Bath Exhaust Fan Vent Through Roof Part II

Last week, we took a look at a modified bitumen roof membrane where vent exhaust pipes were installed through the roof system. When mechanical or electrical or HVAC system elements pass through a roof system, we refer to those elements as penetrations through the roof system.  

Almost all flat roof systems here in DC have some or at least a few penetrations through the roof. In the simplest of flat roof residential row homes, in most cases, there will at least be one or two plumbing vents that go through the roof and at least one chimney. In houses which have been retrofitted, sometimes there will be multiple different types of systems and types of pipes or elements that penetrate through the roof system. Each of those penetrations needs to be treated in a special way. 

configuration of bath exhaust fan vent through roof

There are multiple different types of applications that can be applied to each type of penetration depending on the size and shape of the element that is going through the roof. In this particular case, we are showing round metal duct pipes, of three or four inch diameter sizes that go from the inside of the building, through the attic or crawl space below and through the roof membrane.

Each of these different types of elements may be treated differently and the most common examples, the plumbing drain waste vent pipes often referred to as DWV pipes will normally be treated with a field applied target patch or a roof boot. In the case of a modified bitumen roof system, we will generally field apply a roof boot around the bottom of the pipe in a concentric two-step application on top of the adjacent field membrane.  TPO, PVC, and multiple different types of thermoset membranes have a different application though and these applications generally are a single layer of materials.  However, even in those cases when there is a pre-manufactured target patch or pipe boot, for example, alternative or additional materials will often be applied as well for intended redundancy.

The picture below shows the near finished application of a two-step or two layer modified bitumen target patch.

application of bath exhaust fan vent through roof
In the picture below we are holding up the edge of the penetration patch. You can see a substrate or lower layer below.  This is an APP modified bitumen roof system and this particular type of system is covered with a manufacturer applied ceramic granule. The ceramic granules are applied to resist the deleterious effects of ultraviolet rays from the sun.

Underneath of the top layer, you can see that the granulated surface has been sprayed with an asphaltic primer. That asphaltic primer has a higher cohesion rate than the emulsion on the underside of the roof membrane. The roof membrane is heated with a flame, from a roofing torch and then in a molten state laid impressed into the substrate layer of the roof below.

asphaltic primer bath exhaust fan vent through roof

Without the primer application though the roof membrane will not adhere with the same level of adhesion and cohesion.  Primers of this type must be applied to different types of primed and or galvanized metal terminations or transition elements into substrate layers of membrane with dissimilar coatings.

bitumen membrances in bath exhaust fan vent through roof

In most types of modified bitumen membranes, at least torch down modified bitumen roofing, it’s important to see that a bit of the emulsion on the underside of the roof material has oozed out from the edge of the single-ply roof membrane.   Here in the picture below, you can see a roughly quarter inch thick bead of emulsion has oozed out from the underside of the roof membrane.

app modified in bath exhaust fan vent through roof

SBS modified bitumen is a traditional alternative to APP modified bitumen roof materials.  These two systems can be both manufactured in torch type application like the one shown in these photos here and below, but SBS modified bitumen roofing materials have a much higher amount of intended bleed-out when applied in successive layers.

quality control measure in bath exhaust fan vent through roof

When checking workmanship and even applying quality control measures on your own installation, it’s important to understand these distinctions because there are different allowable tolerances and ranges of acceptability for each type of material classification.

For years, modified bitumen roll roofing was considered one of the best and most modern alternatives to traditional Built-up roofs and traditional metal roofs.  Because of the prevalence in the local geographic area of Washington DC, our company will commonly work with repairs and retrofits in modified bitumen systems. This particular roof is an example of a retrofit because the interior modifications and changes to the layout required the relocation and the addition of mechanical systems which are now exhausting air out through the roof, but in most cases of full roof replacements our company will either recommend a modern thermoset membrane like TPO or possibly modified bitumen systems, depending on the specific configuration of the roof.

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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