The southern exposure will certainly burn shingles out quicker than other exposures, but the deterioration you’re showing is extreme. While not publicized, there are manufacturing defects that occasionally occur, resulting in shingles that deteriorate in an accelerated manner. While I can’t know for sure, the extreme difference in wear between shingles that are so close together on the roof makes me think there’s more going on than simple southern exposure/UV deterioration.
Venting. Much is made by manufacturers about ventilation on shingle lifespan. The single factor effecting shingle lifespan, more than any other factor, is color. Black and dark shingles don’t last as long as lighter colors, with the purported difference being dark shingles get a lot hotter than white shingles. Ventilation does little to effect roof temperature. The idea that some vents will reduce the solar load on a shingle roof is silly. At most, ventilation will reduce temperatures a few degrees.
Ventilating roofs is different for different climates. In cold climates, it’s about keeping the roof deck cold to reduce ice dams at the eaves and minimizing condensation in the attic. In hot climates, it’s about getting the very hot air out of the attic to reduce cooling loads and minimize strain on AC systems. No where is there any engineering indicating attic ventilation is about extending the lifespan of shingles.
That said, the roofing industry has latched onto this as contributing factor in shingle lifespan and performance. A simple bit of physics should tell anyone that a few square feet of ventilation space in an attic in no way reduces shingle temperatures more than a very few degrees. One wonders if the shingle manufacturers are latching on to this for self serving purpose or if they actually think that vents reduce shingle temperature. I don’t know, but I know someone that does.
Go Here and download “A Crash Course in Roof Venting”. It explains all of this in very simple and clear language.
So, I really think the damage on your particular house is a manufacturing defect or some similar inadequacy in material, not crashing snow loads or lack of ventilation. If ventilation was a major factor in shingle failures, we would be seeing shingles failing everywhere and all the time because hardly any roof system is ventilated to currently agreed upon standards.
