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Roof plumbing vent flashing (C) Daniel FriedmanRooftop Plumbing Vent Installation

Best-practices details avoid plumbing vent leaks, frost, condensation

This article describes proper installation details for rooftop plumbing vent flashing or boot flashings.

Page top photo: an aluminum and rubber plumbing vent flashing boot installed on an asphalt shingle rooftop.

This installation looks pretty good, thought that very narrow remnant of a shingle tab to the left of the vent is a bit fragile and may break off.

You can see that in a proper vent flashing installation the top and sides of the flashing are underneath the shingles and the bottom end of the pumbing vent flashing boot is brought out to the top of the shingle course below so that water will drain out on top of the lower roof surface.

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Rooftop Plumbing Vent Installation

Temporary roof vent flashing while waiting for metal roof installation (C) Daniel Friedman Eric GalowHere we provide procedures for proper plumbing vent flashing installation and we describe common mistakes that can cause roof leaks, condensation in the attic, heat loss, mold, sewer gas odors, even mold contamination.

Installation of plumbing vent stack flashing on a metal roof

Below are details from the original plumbing vent installation of a 3-inch ABS plumbing vent through a standing seam metal roof.

We had installed a conventional plumbing vent stack and boot (below left) along with a roof membrane, while waiting for the metal roof to arrive at the jobsite.

To properly seal the plumbing vent to the metal roof we replaced the shingle-roof type vent flashing with the white rubber flashing shown at below right.

That flashing is sealed to the metal roof surface by both adhesive sealant and a ring of metal screws.

Below I'm showing the original installation of the plumbing vent boot on this standing seam metal roof as it was being installed by Eric Galow.

Permanent roof vent flashing for metal roof installation (C) Daniel Friedman Eric Galow

Below you'll see that even with the plumbing vent stack broken and bent over nearly touching the roof surface, the rubber boot installed in the photos above managed to avoid a flood of melting snow into the roof cavity.

This broken plumbing vent was caused by the property-owner's reluctance to finish the job of installing snow guards on the roof.

Rubber boot keeps most water out of the roof cavity even with a broken roof vent stack pipe (C) Daniel Friedman

Details about the snow guard system and its proper installation (done after this damage occurred) are

at SNOW GUARDS on METAL ROOFS

and step by step details of how we repaired this snapped-off plumbing vent without having to cut open the roof from above nor the cathedral ceiling from below are

at PLUMBING VENT REPAIR

Rooftop Plumbing Vent Installation Details vs: leaks, condensation, frost

Question: cause & effects of condensation or frost on plumbing vent in attic

Condensatin & frost on plumbing vent in attic (C) InspectApedia.com Kahn

I noted frost on a plumbing vent pipe. The roof flashing appeared fine. [Photo below]

[Click to enlarge any image]

Plumbing vent boot flashing (C) InspectApedia.com Kahn

Was wondering is just from condensation freezing over or is there a larger issue. - Dov Ber Kahn, by private email, 2020/12/16

Moderator reply: details of plumbing vent boot flashing installation vs leaks, condensate, frost in the attic

It's no surprise to see some frost on the outside of the upper portion of a plumbing vent in an attic in cold weather.

Frost and condensate on plumbing vent in attic (C) InspectApedia.com Kahn

Consider that the copper pipe sticking out that point is put at a temperature below freezing. Moisture in the attic air will then condense on the pipe.

I have a couple of other observations from your photos.

The attic has certainly had a moisture problem although I can't say what the source of moisture is.

Look in the rafter bay to the right of the plumbing vent and you'll my white arrow annotations pointing to lots of dark moisture stained roof decking. [First photo above]

There's also a little bit of moisture staining on the rafter right next to the plumbing vent that says there could have been leaks at that plumbing vent in the past although they don't look like major ones. I might have pull up the insulation to look at the attic side of the ceiling drywall to be sure it wasn't moldy.

Plumbing vent potential leak points: rubber seal aged and too-high exposure along flashing sides (C) InspectApedia.com KahnIt may not be significant but a closer look at the plumbing vent boot flashing (above) shows that the rubber seal has slipped down from its original position, possibly due to snow loading or due to simple aging of the rubber.

Sometimes when there is wet snow on the roof, if the rubber seal has aged, dried, hardened, we may see a bit of leakage around the vent pipe in this location.

That would be in addition to the condensation and frost we've already noted, but it could also be related: if the plumbing vent does not seal around the vent pipe then air leaks at that location increase the movement of moist warm attic air up and out around the vent flashing, increasing the rate of condensation and frost formation on the vent pipe.

Do not face-nail plumbing vent flashing

Also notice in my enlargement of your photo those two nails at the bottom of the vent boot flashing are not a recommended practice as they puncture the metal flashing to form possible leak points.

In your photo we see two face nails, sealed atop the flashing. We can't see if sealant was used below the flashing at those nail punctures but in the attic we don't notice leak stains at that corresponding location. That's probably not a leak point.

Typical plumbing vent flashing installation guides specifically warn:

Do not face nail the plumbing vent flashing.

That sort of leak can happen in particular in a climate such as yours (North New Jersey) where there may be wet snow or even ice dams on roofs in winter.

It's not a major crime and I wouldn't be tearing up the roof to fix it, especially this time of year. But if we see recurrent or worse leaks at the plumbing vent flashing we might decide to seal that flashing more-carefully against side leakage.

Take a look again at the roof deck inside, along the opening cut into the roof to see if the stains suggest that there has been water entering along the vent boot flashing sides.

Shingle tabs should extend below the plumbing vent cut-out

Plumbing vent boot flashing installed too low on roof with respect to shingle courses (C) InspectApedia.com

On your photo of the roof exterior I have indicated in yellow [photo above] that the actual plumbing vent boot flashing is a bit more-exposed along the sides than is preferred to avoid leaks. The top shingle's bottom edge is too high at this plumbing boot installation.

Best practice would be for the vent flashing sides to be below shingles to a point below the lower edge of the vent pipe itself - the "Preferred" line in my photo annotations. .

Below is in my OPINION a better plumbing vent boot flashing installation example, adapted from GAF's PROBlog plumbing vent installation advice.

Plumbing vent flashing installation details from GAF's ProBlog example - cited & discussed at InspectApedia.com

Quoting GAF:

The top shingle’s bottom edge should extend, at the least, to the bottom of the hole cut in the roof deck. Any less may lead to leaking. (Barth 2016)

Notice as well that there are no exposed nails on this plumbing vent boot flashing.

Rooftop Plumbing Vent Flashing Guides & Codes

Photos courtesy of

We discuss how to diagnose and cure drain sounds (gurgling, bubbling, smells, poor drainage)

at PLUMBING DRAIN NOISE DIAGNOSIS (gurgling drains means bad vent piping or clogged drains)

and at CLOGGED DRAIN DIAGNOSIS & REPAIR.

We discuss drainage for plumbing fixtures, such as loose toilets, as sources of sewer gas odors separately

at PLUMBING FIXTURE TRAPS.

Also see FLASHING on BUILDINGS


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