With the sun beating down on roofs year round, or leaves falling and sitting there causing moisture to go under the roof and rot wood, the usefulness of shingle roofs is not a lot over 10 years.  

Also, consider whether moisture is really an issue though.

What I think should be in your tool bag for inspections for those problems is a non-penetrating moisture meter.

- (See Photo 1) - At the back corner of the cabinet below the kitchen countertop there is moisture damaged wood and drywall, there is some biological growth present on the drywall and wood here

An exception would be extensive storm damage that goes beyond the surface and affects multiple areas.The most common repairs performed on roofs include:90-95 percent occur around flashingBlown- Off or loose shinglesPonding water due to insufficient protective measuresPunctures or penetrations relegated to a single areaShingles that are curling upwards may mean there are leaks underneath the surface or that water is getting into parts of a roof where it shouldn’t be going. roofs with curled shingles often need to be replaced within 1-5 years.Complete replacement is typically recommended when:Damage extends below a roof’s surface layerRepairs are becoming increasingly frequentBald spots are showing up where granules( materials that provide added protection to a roof) are missingA roof is more than 20- 25 years old or has undergone too many re-roofs to make re-roofing a safe option againAn existing roof was poorly installed or not installed according to codeRoof replacement doesn’t necessarily mean completely tearing off an existing roof and starting over from scratch.

The following are some property selection guidelines.Older properties require more maintenance than newer properties.Composition roofs require more maintenance than tile roofs.Properties in climates with hard freezes require more maintenance than properties in mild climates.Properties in locations with a lot of moisture require more maintenance than properties in dryer climates.Wood siding requires more maintenance than aluminum or stucco siding.Properties with lush vegetation require more maintenance than properties with little or no vegetation.Locations with high levels of termite activity will require more maintenance than areas with little or no termite activity.The tenant pool has a significant impact on your ongoing maintenance cost.

A bigger issue to consider is where the moisture is coming from and how to reduce it.

I would say any kind of cracks that can potentially get worse, water marks of penetration that can potentially grow mold, things not up to code like hanging wire that is not capped, unstable front porch steps, railing.. things that need to be replaced like windows, doors..   

The joists are dry-rotted, could be all of them due to no moisture barrier between the dirt floor and subfloor.

jetters can open cast pipes or cut thru roots that a cable would not be able to penetrate.