Renovation & Design

Prevent winter roof expeditions in the future

QUESTION: I have lived in a 1950s bungalow since 2001. Over the last couple of winters there have been problems with my plumbing stack filling up with snow and freezing up. The roof is a hip roof and the stack is located on the east side.

This wasn't an issue the first five years in the house, but the last two winters I have had to climb up and clear the stack just about every time it snows, as the toilet barely flushes.

I put in a low-flow toilet on the main level sometime during this time frame, but I'm not sure if the timing directly coincides with the freezing problem. Any insight would be appreciated.

Gord Koshelanyk

ANSWER: With the recent winter weather, resulting in frequent small snowfalls, this issue may become a concern not only for you but other homeowners as well.

A slight amount of snow covering a plumbing-stack opening on a roof may not cause much concern, but it appears you have a more serious issue with it blocking proper venting.

There may be a couple of approaches to resolving this, so that you don't have to risk climbing on your roof in nasty weather just to ensure proper plumbing drainage.

Most older plumbing stacks in homes of your age were constructed of cast iron surrounded with a lead flashing installed on the roof. While this arrangement provided adequate protection from leakage, most of these older vents protruded just above or flush with the roofing, providing little protection from snow and ice.

Surprisingly, these older vents did not often freeze over because of good conduction of heat through the cast iron, combined with frequent hot-water and warm-air generation through the plumbing drains. Many older attics also were poorly insulated and air sealed at the ceilings, further warming the portion of the stack in the attic.

The common practice for older homes over the last few decades has been to dramatically increase the attic insulation to prevent heat loss and cool the attic to prevent premature wear to roofing. Because a substantial portion of your cast-iron stack may be wide open inside the attic, it could be subject to major heat loss as well, since the attic becomes colder after insulation upgrades.

As this occurs, warm vapour from waste water entering the plumbing drains will cool and condense as it enters the attic portion of the stack. If the stack is cold enough, this condensation may freeze inside this pipe, typically near the top. This can reduce the inside diameter of the stack significantly and may prevent proper venting of plumbing fixtures.

As this opening gets smaller, it's also more susceptible to being covered with snow. This can be followed by repeated melting and freezing cycles, eventually icing over the top of the vent above the roof. As more snow falls and outside temperatures drop, this may create a thick layer of ice/snow that will completely block proper air intrusion and vapour escape from this important vent.

While you're right to climb up on the roof to remove the mini quinsy that can form over top of the vent, prevention may be the key to stopping this in the future without risking life and limb. There are two areas that need your attention to make this problem disappear.

First, insulating the exposed iron piping in the attic will help to keep it warmer as it exits through the ceiling. This may be done several ways, but wrapping it with flexible pipe-wrap insulation is often the simplest. Make sure you apply a sufficient thickness, even if it requires several layers, to prevent heat loss. Many types of this pre-made insulation material come with an integral air/vapour barrier which should be installed to prevent condensation and frost.

Insulating the attic portion of the main stack inside the attic will often warm the pipe enough to prevent blockage with ice and snow. If not, hiring an experienced roofer to replace the existing flashing will put an end to your woes. This repair should be done by completely removing the existing flashing and the collar that connects the flashing to the cast-iron stack.

Quite often, the opening of this collar has a much smaller opening at the top than the inside dimension of the stack. Replacing this with a flexible neoprene connecter/flashing will serve two purposes: it may increase the diameter of the opening above the roof and it will allow the installation of a short vent extension.

Installation of a short section of ABS plastic drain piping above the roof should prevent blockage caused by snow build-up on the roof. Combined with the increased inside pipe diameter and warming of the old stack, it should eliminate future winter blockage. This minor upgrade can help prevent debris and leaves from entering the top of the vent and should ensure the top remains above roof accumulations of snow.

Care must be taken not to extend this pipe too high, though, as too much exposure to frigid weather can cool the new vent extension above the roof, simply transferring the condensation-related blockage higher up.

I suspect that reduced water entering your drains from your new toilet may be partly responsible for the vent blockage from interior frost build-up, but that's not the underlying cause of your recent problems. Excess cooling of the old stack in the attic and snow blockage of the flush-mounted vent flashing on the roof are the main culprits. Remediation of these two areas of concern should certainly prevent the need for winter roof expeditions in the future.

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


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