Wood Stair Meets Wall Stucco Problem
These wood stairs look like they where built after they finished
the stucco. The main problem here is the wood touching the
stucco can absorb moisture and this could lead to wood rot
problem gets worst if the ends of the wood treads and risers are
Paint can provide a water resistant and sometimes waterproof barrier, but this won't solve your problems all the time. Keeping a small space between the two will be your best bet. I've seen paint under these conditions separate from the wood, for a variety of reasons, making it difficult to create some type of separation that wouldn't allow moisture to absorb into stair treads or other parts of the stairway.
This problem could have been prevented with a 1/2 inch space in between the wood and the stucco. This will allow air to circulate between the wood and the stucco allowing the moisture to evaporate faster.
Here's an architectural section that might provide you with a little more
insight on this particular problem. The black arrow is pointing to a small
gap between the wall stucco and wood stair parts.
Any amount of debris that actually gets trapped in between the wood and stucco will only add to the problem and it's only a matter of time before dirt and particles of leaves get trapped in this area, providing a bridge for moisture that accumulates in the wood or stucco, to transfer back and forth.
Another problem will be that the area in between the wood and the stucco can remain moist for long periods of time, without direct sunlight or higher than normal temperatures, allowing it to dry out. For example, the visible areas might look dry, but the areas that can't be visually examined, might not be, allowing moisture to remain in these areas for longer than normal periods of time.
The bare minimum gap for something like this would be a half-inch, but I
would like to see at least one inch and this area will need to be inspected
regularly to make sure debris isn't building up in between the wood stairway
and the exterior stucco wall.
It won't do you much good to have a 1 inch gap filled with debris, allowing moisture to travel back and forth from the wood to the stucco.
A recommended maximum distance for something like this would be 2 inches, even though you could probably go up to 3 7/8 inches, but that could be pushing it. Don't forget to validate this information by checking with your local building department officials to make sure your stairway meets local building codes.