Question: I have been having problems with the top of my stack getting covered in snow this winter. It is six inches tall and I was going to extend it this spring. I was going to add a 12-inch piece to the existing pipe. My concern is that the pipe will freeze with ice if it is too tall. Is there a cut-off height to solve my problem? —Thanks Doug A.
Answer: Roof plumbing vent terminations being covered with ice and snow are always a potential problem in our area due to our extreme winter weather conditions. Extending the height may prevent it from becoming capped with snow, but at the expense of potential freeze-up, which would be more detrimental. Insulating it better in the attic, and/or periodic manual snow clearings are a better approach.
I have received dozens of e-mails like yours over the years and answered many in previous columns, and I would like to wish everyone a Happy New Year for 2023 and thank all readers for their continued support and outstanding submissions. A straightforward solution like your pipe extension warrants a new approach. Gluing on an extension to the short ABS vent pipe above your roof may indeed be enough to prevent the top from being covered with blown snow. That will certainly depend on the location of it on your roof. If the vent termination is near a valley, is on the leeward side of the prevailing winds, or your home is surrounded by taller houses on both sides like mine, it may still not be sufficient. If the vent is well situated, with no factors to increase the snow drifting over top, then adding another six inches or so should solve your problem. The unfortunate part about that solution is that its success may also cause a further issue which can be more problematic.
Most stacks that are too low to the roof, or poorly located as discussed above, may have the tops covered with a few centimetres of snow after heavy storms. That could potentially cause the vents to malfunction, but that is in rare cases. Even with a small amount of snow on top, the heat from the escaping sewer gasses inside the pipe should quickly melt it enough to prevent a full blockage. Even if there is a small hole in the snow capping the pipe, enough air should enter the vent for proper drainage, while also allowing escape of noxious sewer gasses from the plumbing system. If it is completely sealed, there is a risk of the melting snow freezing up on very cold days, but that is often a short-term issue.
The simple solution to your issue, as long as you don’t have the pipe completely blocked above the roofline on multiple occasions, is twofold. First, adding insulation around the stack inside the attic may help. This should prevent that portion of the vent pipe from dropping below freezing, even in the coldest weather. The extra insulation should be continuous around the entire pipe, which may help keep the escaping gasses and water vapour from freezing up and blocking the vent below the roof. If the temperature of those sewer gases is raised a few degrees in that location, it may be enough to completely melt any snow that covers the pipe top.
Secondly, and perhaps the most simplistic approach, is to climb up on your roof immediately after a heavy snowfall and clear the area with a plastic shovel. That is only an option if your vent termination is on an easily accessible portion of the roof, with a moderate pitch and low height. If you have a two-storey, or higher, home that may be out of the question. Fortunately, higher homes like those have more of a tendency to have clearer roofs than bungalows, as much of the snow blows off. Also, if you have a very steep roof, regardless of the height, it may too dangerous to attempt snow removal from above. A long snow rake designed for roofs may be another viable option in that situation.
The real risk with adding more piping above the roof is exactly what you are concerned about. The higher the vent is above the warmer attic, the colder it will be at the very top. The attic not only allows insulation of the ABS piping, it prevents cold winter winds from hitting the exterior surface. Those two factors together, more heat loss the higher it extends above the attic and colder surface temperatures, can be a double-whammy in creating a bigger problem. Any snow that does blow over the higher pipe may more easily plug it. Also, the melting snow from the warm gas emissions is more likely to freeze, especially at night when the sun is down and ambient temperatures drop. Ice buildup inside the pipe is the truly risky situation, as it may not easily melt, causing a substantial or complete blockage of the vent.
There are also several types of devices available to prevent ice blockage or help warm the vent, but their reliability is questionable. The only one that may be successful is wrapping the pipe with an electric heating cable designed for that specific purpose, but that can be costly to run, a waste of electricity, and difficult to monitor.
Your efforts to prevent your plumbing stack termination above the roof from being snow-covered may be better served by improving its insulation inside the attic and more frequent snow removal, rather than extending the length. The risk of your suggested method is trading one issue for another, with the newer one being much more problematic.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)(cahpi.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.