Renovation & Design

Stains in attic not always cause for concern

TRIBUNE MEDIA MCT/Owens Corning�s Attic Stairway Insulator is a tent made from insulating material and reflective foil that covers the stairway opening. Uninsulated attic hatches are a common spot where heated air escapes into an attic. (Owens Corning/MCT)

Question: Last winter I took a look into the attic and noticed black stains and water marks on a portion of the sheathing material. This summer I cleaned the portion of the black stain with a dry mop, as much as I could reach from the hatch opening. While the black stains are either mould or just black dust from attic insulation settled on the wet surface in winter, I am concerned with the water marks. Are they from moisture too or from a possible leak? I probably had an ice dam this winter in that area of the roof. I would appreciate your opinion. Thank you in advance, Eugene.

Answer: Often our homes may appear quite problematic, especially to the untrained eye, but in reality, what's occurring may not be much of a concern. Attic stains may fall into this category, depending on the severity.

Almost all attics in homes more than 25 years old will have some moisture stains on either the sheathing or the framing. In a reasonably well-insulated and vented attic, stains may appear due to condensation. This happens due to trapped damp air, often in the late fall or early winter. When this humid, relatively warm air cannot escape the attic space before the temperature drops, condensation can occur on the coldest surfaces in the attic. Those often are the underside of the roof. When this phenomenon occurs, the sheathing or framing may become slightly wet on the surface. If the temperature drops below the freezing point, often at night after the sun goes down, this moisture will freeze.

Looking up into an attic in the days or weeks after the first few sub-zero days will often reveal a layer of white frost in some areas. This frost will normally melt on warmer winter sunny days, but may reoccur at night when the surface temperature of the roof drops. Also, a heavy layer of snow on the surface of the roof will insulate the sheathing and may keep the frost from melting, altogether. If the frost does melt, it will further wet the wood in the attic, staining it. Small stains may only be this surface moisture bleeding out minerals or dirt from the wood and are often brownish in colour. These stains may turn darker in colour over the years, as the frost/melt cycle reoccurs. In many cases the continued wetting of the sheathing will cause it to rot, but that may take several years or decades, depending on the attic ventilation.

The other two possibilities for the stains in the attic are excess air intrusion from the living area in the home and leakage through the roof surface. If the attic floor is poorly insulated or air sealed, warm air can certainly leak in from the heated living space and cause some major moisture issues. That may be visible through your attic hatch by a trained professional as much more than a few stains. The remediation for that issue is more complex and will be left for another discussion, as it has been addressed many times in previous articles. Roof leakage can often be seen as dark black stains, concentrated and radiating out from one or more individual locations. These are commonly seen underneath roof vents, plumbing stacks, vent hoods and around chimneys. If these isolated spots are seen away from these roof penetrations, then the roof may be damaged and leaking.

Attempting to clean the wood surfaces in your attic is a futile effort. While you may have succeeded in removing some surface dirt or mould, that will not help extend the life of this material if it continually gets wet. If the stains are only surface ones, caused by periodic frost accumulation, there is no need to do anything. If your attic has reasonable ventilation, it should prevent the frost or condensation from remaining in that space for more than a short time. As long as the wooden framing and sheathing is allowed to dry after a brief wet period, damage may not occur, except after many years.

I commend you on attempting to be proactive in preventing a problem within your attic, but your remediation methods are misguided. Rather than attempting to mechanically remove some stains, which will be difficult and useless, focusing on reducing the moisture in the attic area makes more sense. Ensuring soffit vents are not blocked or painted over and roof vents are undamaged and cleared of snow is a much more worthy effort. If you did have ice damming it may be a sign there is a problem with these issues, so improving them will be more worthwhile.

Seeing some stains on the underside of the roof sheathing, or framing, may not be a cause for serious concern. Cleaning them off manually will not be worth the effort, but further improvements to insulation and ventilation may prevent the problem from getting out of hand. If the stains are very dark, isolated, and radiating out, hiring a professional to repair or upgrade the roof may be necessary.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past 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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