Plumbing Vent or Stack Flashings

Home Inspector with Aldrich's Home Inspections, Inc.

One of the most common areas for a roof to leak is around penetrations. Therefore, it is very important that the flashing around those areas be installed correctly and maintained properly. Plumbing vents or stacks are one such type of roofing penetration subject to leaking. Below is some useful information on how to correctly install and maintain plumbing vent or stack flashings. 

If your roof has a pitch or slope greater than 2/12-that is, 2 inches of rise or height gain for every 12 inches of run or horizontal distance-then there is a pretty good chance your roof is a water shedding roof not a waterproof roof. 

Supplied by With all water shedding roofs, the roofing and associated flashing materials must be installed like the feathers on a duck so the rain water will run off the roof instead of into the structure. This is the reason why most roofers start installing the roofing materials at the gutter edges and then work their way up to the peaks. Think of the gutter edges as the tail of the duck and the peaks as the head. If/when a portion of the roofing system is not installed properly, such as missing, incomplete, or improper flashings, there is the potential for a leak.

Nine times out of ten, the leaks or failure points of sloped roofs occur in one of two areas:

  1. Around the penetrations through the roof, or
  2. In the valleys 

For the purpose of this blog entry, I am going to concentrate on plumbing vent or stack flashings. However, many of the concepts and common problems found can be true for other types of penetrations as well. 

Materials Used
The material used for the plumbing vent or stack material will greatly depend upon the age of the structure and whether or not the plumbing system has been redone at some point over the years. The most common material used in today's construction is Acrylonitrile Butadiene Styrene or ABS. Other stack materials could include cast iron, steel, aluminum, or copper. 

The flashing material used depends upon the type of stack and roofing materials. Some common flashing materials include steel, rubber, lead, copper, aluminum or a combination thereof. 

The most common stack flashing material I see in my area-that is, the North Olympic Peninsula, which includes Port Angeles, Sequim, Port Townsend, and their surrounding areas-is rubber or neoprene, which is most often used in conjunction with ABS stacks and asphalt shingled roofs.

Below is the basic installation process for a plumbing vent or stack flashing on an asphalt shingled roof followed by a diagram that helps illustrate what a proper installation should look like.

  1. Starting from the gutter edge, the roof shingles are installed up to the location of the plumbing vent or stack.
  2. A roof shingle is cut and slid over the stack.
  3. The flashing flange is placed over the stack and sealed or nailed in place.
  4. The installation of the roof shingles then continues so that no more than the lower half of the flange is exposed. 

Neoprene plumbing stack flashing

















Common Problems
Below are some of the common problems that are found with these types of flashings. Each one of the following conditions can lead to possible leakage. 

Rusted Flashings - Rusted flashings can be caused by a number of things:

  • Age
  • Failure to maintain the protective coating on the flashing
  • Incompatible materials; tar or mastic over the flashings

Damaged Flashings - Damaged flashings might be caused any one or a combination of the following:

  • Snow or ice accumulation on the roof
  • Animal activity
  • Foot traffic
  • Careless roof work nearby
  • Replacement of a pipe or stack without replacing the flashing
  • Deterioration of the flashing due to age.

Vertically Misaligned - Flashings that have been displaced vertically-that is, up or down-are usually the result of either the stack or the roof deck moving. If the roof deck were to drop relative to the stack, say from a snow load, then the flange might be pulled up off the roof. If the opposite were to happen and the stack were to drop relative to the roof deck, then the flange might be deformed causing a recessed area around the stack. The below diagram may better explain these conditions. 

Plumbing stack flashing - stretched or buckled

















Installation Problems - Most often I see installation issues, some of which are listed below.

  • The flashing was never installed.
  • The wrong flashing material was installed.
  • The top half of the flashing flange is exposed above the roofing material.
  • The bottom edge of the flashing flange is covered by the roofing material.
  • The flashing is located in a valley.
  • There are exposed fasteners that are not sealed or that have failed sealant.

If any of the above conditions are found, I recommend having a qualified individual, such as a licensed roofer, further evaluate the condition and repair as needed.


Re-Blogged 1 time:

Re-Blogged By Re-Blogged At
  1. Wallace S. Gibson, CPM 01/12/2010 09:37 PM
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Bridget Cella
Re/Max Connection - Sewell, NJ
e-Pro, Realtor

Nice visuals.  We have had a lot of rain here in New Jersey this winter season - lots of flooding and leaking roofs I am sure.

Have a great year.

Jan 12, 2010 01:03 PM #1
Morgan Evans
Douglas Elliman Real Estate - Manhattan, NY

I'm quite impressed with the level of detail and thoroughness of this explanation of water penetration.  This is definitely an area where I would leave to the experts when putting up a new roof or getting a home inspected.

Jan 12, 2010 01:05 PM #2
Sherry Siegel, Managing Broker, EcoBroker, ABR
BrokersGroup, serving Sequim and Port Angeles - Sequim, WA

Something of the sort happened on the roof where we live, allowing rain to find its way down the inside stovepipe. Water had puddled a bit by the time I got home. We were having one of those ferocious windy storms, too. The roof had really reached its shelf life. Your explanation and diagrams give me a fairly good idea of what I would have seen had I climbed up to look.

And Jason, really appreciated the visuals you used at the 1st Time HomeBuyers class this past weekend. I don't think anyone in there would forgo a home inspection after looking at those photos.

Jan 12, 2010 03:03 PM #3
Steven L. Smith
King of the House Home Inspection, Inc. - Bellingham, WA
Bellingham WA Home Inspector


That is a nice post. Well written. I got a kick out of comparing the roof to the duck. Just a ducky idea I think.

Jan 12, 2010 03:17 PM #4
Charles Buell
Charles Buell Inspections Inc. - Seattle, WA
Seattle Home Inspector

Good post Jason, I would add that that these neoprine/rubber type flashings work fine on roofs that have about the same life expectancy 15-20 years---but I hate it when I find them on roofs rated for 50 years or more:)

Jan 12, 2010 03:34 PM #5
Darren Miller
About The House - Succasunna, NJ

Very nice Jason-

The builder asked why I called this out on a brand new home...

Because it's not right!


Jan 18, 2010 09:26 PM #6
Gene Allen
Fathom Realty - Cary, NC
Realty Consultant for Cary Real Estate

Here is a nonsensical question.  Why do they put 10 year stack flashing on a 20 year roof?

Jan 23, 2010 12:36 PM #7
Jason Aldrich
Aldrich's Home Inspections, Inc. - Sequim, WA
Sequim, WA Home Inspector

Thanks, Darren. It is amazing how some folks think caulk or mastic are the next best things since duct tape.

Gene - I can think of two reasons--"they" are either cheap or don't know any better.

Jan 25, 2010 11:12 AM #8
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Jason Aldrich

Sequim, WA Home Inspector
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