House whodunit: What's causing black mold under roof sheathing?

Dear Jerry: I have inspected hundreds of roofs, but this one has me puzzled. The house is a two story, about 20 years old.

On the east side of the house only, the plywood for the first three trusses is black for about 6 feet up from the eaves. Then, it skips three or four trusses and then there is another panel of black mold. The west side is fine. The east gable wall is black, the west gable wall is fine.

The roof was supposedly replaced two years ago. There are insulation baffles at every other truss and a continuous ridge vent. fiberglass blow-in insulation was used.

Any ideas or help would be appreciated.

— R. S., via email

Dark areas on the underside of roof sheathing can sometimes be tricky to diagnose. However, there are a few places that I would suggest you research further.

The dark stains on roof sheathing can usually be traced to one of two things: excessive humidity inside the house making its way to the attic, where it then condenses on the structural members during cold weather, or some type of roof leak.

Let’s look at the inside issues first. Excessive indoor humidity can usually be traced to over-use of a furnace humidifier, poorly vented bathrooms and kitchens, and/or lack of air movement within the house. Lack of proper ventilation in the attic can also result in condensation stains.

First, I would want to know if there is a bathroom under the areas where the stains are. If so, is the bathroom properly vented by a fan to the exterior? If not, then that’s a significant clue. If the bathrooms are vented, where is the duct? Is it in good repair and where does it terminate? I’ve seen vent fans that were installed in the ceiling, but never attached to a vent duct. Others may have their ducts terminated a few feet from the floor of the attic, in hopes that the moisture will rise and exit one of the roof vents.

Another possibility is that the fan duct was routed to the eaves of the roof, and terminated at a soffit vent. In this case the moist bathroom air exits the soffit only to be sucked back into the vent due to the natural air currents from soffits to roof vents. This too, would account for the stains in the areas that you mention.

If the stains are directly above a bedroom where people sleep, eight or more hours of breathing can release enough humidity to contrast with other areas of the house. If there is a forced air heating system, running the fan in the “on” position rather than on “auto” will help equalize the humidity between all rooms. Plumbing vent pipes, furnace vent chimneys and other basement-to-roof piping should be checked for integrity and for gaps that could allow basement and indoor air to flow up past gaps around the piping into the attic. Prevailing westerly winds may well cause enough air movement in the attic to darken the east gable wall and not the west one.

Moving on to a possible external cause for the sheathing discoloration, stains near the eaves can often be traced to water infiltration from ice damming. If the roof has an irregular roof line with a cross-gable or dormer or two, these areas can not only block air movement, they can at the same time cause ice to form at the eaves. I would want to know if there has been any history of ice dam issues.

Other roof water infiltration can usually be traced to poor flashing around chimneys, plumbing vents, roof vents, etc. may penetrate the shingled surface. Missing or worn out roofing material is another cause. Since water can travel from the source of the leak to a point where it passes through roof sheathing, the leak is not always where the stains appear. Just because the roof was replaced a couple of years ago, does not necessarily rule out an external cause of the stains, but I personally think it’s an internal not an external issue.

It may take a while, but with a bit of detective work, I think you’ll pinpoint the cause and hopefully be able to correct it.

Jerry Ludwig is a former contractor and home inspector. He is an honorary member of the American Institute of Architects. Email your house questions to [email protected] or write him at P.O. Box 25510, Rochester, NY 14625. Please include the year your house was built and the town where you live.