That the shingles at the peak are in better shape, make me also agree with the others about ice vs heat. I see no ridge top vent, so the heat would still be highest at the peak.
I am wondering about the quality of the shingles as well.
I am guessing there is not enough temperature difference for your venting scheme to work. Look up stack effect for why exterior chimneys do not work--but in that case, the air has less bends, longer column and perhaps more temp delta.
IMHO, you need to get air flow from the neighbors roof section.
In that first photo, is that part on the left your roof too? the same color section. Why is that and the top row ok? I would at the least rip out the vents you have--the other section did better without. A proper venting setup would have soffit vents and then a gable or a ridge.
The part on the left is the neighbours roof. His shingles are much newer.
I agree my roof would probably have done better without the vents and I will be taking all but two top ones off unless I get expert opinion to the contrary.
I can easily put a ridge vent in but my difficulty is the intake cool air.
IKO organic shingles, eh? Have there been any recalls? Complaints? Why organic instead of fiberglass?
You don't need continuous soffit venting....just soffit venting period. In solid wood, you can drill holes sized for round air vents or cut squares/rectangles if room permits. ANY lower venting is better than zero.
Camera optics may explain but it looks to me like your roof sags from left to right. Further, it looks like the damage is worse on the left side, than the right. In an older home, I'm guessing it was stick framed. So the rafters might require some additional bracing.
I had 33yr old cheap 3-tabs on a 5/12 pitch, in Texas heat, that didn't look nearly as bad as your 15's. We don't have ice down here though. You have potato chips.
Due to its Southern exposure and pitch, your lower vents won't work as intakes. Did your neighbor have this problem prior to replacement? How does your decking look from inside the attic? Is it ply or chip-board? I'm guessing you have an attic moisture problem could be due to a roof leak or from humid living area air rising up into the attic, then condensing on a cooler night-time surface.
Fifteen years ago, most shingles used here were IKO and BP organic. Certainteed had a very small presence though they have been growing since then. Fibreglass shingles are also becoming more popular.
The roof is level, it is the camera angle that makes it look sloped.
I don't remember my neighbour's roof looking this bad prior to it being reshingled.
Decking is the original tongue and groove boards.
I have not been up in the attic in a while. Could attic moisture cause deterioration of the shingles?
I can guarantee it's the IKO shingles. I had mine [censored] out after about 7 years. Looked like yours, peeling aggregate and just falling apart. When I installed them on the main part of the house in 1989 they lasted 20 years. When I put the same shingles on the garage that I added 2 years later, they lasted 7-8 years. They told me this and that about venting issues. They were full of [censored]. Long story short IKO went with [censored] ingredients. Got that through the grapevine. I have since went with GAF. If there is a next time it's steel.