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Repairing cracked vent pipe an easy fix

Question: I am wondering if you could explain an unusual pipe I have seen in my attic. My house is a bungalow, built in 1986. This was in the attic and I found a small area, three square inches, of frost on some blown in insulation. I swept away the insulation and found an ABS vent pipe with a very fine crack in it. I’m assuming the pipe leads to the exterior roof vent. I am not sure how long the crack existed.

What would cause the pipe to crack? Since it’s a vent pipe I would think there wouldn’t be enough water to accumulate to cause it to crack.

I should note that in the past the vent from the roof has frozen on me.

K. Wazney.

Answer: Fixing a damaged vent pipe in an attic of a home your age should not be too difficult and should be the focus of your endeavours. Trying to find the cause, when it may have dated back more than three decades, may be a waste of your efforts. Calling a plumber to enter your attic and replace the cracked ABS pipe, before it causes any serious issues, will nip it in the bud.

Finding a direct cause for a small amount of frost in an attic can sometimes be difficult, but you seem to have done just that. Typically, frost forming on the underside of the roof sheathing, or elsewhere, is caused by air leakage from the house. This may occur at any location where the air/vapour barrier is damaged, or other poorly sealed areas. This can frequently happen around the ABS plastic vent pipe for your plumbing drains, but is normally located where it breaches the attic floor. If the polyethylene sheathing is not properly secured and sealed to this pipe, air can rise up around it. If the gap is significant enough, it can allow this moist air to enter the attic, hit the wooden components, and condense. In very cold weather, this condensation will quickly freeze, leading to the visible frost.

Because the frost in your attic was seen on the loose fibreglass insulation, it easily could have had this same cause, but did not have enough heat energy to rise up to the underside of the roof. The fluffy insulation may have slowed down its rise, allowing the condensation to occur in the fibreglass, itself. You were wise to crawl inside and pull away this frosty material to look for the source. Unusually, it appears that the moist air actually leaked directly out of the pipe rather than around it. Even though it appears that the crack is only hairline in the black plastic, it may be enough for gas to escape. This may be why the frost is only seen in the insulation, due to the limited leakage.

Because this air leakage problem is emanating from your plumbing waste system, it is somewhat more urgent to fix than a standard attic air leak. This is not only because the air leaking from your pipes will have a very high moisture content, but also contain noxious gasses from the drains. These gasses should be expelled to the exterior of the home as quickly as possible, to prevent any problems. It is unlikely that these could re-enter the living space and make any of the occupants’ sick, but it still should be removed from the attic ASAP. The simplest way to do this would be to cut out and replace the damaged section of plastic pipe.

The possible cause of this damaged pipe may never be determined, but it is likely not something to be at all concerned about. Because of the thickness and durability of ABS pipe, it is rarely damaged unless subjected to extreme pressure, or other physical abuse. There should be nothing inside the attic that would exert that kind of pressure, because the boot sealing the top of the pipe exiting the roof is flexible. That should allow the vent to move up and down, which will prevent pressure-related damage. Unless later workers inside the attic, such as insulators or alarm installation technicians, damaged it during retro-fit work, it may not have a recent cause.

My guess is that the pipe was originally flawed from the manufacturer, or damaged during the original construction of the home. It is not unusual for a minor defect in a building material to cause a more serious issue years after installation. If a tradesperson working on the home accidentally cut the surface of the pipe before it was used, it may have gone undetected by the plumber putting it in. That cut may have eventually opened, and allowed the gasses inside to escape. Another possibility is that some chemical compound was spilled on the pipe, again during the construction process, that weakened that small section of plastic. That could have included some type of solvent, adhesive, cleaner, or other chemical used by any of the trades in the building process.

Regardless of the cause, the situation now calls for action to be taken to prevent sewer gas and moisture leakage into the attic or further deterioration to the pipe. Cutting ABS pipe is quite straightforward and is often done with a hacksaw or fine-toothed reciprocating saw blade. Cutting out the damaged section should be quite easy, as long as it is readily accessible. Once removed, a new piece of ABS pipe can be put in its place with a pair of coupler fittings and proper adhesive.

Trying to determine what caused the hairline crack in your attic vent pipe is not likely worth the time involved. Hiring a Red Seal plumber to go in, remove and replace the damaged section, and eliminate the vapour leakage causing the frost is the proper use of your resources.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)( 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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