Help! -- what is causing these shingles to fail?
Our roof is 12 years old, with 20 year BP shingles. The shingles are now very badly worn in some places (curling and disintegrating) and yet perfect in others. Bad ventilation is the first thing people usually talk about when shingles fail, but when our roof was installed the roofers put two extra roof vents in, saying this would bring the roof up to code and provide good ventilation.
What has me concerned and puzzled is that, contrary to the insufficient ventilation theory, the worst shingles by far are the ones where water run-off concentrates when it rains. For example, our house is a side-split, and the worst shingles of all are in a path about two feet wide that runs almost from peak to eaves down the lower (kitchen) roof where it butts up to the siding that rises to the upper roof. As you can see, we had to have a large patch put over this area while we have been negotiating a guarantee settlement with BP (which has been taking a while!).
The water run-off is being channelled to the 2 foot wide section from a portion of the upper back roof which slants down toward the front of the house where it meets a valley that then directs run-off down the lower roof along the 2 foot wide path I mentioned, toward the back of the house. You can see the patch that runs down the lower roof at the extreme right side of this picture.
All of the shingles within that two foot wide path are in terrible shape, from the upper valley to eaves. The shingles on either side of the valley flashings are bad too -- again, this is a spot where water run-off is concentrated.
Does this sound like a ventilation problem? Is this kind of excessive wear from water run-off normal?
Another bad patch of shingles, one that wouldn't be caused by run-off, is the lower section of the lower roof, which you can see on the right side of the first photo above. I assume the problem in this area is caused by bad ventilation, but why wouldn't all the shingles on the back roof be affected more or less equally? If the heat was going to concentrate anywhere, why wouldn't it be the shingles at the peak of the roof that are affected, since this is where the heat rises to and becomes trapped? The shingles on our peaks are in very good shape.
I should add that, as far as I can recall, our previous roof didn't have these same problem areas. Those shingles seemed to wear more evenly, altho there was a modest difference between the front/north side of the roof and the back/south.
I would be interested in peoples' thoughts about what is causing our shingles to fail and, more importantly, how these problems can be avoided when we replace the roof.
Doesn't look like a ventilation problem.When you have excessive water run-off on an area of roof it will wear down that section prematurely.You can prevent this on your new roof by noting where the majority of water hits and put a metal pan or trough over that area.If an upper roof has a downspout dumping onto a lower roof,we always add a downspout down to the main gutter.
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