drain-waste-ventplumbing
I'm doing roofing, and looking at a roof vent pipe (e.g. a plumbing drain waste plumbing penetration). It is 1.5" galvanized pipe original to the construction. I see it emerge from the top plates and up to the roof.
I can match up all the other roof vent pipes with their fixtures below, but not this one. I think it might be abandoned.
How can I determine reliably where it goes, and if it's still part of the home's plumbing system? All I can think of is to smell for sewer gases.
If you have access to the underside of the roof, removing the pipe and reconstructing the roofing is not difficult. Such a repaired roof is much less likely to leak than with the pipe sticking through over the years.
By the number of steps, this might seem daunting, but each is very simple and an intuitive step in a lasting, trouble-free repair.
The basics are removing the bad stuff, repairing the hole, and renewing the roofing to make it weather tight.
After removing the pipe and flashing, clean up the hole through the roof decking, possibly called cladding or sheathing, (assuming the roof is not wooden shake or shingles) by cutting through with a hand saw, jig saw, or judicious use of a circular saw and make the hole rectangular (if not already).
Working from underneath, cut a piece of plywood of the same thickness as the decking to fit inside the hole to make a filler piece for the decking/sheathing/cladding.
Create support for the filler piece by cutting some more plywood, or a 1x4 (maybe two) to fit snugly between the roof joists and fasten into place by screwing/nailing through the joists into the plywood or board supports.
Topside, fasten the decking filler to the supporting layer: construction glue, short nails, or screws. It just needs to be strong enough to shrug off walking on the roof.
Peel back and remove a row or two of roofing to access the tar paper and make it so a nice new piece of roofing fits cleanly. At the up-slope edge of the row above, try to loosen and remove the roofing nails holding the shingles with a wide flat-blade screwdriver or thin pry bar instead of simply ripping the roofing material. If there is more than one layer of shingles (up to three layers are allowed), forget going for the tar paper: instead, make some "roofing filler" with scrap shingles and fill in the hole in the roofing to make a consistent support surface for the "finish layer" of roofing.
Obtain new tar paper and roof shingles. Probably you will have to buy roofing in units of 33 square feet (1/3 of a "square") and tar paper in obscenely long rolls. It might pay to go to a local hardware store to see if they will sell it by the foot. Maybe you want to use it up a whole package of roofing by replacing more of the roofing. If so, remove roofing up to the first ridge or peak.
If the old tar paper is exposed, Add new tar paper by tucking it under shingles above and at sides and lightly tacking it down with a few short staples. Cut off the excess, but keep 6 inches over the lower row of shingles (down to just above the tar strip).
Working from the bottom, follow the existing pattern of shingle placement and overlap. Usually each row is 1/3 to 1/2 offset of the width of a shingle to stagger the side joints (to protect it by the next row above). Place nails 1/2 inch upslope from the tar strip (black goo stuff near the middle of the shingle). A maximum of three nails per shingle is all that is needed. The next row will add up to three more nail fastenings which is plenty of holding power. The black goo will melt on the next hot day to secure the shingles together.
At the top where the new roofing joins the existing, slide the new roof shingles under the existing row and gain access to the nailing strip by gently lifting and curling the lower edge of the row above it. Nail carefully.
If there is any doubt about an exposed seam, use a suitable roof patch caulk. If doing this during cold weather and it will not be warm before the next wind storm, it might be worth judicious use of a torch (directed flame) to heat the lower edge of the shingles to get them to stick together a little. Given time, they will adhere on their own adequately.
I used a quick and very cheap solution, taking a standard plastic elbow and jamming it on to form a vent cap:
This will not affect vent operation, but will keep a large fraction of the rainwater out of the pipe, potentially extending the pipe lifetime by a few decades. This also has a presumably minor benefit of keeping rainwater out of the sewer.
Best Answer
One thing you can do regarding the question if it an active part of your plumbing is to turn on your water sources one by one. While water is running down the respective drain put your ear to this unknown vent pipe top and listen for the sound of running water.
One way to make this process efficient is to have two people talking on cell phones. One person is on the roof to listen at the pipe. The other person operates each sink, toilet or washer in turn while communicating with the person on the roof.
Of course use common sense safety strategy climbing to the roof, especially if it is particularly steep of high from ground level.