Opinions on Roof Sheathing

Dark sheathing stains on just one roof slope. Opinions for cause, condensation? :slight_smile:




Could be a combination of several factors:
No tar paper, improper ventilation and the roof not getting enough sun to dry out.
What was the moisture reading?

I’ve seen it associated with poor ventilation (no ventilation).

Hard to tell in these pictures. If the color is a dull gray, it could be that the sheets were out in the elements in a storage yard. How prevelant was the coloring?

You may see that kind of staining on north slopes as they don’t get the sun to dry them out. But…where did the vapour for condensation come from…(1) just natural upward winter moist air leakage from the house though ceiling penetrations; (2) outside dryer vent air ending up in the attic (3) outside bathroom eave vent air ending up in the attic

What is the insulated flex pipe in picture 2 serving?? Is it airtight?

· Improve: We noted that the exhaust duct for the (main floor bathroom) fan discharges the air through the roof soffit. This can promote mould growth in the roof space as a result of air flowing from the soffit into the roof space. It is better to run the exhaust duct through a gable end in the roof space or through a wall or roof. Equip the duct with a backdraft damper provided with weather-stripping and a counterweight or spring to prevent air return (“Clapet Daoust” type of damper or equivalent). Ensure that the exhaust outlet is not blocked, that the backdraft damper is working and closes properly, and that the air is discharged directly to the outside through a sealed, insulated duct. Make certain that duct is the size recommended by the manufacturer and is a short and straight as possible. Seal all leaks.


If all vents and exhausts are terminated propoerly it appears to be a case of poor attic ventilation.
typically the requirement is to have vents installed in the upper 1/3 of the roof capable of venting 1/150. This can be reduced to 1/300 by the use of vapor barriors and soffit vents

If staining is uniform (that’s how it looks in your photos), then it’s condensation. If staining were localized it would more likely be leakage. Good chance that Brian’s right about only one slope getting enough sun to dry it out.

If a bathroom vent terminates in the soffit and the soffit provides intake air for ventilating the attic, then moist air is being pulled into the attic. That might be enough to cause this kind of problem depending on the average relative humidity and the hourly rate of air exchange in the attic. Also whatever other sources might be putting moisture into the attic. Stack effect can carry quite a bit of moisture up through holes cut in drywall for recessed light fixtures.

Bathroom vents must terminate at the exterior. Not at soffits or beneath roof vents (another common defect).