Optimizing Shingle Roof Elements for Flat Roof Durability
This past week, we looked at an example of a shingle roof exhaust duct termination installed on a flat roof. Today we’re going to look at additional examples and other types of elements that penetrate through common low slope or flat roof systems which should be extended higher above a low-slope roof deck.
In the next picture below, we show an example of a PVC vent pipe, which is part of the plumbing drain, waste and vent system, installed over 2 ft above the low slope roof system. Plumbing vents do not have the same type of concerns as air exhaust ducts. In the case of an air exhaust duct, if snow or water gets into the ductwork, it’ll generally cause leakage into the interior of the building.
Water, either in the form of rainwater in a liquid form or in the form of ice or snow, will melt on the inside of the building and even if it’s within an air duct, that water will generally leak from the joints of the air duct because although there might be some tape or sealants applied at those joints, those types of sealants are generally not intended to be waterproof. Also, several types of air ducts have insulation on the inside of the air duct and or have seams along the length of the air duct which will also allow water to leak through.
When that water leaks through the duct, it will enter into the plaster or drywall ceiling or the interstitial insulation in a roof attic. That moisture and or water buildup, after it leaks through to the other building materials, will generally cause rot, deterioration, water damage, and or mold or mildew growth.
Those types of concerns are generally not a problem with a plumbing vent system though because unlike a air duct, plumbing vents are intended to be capable of handling draining water without leakage. In other words, at the joints of those pipes, the sealant or joining adhesives or cements are generally strong enough to be watertight. One of the more important reasons though, that plumbing vent pipes like the ones shown above should be installed several inches above the low slope roof is that when covered with a thick layer of snow, the pipe can become blocked with both snow and or ice that prohibits air from entering into the pipe and then causes a siphoning effect.
Siphoning happens when water is drawn out of a trap due to a pressure difference. In typical plumbing systems, traps are somewhat simple but effective U-shaped sections of pipe intended to stay filled with water that work as a barrier, preventing sewer gas from entering the interior of the building. In most cases, at buildings here in Washington DC, the vent pipes run all the way from the area near the plumbing drain on the inside of the building up through to the outside of the building, generally through the roof.
When a vent pipe is so low that it’s easily and often covered in a blanket or layer of snow, it becomes blocked. Blocking the top of plumbing vents can create a vacuum effect in the drain pipes, pulling water out of the traps and allowing sewer gas to escape into the interior of the building.
Sewer gas is a mixture of gasses produced by the decomposition of organic matter in sewage, you can just think about it, from a common sense perspective, the things we flush down toilets and through our kitchen sinks run into the sewer system. The gas produced from the decomposition of these materials can be harmful to human health, as it contains a variety of pollutants and pathogens that can cause respiratory problems, nausea, and other health issues. As well, breathing in sewer gas can be particularly dangerous for individuals with compromised immune systems or respiratory conditions.
Sewer gas has a strong, unpleasant odor that can permeate the entire house, and exposure to sewer gas can lead to respiratory problems, headaches, and other health issues. These problems are easy to avoid by simply installing and flashing the plumbing vent pipes and penetrations through the roof system properly. A properly trained roofing contractor can install a proper flashing assembly around these penetrations.
The next picture below shows a properly installed plumbing vent termination, you can see the plumbing vent is installed over a foot above the adjacent roof membrane. Instead of having used a residential boot to terminate the membrane, instead a TPO concentric boot has been installed, as required, properly.
The next picture below shows a different but similar issue where a dryer or bathroom exhaust fan vent which has been installed through the roof. The example below shows a single exhaust vent termination, but it’s actually a group of them on this roof and at least one of them is used for a dryer vent but all of them are installed wrong in the same way.
They have a grill or screen mesh at the opening. This mesh will help to keep rodents out of the ductwork, but dryer vents are not allowed to have a mesh installed in front of them because they can clog up with lint and if a gas dryer is used, the vent can cause potentially significant problems.
If you look closely at this particular example, since the vent is intended for a pitched shingle roof, it does not extend properly above the surface of the rooftop and therefore the roof membrane has no area to properly terminate against the side of the ductwork. Here, where the roof membrane terminates at a horizontal edge, the uncovered galvanized metal, in close proximity to the wet area at the horizontal surface of the roof, has damage from oxidation.
The oxidation causes a different problem which leads to deterioration separation, essentially delamination between the modified bitumen membrane and the ductwork itself which then leads to leakage.
When you look very closely, as shown in the zoomed in picture below, you can see how water can get between these separating materials.
The last picture below shows a similar incorrect installation, but this time it is at an aluminum painted older roof. The problem is prolific here in Washington DC and the fix to this problem just requires properly trained contractors who care about doing things right.
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.
