that there is a seam directly in the horizontal field of the membrane and in this case at least the seams at the exterior outer edges of the skylight curb. Here we consider that better because in the case of an overly flat surface, it’s better to not have seams in areas where ponding would occur.
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.
