The first and most important thing to do once decay is discovered, is to figure out where it started.
If it is not wood destroying insects, it is water. So we have to figure out where the water is coming from and fix the leak or water penetration. If it is insect damage, call professional exterminators first!
For water damage, in order to check for the origin of the problem, start by checking for the obvious – roof and plumbing leaks, and missing or punctured flashing.
If the problem is not obvious, look for stains and follow any dripping tracks outside and inside of walls.
Another very common problem, is the size of your eves. Are your eaves wide enough to prevent water from cascading down sidewalls?
In addition gutters are a big culprit. Are gutters poorly maintained or missing?
Grading create other water leaks inside that create dry rot and other major problems. Do finish grades slope towards or away from the foundation? Are foundation cracks not waterproofed, if not, is it admitting water?
Other reason why carpentry is needed to repair structural or non-structural lumber, is when water pools on or is too close to it. Is untreated wood in direct contact with concrete, masonry, or soil? Check to see if crawl spaces have soil covers, and if venting and/or insulation is present, adequate, and properly placed.
Damage at the top?
The same goes for attics and spaces in between roof and the ceiling of your home.