On an addition and remodeling project we are working on we discovered some problems from years ago. The home had a roof that joined against a wall. These are tricky areas to flash properly in that you need the roof water to exit the roof without any chance to get behind the siding. This usually requires the roof step flashing to curve and be brought out to the face of the siding. On this project when we opened up the wall from the exterior we found that water had been collecting behind the siding. This caused the wall sheathing and the wall studs to decay. There was also a small area (the dark area at the bottom of the photo) where the floor joist was completely decayed.
The homeowners were not aware of any problems in the wall. I suspect this was the case because the water most likely did not enter the wall during all rain conditions. It may have only entered during wind driven rain from a certain direction. Or a winter ice dam could have caused melting snow to find its way into the wall, but not during warm weather rains. Water does odd things and behaves strangely sometimes.
A good sign that there is moisture in a wall is if paint on wood siding continually peels and blisters. Once water is in the wall it typically drives itself out during dry periods. It can really only go out through the exterior finishes, or in through the interior finishes. On many homes it is easiest for water to exit the wall by going through the siding. That is why it isn’t always easy to spot problem areas like this.
In this case we are scheduled to remove this section of wall to open up the new room to the house. We can correct the damage without too much difficulty. But if we were not constructing an addition it would be more intensive to correct. And if we were not constructing the addition the leaky wall would most likely not have been discovered.