Plumbing vent caps are installed during construction to pressure test the drain-waste-vent plumbing system before it’s filled with water and used regularly. Once the test is passed (e.g., the pipes hold pressure and don’t leak), these caps are supposed to be removed to ensure proper drainage within the home. This vent cap was discovered at a 1999 home on Orcas Island in the San Juan Islands. Surprisingly, I call this out all the time. If the vent has been abandoned, the cap is appropriate; but this usually isn’t the case. Usually, the plumbing contractor forgets to get back on the roof to remove caps. So, if you see caps, I’d remove them if the pipes aren’t abandoned.