Recently I was helping a friend of mine nail sheets of felt on a roof, and we finished applying the felt in the afternoon when it was warm. We came back the next morning to nail the shingles on the roof, and the felt was bubbled up. It about 20 degrees colder that morning than it was the afternoon of the day before, when we nailed the felt on the roof. He told me that the felt bubbled up because it is cold outside. I know that most materials shrink when they get cold. However, if the shrinkage was caused by the felt's contracting due to the cold weather, I don't understand why it would bubble up. Let's assume that there is a sheet of felt that is 10 feet long by 10 feet wide. Why wouldn't the felt just shrink to become, say, 9.99 feet long by 9.99 feet wide square, but still remain flush with the roof surface without bubbles? If the length and width of a sheet of felt becomes smaller, but the extra length and width just become bubbles, the felt isn't really getting smaller per se. It's just being crumpled up. Assuming that felt shrinks, as opposed to becoming crumpled up, wouldn't that just make it have a smaller length and width without bubbling up?