said by MaynardKrebs:a) Shingles need to be replaced when you see about 5% of them starting to curl - the rest of them will follow in short order (1-2 seasons at most).
The shingles were most likely 25 year rated (and therefore nearing end of rated life) but they are holding up really well with no curled shingles anywhere.
said by MaynardKrebs:b) NEVER install shingles where the slope is less than 4:1 Use a proper low-slope roofing material in these areas.
I didn't measure it (there is no level reference to get a good measure of the height) but it seems to me that there are 5 to 6 feet horizontally for every 1 foot gain in height. I admit that I could easily be deceived by the way the roof looks (optical illusion). For many years I didn't know my roof had any low slope area at all but when I actually climbed up it seemed like that part was completely flat.
said by MaynardKrebs:c) In areas with high winds, always install a ice/water shield membrane over the entire roof before shingling, to prevent wind-driven rain from being forced under the shingles. It's also prudent to install an ice/water shield if you have raccoons habitually on your roof - they're known to rip shingles off.
We do have a significant raccoon population but they seem to spend more time on the ground and in the storm drains then on the roof. I never heard of shingles being ripped off here.
Any animal damage to the roof would be more likely from squirrels. The water being pushed/forced under the shingles is my suspicion as well. Ice isn't really an issue here but a good water seal will be a priority when the roof is being replaced. Somehow it seems weird to me to put a perfectly watertight membrane on the roof and then pierce it with hundreds of roofing nails. I guess there is no other way to keep the shingles in place ?
Since the low sloped area of the roof faces south I'm tempted to install a solar array once the roof is fixed (there is a tall tree in the neighbors yard that may cast a shadow over it).
Since pictures say more then a thousand words:
Google Maps
Colored for clarity. Red = low slope
I have colored each area of the roof in a different color to make it clear where the ridges and valleys are. When the home was originally build in the 50s all parts of the roof had a good slope. The red area was much smaller, probably with the same angle as the yellow area on the opposite side. However later another room was added behind the garage and without changing the ridge line the red portion of the roof was stretched out to cover it. This caused that portion of the roof to be very much lower sloped. The circled area with the question mark is where the leak appears to be. There are no roof penetrations (vents, chimney, antenna mounts, etc.) near that area.