Damaged Roof Plywood
This roof plywood damage isn't uncommon
if your roof isn't maintained or replaced when it it can no
longer be repaired.
The wood damage was caused when the roof shingles at the edge of the roof were damaged, allowing water to roll underneath them.
The biggest problem with water underneath your roofing is that it's normally easy for the water to seep into the cracks and on top of the wood framing and sit there for long periods of time.
The roofing material acts as a moisture barrier and will rarely let moisture evaporate that is underneath it. This isn't good for the wood of course.
In other words, the damaged roofing materials allow the water in, but don't let the water evaporate out. If you look at the roof rafter, you will notice a fungus almost covering the entire bottom of the rafter. Wood fungus is a good sign that no one was maintaining this property.
Home Repair Tip: The damage plywood will need to be replaced, this will involve removing the roofing, removing the plywood and then you're going to need to check for additional roof rafter damage.
The roof rafter in the picture above will definitely need to be claimed and then closely examined to see if it is still structurally sound.
The damage at the edge of the roof is extremely common and these areas seem to suffer from not only wood rot, but termite damage. Don't forget that moisture or wet wood often attracts termites.