Understanding Goosenecks at Rooftop Pipe Penetrations
Here on our website and in our blog, we have looked at the whole spectrum of different types of flat roof penetrations. Today, we’re going to look at how goosenecks can be used for rooftop exhausts to prevent type penetrations to allow the passage of air with generally lower risk of water infiltration. A list of the majority if the types of typical rooftop penetrations follows:
- Vent pipes may include plumbing vents, roof vents for attics or crawl spaces, and vents for kitchen or bathroom exhaust fans.
- Chimneys and flues are most often used for fireplaces, furnaces, or other heating appliances.
- Skylights and roof hatches provide natural light or access to the rooftop.
- HVAC equipment can include air conditioners, heat pumps, and other heating and cooling units.
- Electrical conduits and wiring are used to carry power to lights, outlets, and other electrical devices.
- Drainage systems include roof drains, scuppers, and gutters that passively shed water from the roof surface. Gutters do not always necessarily penetrate through the roof system, but here in Washington dc, especially with historic styles of architecture, there are several types of built-in gutters and or elements that actually integrate through the typical plane of the roof membrane.
- Solar panels and other renewable energy systems generate electricity or heat from the sun.
- Signage and other rooftop gadgety gizmos (non-technical name) can include signs, antennas, satellite dishes, and other equipment that is mounted on the roof.
A typical gooseneck vent is shown in the picture below. We specifically mentioned that goosenecks can be used for venting elements of exhaust air or plumbing gases, but they can also be used to allow loose elements to pass into the building from the outside. Some of these loose smaller elements are collections of individual wires, linsets thermostat or control wiring, and refrigerant lines. Often, these type of smaller elements pass through collectively into a pitch pocket, but goosenecks provide an alternative.
There are several technical comparative advantages to goosenecks. In some cases though, pitch pockets are simply better from a functional perspective. Unlike gooseneck, a pitch pocket inherently creates a seal around individual smaller elements. Often pitch pockets are used to seal around elements that don’t fit neatly into typical pipe penetrations where flashing can be applied directly to the element passing through the roof membrane.
Even though pitch pockets inherently seal to the penetrating elements, goosenecks can also be sealed at the inside of the pipe of the gooseneck. In cases like this, a gooseneck may be not really being used as a pipe, but instead as a sleeve for other elements to pass through. Often, materials like Schedule 40 PVC DWV piping can be used as a sleeve.
The particular gooseneck type installation shown in the picture below is a iron screw pipe, traditionally used often for gas supply piping. Here though, the pipe sharing may be used as a gas vent, not an exhaust necessarily, but as a passive ventilation required for interior mechanical equipment.
It may be counterintuitive, but often it’s easier to flash a modified bitumen type membrane to a larger pipe than a smaller pipe. In this case, the outside diameter of this pipe is only 1.5 in. Typical vent or exhaust piping is generally three or four inches or larger. In those cases, it’s actually easier to wrap a modify bitumen concentric target patch type overlay to a larger round penetration then to a smaller pipe.
When installing a modified bitumen membrane flashing around a pipe, to terminate a roof membrane it needs conform to the curved surface. With a larger diameter pipe (like 3-4 inches), the curve is more gradual, which means the membrane doesn’t have to bend as sharply to follow the radius of the pipe. This gentler, more graduated curve makes it easier for the material to lay flat or at least tightly and adhere to the pipe substrate without creating wrinkles or voids.
In contrast, a smaller 1.5-inch pipe creates a much tighter curve to which the membrane must bend more sharply to conform to the tighter radius, which can cause the flashing to wrinkle, buckle, or create small gaps where lamination is difficult where water will enter.
This is particularly an issue with modified bitumen membranes since they are relatively thick and somewhat rigid material. As well, many different types of single-ply membranes are also relatively thick, compared to the radius they must conform with in smaller pipe penetrations.
Although, above, we’ve been looking at pictures and discussing radius type pipe penetrations, the picture below shows an alternative square duct.
Although we’ve been looking at several different examples of modified bitumen type membranes with different types of penetrations, the next picture below instead shows a thermoset roof membrane with a gooseneck penetration. This particular gooseneck happens to be installed between a rooftop mechanical equipment dunnage system.
Many different types of piping, without a gooseneck to prevent precipitation entering in regular rainfall and weather events, internal elements of the building can actually be damaged. The next picture below shows a typical plumbing drain, waste, vent pipe, often referred to as DWV piping. Although this particular vent is open in the upper horizontal position, it does not cause any problems with precipitation because precipitation will just run down into pipe and then continue from there into the public drain system, or a private septic system, if built properly.
The next two pictures show a very similar type of installation, but interestingly or oddly, the roofer installed these elements through a pitch pocket instead of a typical situation where a concentric target patch or field built custom flashing system is installed.
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.
