Organic shingle fallure pattern: what causes this condition?

I don’t know the answer to this one.

My theory is that heat rising to the peak causes the upper deterioration and ice dams cause the lower deterioration, except I think the one building is an unheated garage. Still, it probably warms up inside from the sun.

Does this happen in climates where ice dams arean’t a problem?


Hot & cold - Copy.JPG

One word…ventilation.

How was the sheathing underneath?


I think you’re probably right jeff, but why isn’t it uniform? Why the damage near the bottom?

Vaulted ceilings and no rafter bay ventilation.

Actually it is uniform…along the top and the bottom.

You apparently have gable vented louvers…which probably is trapping the heat along
the eaves while at the same time stagnant air or no cross ventilation existing is causing the shingles near the top to deteriorate…if not for the additional layer under the ridge caps I suspect they too would be deteriorated.
Organic shingles also tend to blister more than your fiberglass shingles…taking into consideration there is venting problem (condensation etc) I am sure this also has contributed to their failure.

Is it a true vault or are they scissor trusses?
Also was that the east side of the house, how the the other side of the roof look?
The roof looks likes it about 15 - 20 years old but then again its hard to tell sometimes. I just inspected a roof that had 3 tab shingles that have last over 30 years…I was amazed at their condition. If not for the poor venting and numerous hail storms we have had the owner could have probably gotten another 5 years or more.

Any pictures of the interior…including infrared on the ceiling?


Organic shingles?? are they available in the safeway produce department???

Uniform across the roof.

  1. Vaulted ceiling- no ventilation, photo of roof peak below. Condensation not much of an issue with this place I don’t think. Very dry here (except for the unusually foggy day of the inspection)


Hmmm… seems to me the problem area is approx where the full logs are located, am I correct???

The plot thickens…

It’s above the outside wall, which is why ice dams comes to mind.