Wet vaulted ceiling.
Q I have vaulted ceilings in my house in central Minnesota. What would cause these two stains?
Jeff Brooks, Alexandria, MN
A You almost certainly have condensation occurring in the wall, probably near the peak of the cathedral ceiling. An air leak is probably allowing moist interior air to flow into the wall cavity, where it's condensing and freezing on the inside of the exterior sheathing. During warmer periods, the ice melts and water seeps out through joints in the plywood sheathing, creating stains.
Remove a few siding panels near the peak where the moisture starts. If our hunch is correct, joints in the sheathing will correspond with the moisture, and the sheathing will be damp.
If the sheathing has begun to rot, replace it. This will allow you to examine the framing and the wall/ceiling/ roof junction to spot the problem. If you have a small attic space above the cathedral ceiling, adding roof vents may help flush out moist air.