Asphalt shingles can buckle across each course (row of shingles) of the roof.  This is caused by movement of the roof deck below and occurs in roofing systems whose roof decks are made of dimensional lumber. If you find that their is any signs of buckling call your roofing contractor immediately as this can be very hazardous.

How Shingles Buckle

In a roof with a dimensional lumber roof deck (also called a board or plank deck), the wooden boards tend to shrink or swell according to their moisture content.  Other boards that run adjacent to these roof deck lumber boards also shrink and swell, but at different rates.  Since shingles are typically secured by two rows of nails that go into the lumber decks, a single row can actually be fastened into two separate boards.  This, coupled with the difference in shrinking and swelling between the boards, is what causes shingle buckling.

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