Key Insights into Rear Roof Termination for Flat Roofs
In the roofing business, here in Washington DC, everyday, we see things that are a little bit different and weird. Often we see work that’s been done by other contractors or supposed roofers who do things that are inadequate or inappropriate. At this particular roof, we saw something incorrect done in the past by other contractors. At least, in this case, we will show an example of something that while not correct, almost does the job, almost good enough.
In the picture below, we show an example of a modified bitumen torch-down roof membrane with a typical rear termination. This layout is relatively common, here in Washington DC, where most buildings in the historic areas of the city are built side-by-side. These buildings are typical row homes. They share a common demising wall. In most cases these demising walls are built from a double wythe brick masonry partition, and run all the way from the foundation to and through the horizontal field plane of the roof system itself.
Austin, the rear termination of the roof membrane is left improperly finished. The proper installation of a drip edge with a properly seated roof membrane, join and seem together is a detail of work that is a bit onerous and complicated. This particular type of work is especially difficult for amateurs or people who are not familiar with the proper installation techniques in the requirements of the building code.
The overall layout of the entirety of the main roof system is shown in the picture above. When you scan the surface of the roof membrane, you’ll notice that the layout and configuration includes relatively common elements. There are two abandoned chimneys which have been roofed over, and an old access hatch which is also now been abandoned and roofed over.
This field area of the rooftop surface also includes a single plumbing vent pipe, a historic skylight, and a contemporary AC condenser unit. These elements are all relatively common and found on most roofs, but each roof can vary a bit from one to the next. For example, it’s typical to find more than one plumbing vent on the average roof. The location and quantity of plumbing vents depends very much on the layout of the interior rooms which use plumbing drains. For example, a kitchen, bathroom, and or laundry room will all require a vent because most of them will have a drain which is part of the plumbing system in each of those rooms.
When a drain is present, the building code requires that a vent is installed to allow the ventilation of sewer gas and to allow the water to flow smoothly from the house into the public drain system. If all of the kitchens and bathrooms happen to be in a stacked type layout where one sits basically over the next, those vents can possibly be combined before they go through the roof system. This grouping or joining can possibly minimize the amount of roofing penetrations which require detailed flashing.
Here, when the plumbing vents come up and through the roof system, they must be flashed and treated properly, the setail of flashing at these locations is more tediousthan at other areas of the field of the roof. In most cases, with a modified bitumen roof system, the flashing will require torch welding a concentric two-part sleeve around the pipe penetration.
The first part of the flashing, in this case a flexible modified bitumen material, same as the remainder of the main roof membranenwhich runs upwards and wraps around the pipe itself. The second part is redundant but important because it overlays the base of the first part which has cuts in or through the membrane to allow it to follow the contour between the vertical part and the flat portion of the flashing.
The AC unit should be set on a sacrificial slip sheet when set directly on top of the roof membrane. It almost all cases it’s better to set the AC unit up on an independently supported dunage system or a pair of steel beams to separate the bearing of the AC unit from the field of the roof.
In some cases though when framing below the roof membrane is adequate to support the AC unit and when the AC unit is small enough not to cause problems with the structural bearing support below, the AC unit can be set almost directly on the membrane of the roof itself. Here though and in almost all cases, when the AC unit is set directly on top of the membrane, it should be separated with a sacrificial portion of membrane to prevent excessive accelerated wear related to ongoing vibration as the machine runs in repetitive cycles.
While it’s not at all technically proper or correct to fasten through the vertical part of the rear termination of the roof membrane, that’s what’s been done here, as shown in a more close view in the next photo below. Although it isn’t correct, it does work better than nothing. Some other contractors who, for whatever reason do not work properly, will often not adhere or set the roof membrane with any fastening at all.
It also should or could be noted that while this isn’t correct at least where they have installed fasteners through the membrane, these fasteners are driven through a vertical layout, not through a horizontal or near horizontal positioning of the membrane. Although this is not right, it’s better than some instances of improper installation.
In this coming week’s blog article, we will look at other photos from this roof and at others which are examples where this type of work has been entirely omitted.
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.
