Renovation & Design

Vent blockage a common cold-climate problem

QUESTION: I enjoy reading your column and hope you can make a suggestion that will eliminate a problem I am having. I have two roof vent pipes installed on the north side of my home and they face the prevailing northeast winter winds. One pipe is closer to the edge of the home while the second is close to the peak. I increased the height last year and while it helped -- it only froze once last winter -- the pipes still froze solid during the coldest part of the winter. I increased the height as snow was piling up over the pipes.

I have to climb up on the roof, which is starting to get dangerous as I am 70 years old, and take a screwdriver and hammer to break open the ice and then pour hot water down the pipe. No one else seems to have this problem, or at least I have never seen anyone up on the roof during the winter. When the pipes freeze you get a sewer smell. I paid for a service call from a plumber in Steinbach and the fellow did not know what was causing it, but I still had to pay him for nothing.

Hope you can help,



ANSWER: Freezing or blockage of plumbing vent terminations above roofs is a fairly common problem in our area. This can be due to a number of factors, the most significant being our extremely cold winter weather. I will explain the reason for the location of these pipes, causes of your problems, and a couple of possible solutions.

The purpose of the rather strange practice of installing an open pipe just above the roof line in our homes is twofold. The first is to provide adequate air into the top of the plumbing drain system in our homes to ensure they don't block up due to air being trapped inside the pipes. While this is a fundamentally critical function, it may not be the most important.

The second reason for venting to the exterior of the house is to prevent sewer gas from leaking into our living space via the plumbing drains. Terminating these pipes above the roof ensures any vapours that potentially come up through the drains from the sewer will harmlessly be released outside rather than penetrate our indoor air.

To provide both of these functions, terminating the drain vent high up, above the roof, is necessary. Unfortunately, this location may be prone to blockage from snow, ice and debris such as leaves and roof litter.

The reason the top of these pipes normally will remain open and free of ice is due to the heat of the rising gasses from the drains. When hot water is run in any of the plumbing fixtures in the home, warm air will rise up through the vent pipes and normally melt any thin layer of snow or ice on top or prevent it from forming altogether.

The first key to preventing blockage with ice and snow is stopping partial blockage with leaves or debris. If the entire area at the top of the stack remains free and open, there is less chance of ice formation.

To prevent this hazard, inspection and cleaning the top of the vents in the late fall, after all the deciduous trees have lost their leaves, is fundamental.

Even with regular maintenance, icing over of low roof vents can occur. One possible solution is to do what you have; extend the vents higher up to prevent coverage with snow. While this may help prevent snow blockage, it may actually hinder the true solution to the issue.

The real cause of ice formation at the top of your vents is the loss of heat inside the pipe as it rises through your cold attic. If the pipe does not remain warm enough, the large amount of moisture in the air rising up toward the roof can condense and then freeze before it exits to the exterior. If you have older cast iron stacks, this is even more likely due to good conduction properties of the metal. An iron pipe will lose heat more rapidly than other ones, but even a more modern ABS plastic pipe can be subject to freezing.

The real key to preventing excessive heat loss and freezing of the wet air inside your stack is better insulation. Insulating the exterior of these pipes, mainly as they progress through your frigid attic space, may prevent this excess heat loss. While there may currently be a thin layer of fibreglass or other pipe wrap material around these stacks, it may provide only minimal help. Covering the stacks with better-quality or thicker layers of insulation may be the only thing required to prevent freezing.

Adding more insulation around the bases, where the vents enter the attic, can also prevent quick cooling. If you succeed in this regard, the ice and blockage problems may disappear.

In some homes, even better insulation inside the attic around the vents does not prevent blockage during the coldest weather. If your home is one of these exceptional units, due to location or other environmental abnormalities, further measures may be required. Installing a specialized heat trace cable outside of the pipe, embedded in the insulation surrounding it inside the attic, may be the final remedy. I would only recommend this as a last resort, as it may be tricky to install and somewhat costly to run.

While you have taken some measures to prevent snow blockage of your vents by extending them upward, that may not be the best solution since it's not effective in the coldest weather. Preventing excessive heat loss inside the attic with better insulation around these stacks should be the proper solution to your problem.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors -- Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at



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