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