Terry's Tips are personal observations based on my experience and seeing the cause and effects of things. There may be many ways to address different problems and all opinions are welcome. I am always open to suggestions and certainly want you to correct me on any mistakes that might be written.
One of the most common places that I find moisture issues is where a roof line meets a wall. It seems simple but very rarely is kick out flashing installed in this location. What happens is as water runs down the roof it is directed by the step flashing to the bottom edge. The step flashing should be installed under the building paper and the siding on the wall. It then is interwoven with the shingles on the roof. This stops water from entering this joint between the roof and wall. This is usually done right because if it was not you would see moisture damage at your first rain storm. The problem is at the bottom edge. With out the kick out flashing to divert the water out and away from the siding it can be directed behind it. Obviously we are not talking about large volumes of water here. This is why the problem doesn't show up right away. By the time the problem shows itself it usually has been going on for years. Carpenter ants may be your first clue in these locations that there is a moisture problem. You may also see tannin stains and drip marks coming from behind the siding. If enough moisture is getting behind you can even see stains coming from behind the siding on the foundation and in the basement at the sill area. Depending on the installation details under the siding you may be lucky and there is no moisture damage. Vinyl and aluminum siding have large air spaces behind the siding and weep holes to allow moisture out along with decent air circulation. This can be enough to allow the moisture to dry out every time it gets behind. Also the orientation of that side of the home has alot to do with it. If the same detail is missed on the south side the water may be getting behind but the sun is drying this area out. If you have alot of shade and northern exposure these same areas tend to stay wet much longer. Wood siding materials are usually installed directly on building wrap or paper. This helps trap the moisture which can lead to eventual decay. This is why I always recommend a rain screen approach if installing wood or cement sidings. A rain screen is nothing more than providing an air gap behind the siding. There are many ways to do this that will work. Take a look at the images below to visually see what I am talking about.
This is what I typically see.
This is what the flashing should look like.
Moisture meter confirms what is suspected but we see other clues that moisture is behind the siding.
Tannin stains are just another clue as we can see the moisture came from behind the siding.