Bath Exhaust Fan Vent Through Roof

One of the most common retrofit type procedures needed on low slope flat roofs in Washington DC historic neighborhoods like Capitol Hill are installation of dryer and bathroom exhaust ducts. These ducts have different building code requirements but from a roofing perspective they’re installed in a very similar way.  Often even after a relatively new roof has been installed after a retrofit happens on the inside of the building, this new duct work will have to be installed to the outside of the building. Often as well in home inspections it will come up and be recognized that there is an exhaust fan or a clothing dryer on the inside of the building but no duct to the outside of the building. In those type of cases, it comes to the new homeowner or even a selling homeowner’s attention for the first time that this exhaust system should have been there from the beginning but was never properly installed. In those cases clients will call us and ask us to install these type of systems for the first time. 

bath exhaust fan

In those cases, it helps to understand a little bit about what’s required to install and small air exhaust duct from the inside to the outside of the building.  In most cases, with low slope or flat roof buildings in Washington DC there’s a relatively thin attic space, often referred to as a crawl space, above the top floor of the building in an interstitial space between the roof and the top floor ceiling. 

A core sample on a roof can be cut with a variety of different tools, some of the most common ways of doing a core sample are by using a razor knife with a hook blade to start and a longer blade to finish or even using a keyhole saw.  Core samples on flat roofs can also be taken with a oscillating multi tool or in this case we’ve used a hole saw the hole saw is particularly apropos for this particular type of installation because the dot which comes through the roof will fit the hole saw Dimension very tightly. most hole saws are sold in inch and half inch increments, over about the dimension of about 2.5 inches.  In size is smaller than 2.5 in, Wholesales are generally sold in smaller increments. particularly a 2 and 1/8 in hole saw blade is very common because most modern Standard door slabs are precut  to fit a 2 -1/8 inch cylindrical bore lock set.  For this reason, that particular Dimension is very common and popular in wholesale sizes because it’s so often used by Carpenters on interior construction work like door installation and or door modification. in this particular case though with a 4 inch dryer exhaust, the minimum applicable size allowed by building code.  In our DC area, the 4-in hole saw is perfect for opening up the roof deck. most roof decks in Washington DC are built with 1x decking material, approximately a real dimension of ¾” thickness wood.

You can often identify the location of the roofed rafters, below the roof deck, by recognizing the nailing pattern on the boards above. if the roof decking is built at the same age of the house in historic times, the wood decking is often built with 1×6 boards.

The picture below shows a close-up view of the hole saw cut out through the first layer of cellulose recovery board. recovery board is like an insulation board but has a dual purpose of both providing a small degree of insulation with helps in stabilizing the interior temperature of the house but also prevents condensation. the second main purpose of recovery board is that it provides a more consistent substrate for torching and or adhesion application of the roof membrane.

fixing of bath exhaust fan vent through roof

Once the holes have been properly cut and cord through the rooftop and the rooftop deck boards, a pipe sleeve can be installed through the roof system and through the framing system into the attic space. In most cases, the only way to really make the connection properly on the inside of the attic is to remove the inside fan, in the case of a bathroom exhaust.   In the case of a dryer exhaust, in most cases it’s either better to install a continuous pipe from the inside Space through the attic and to the space above the rooftop or to fit and crawl through the attic space to the point of the dryer exhaust and make the connection and or secure the duct from within the attic space. not all parts of a typical roof attic can be accessed by a full-sized adult.  In most cases, at the farthest rear parts of the attic space, the space between the rafters and the ceiling joists below is so thin that a full grown person cannot fit. in those cases, the ceiling may have to be cut or all of the connections have to be made from below or above and then slid through without disconnecting the components. 

In the picture below, you can see the exhaust ducts have been installed through the attic space and into the space above the rooftop. in this case, it’s better to install the ducts relatively high because it’s important that they are above the minimum snow load and requirement height by the building code.

fixing of bath exhaust fan

In both the picture above and the picture below, you can see a third pipe farthest to the left as shown in the picture below. this pipe penetration is the original cast iron Plumbing stack, also referred to as a plumbing drain, waste, and vent pipe.

bath exhaust fan vent

This particular roof membrane system appears to be almost white or silver in color but it’s actually a typical modified bitumen smooth system with a white or light color paint on top of the roofing membrane. It’s important to keep a modified bitumen smooth rooftop surface coated with a coating that’s maintaining good condition. The coating itself is one of the few defenses the membrane has to prevent accelerated deterioration from exposure to ultraviolet rays from the Sun.

bath exhaust fan vent through roof

In this coming weeks blog article will take another look at this roof and show more details of how the target patch flashing is applied to a modified bitumen membrane.

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, and you can call us at (202) 840-8698 and email us at

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