Roof leaks are associated with problems in the roof covering, but we have various vents disrupting this surface. Plumbing vents are one of those penetrations. Older lead flashing on plumbing vents are giving way to the new plastic vents, which may not be so great.
I do home inspections during the rain. I joke that it is the best time to find a leak, which does happen, but going up to the roof in the rain is not the safest proposition. I do not advise people to go on a roof during the rain, yet I have done so during inspections. The one benefit is that you begin to see what you suspected. Deflections in the roof sheathing do funnel water like a small creek, causing more damage to shingles in the “creek bed” than the surrounding surfaces. You can discover that your assumption about where sealing/caulking needs to be done was correct as you see the water hold on to vents or slight nail depressions. I also had an assumption about a particular type of plumbing vent confirmed.
A builder thought that I was being too detailed when I made a comment about a plastic flashing on a plumbing vent. “Home inspectors have to find things to justify their cost, so they point out things that are not really problems.” (How many home inspectors have heard that line?) In this case, I had pointed out that the installation of a plumbing vent had been done incorrectly. The fix was not going to be too hard to accomplish, but this builder wanted to create the idea that I make a big deal out of nothing. I am sure that sellers looking at my reports could feel the same way as well. However, I know that small problems left untreated can lead to a big mess. When I do roof inspections, I do have the tendency to be wary of plastic flashing on plumbing vents, because I do find more leaks with them than I do with lead flashing on plumbing vents. Lead flashing is not perfect, but I encounter fewer problems with them. These newer style vents can be effected by bad installation or movement in the house.
The main problem with the plastic flashing is that it does not last as long as lead flashing. The plastic relies on a tight seal on the vent pipe to stop leaks. This seal can break through exposure to the elements outside faster than the roof covering. The seal can also be damaged through excessive movement in the home. Another problem is that the bell part of the plastic can bend down to form a cup around the vent. This happens with a bad installation job, but I have seen this with movement or objects hitting the flashing. In the above photograph, I was on the roof during a rain. You can see a little pool of water being held in the flashing. This can begin to leak through the roof surface, down the vent pipe. Logically, I knew that this would happen, although I have had others tell me that I am making this up.
How can I repair these plastic flashing pieces on a plumbing vent? I have seen the broken seals caulked, and this has worked. I have seen people use foam insulation on the interior, but this is a temporary fix. The best repair may be placing a new plastic cover over the older one, gluing the cover on with caulk. As for this cup effect seen in the photograph, I would suggest going into your attic. From below you can push the flashing back up into place. Then you can check for traces of light, which will show you there is a gap (the pvc vent pipes will glow with the sun light). Gaps can be caulked on the roof. Pulling this flashing bowl up form above may lead to more problems. You may not see the gaps, so that is why repairing from the attic would be better.
I am sticking with lead flashing, but I know the plastic flashing is here to stay. I am sure that this type of flashing will improve over time. I do feel that this something homeowners or home buyers should check. This can happen to these vents at any time, and a small leak left unrepaired could lead to more damage than you may expect.