Residential Shingle Exhausts: Adjusting for Flat Roof Efficiency
It’s relatively common, here in Washington DC, that a variety of different types of professionals and non-professional DIY type enthusiasts will work on flat roofs to make minor or improper moderate size repairs to flat roofs. Today we’re looking at a relatively common problem and issue on the flat roofs of DC that could easily be avoided by using a professional roof specialist contractor. Often people who are less experienced with roofs, when working on rooftops here in DC, will use residential roof products instead of commercial roofing materials.
It can be a little bit confusing because many of the historic row homes in Washington DC are in fact residential properties, yet, they are built with low slope roots and the vast majority of low slope roof systems are classified or considered to be commercial systems. The industry makes a relatively clear distinction between residential and commercial systems in the roofing industry.
Most low slope or flat roofs are actually commercial systems, as designated or dictated by the manufacturers and most shingle roofs are considered residential. However, there is a bit of ambiguity in the historic buildings of Washington DC, where flat roofs are used on residential properties, so it can be a bit confusing.
The picture below shows a clothes dryer exhaust duct and a bathroom exhaust duct termination on a flat or low-slope roof, here in Washington DC. Both of these elements are installed way too close to the low slope, near horizontal, roof deck surface.
It’s probably best to draw the distinction between residential and commercial roofs, by dividing the classifications by slope. It’s not just as simple as shingles versus low slope systems, because in some cases there are alternatives to simple asphalt shingles, even on moderate to high slope roofs. Standing seam metal, for example, can be used on both a low slope and high slope roofs.
Even typical low slope modified bitumen roof membranes can actually be applied to higher slope roof systems. It’s just less common to see a modified bitumen membrane on a moderate to high slope roof but there are examples where these types of membranes, intended for low slope roofs, have been installed on moderate or high slope roofs structures.
We’ve looked at this issue before, here, on our website. In many cases, pre-manufactured shingle roof flashing boots have been mistakenly used on flat roofs in DC. We’ve also looked at examples where shingle roof exhaust terminations have been installed on low slope roofs and we’re going to show some more examples of that type of issue, here from buildings in Washington DC, in this coming week’s blog article.
The picture above shows the opposite condition, in this example, above, the exhaust ducts have actually been installed over 2 ft above the low slope roof system, as they are intended to be built. This helps for several reasons. The main reason is because typical snowfall or typical snow buildup will not as easily enter into the ductwork system.
To DIY homeowners or less experienced contractors it may be tempting to use readily available residential products, such as shingle roof exhaust terminations, on these low-slope roofs, but using these purposely manufactured construction products incorrectly can lead to leakage and associated problems. Residential products are designed for steep-slope roofs and usually have shortcomings which cause several problems when used on flat or low-slope roofs.
- Improper Flashing. When used as design, without an extending flange or an integral area to connect to a low slope roof, residential roof products often cannot be properly sealed to withstand the issues related to minimum ponding occurrences, even when a flat roof is built properly to have the minimum required grade at all points.
- Snow Build-up. Often, elements like air exhaust terminations, when it’s installed on a flat or low slope roof, will be under the minimum required height above the rooftop and will allow snow buildup to enter into the exhaust termination in an unnecessarily higher occurrence of snowfall incidences.
When installed on a low slope or near horizontal rooftop, these residential products are often not not adequately sealed to withstand the ponding water. A proper connection may require an extended area of an integral flange at the base of a duct termination. Residential products are not built in this way because the grade or slope on a typical shingle roof, for example, will be sufficient to shed water more rapidly than a low slope roof and therefore an extended flange is often not required for installation on a shingle roof or a residential type roof system.
Low slope or flat roofs also have additional common thermal stressors that will cause relatively small movement in the membrane, but with a reduced size of overlap and bonding, low-slope roof membranes can separate from the small flange at the base of a residential rooftop duct termination, for example. This issue can come up and be a problem in more than just the area of root duct terminations, but that issue is particularly pronounced in connecting rooftop terminations to flashing at low slope roofs.
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.
