Truss uplift usually occurs during cold weather months and is most often caused by non-uniform cooling of the truss components. The top chords and web members contract slightly due to the low temperatures in the attic while the bottom chords, being covered with insulation and closer to room temperature, contract very little or none at all. This causes the bottom chords to be longer, relative to the rest of the truss, than they should be - and they bow up. When warm temperatures return to the attic (and all the parts of the trusses are close to the same temperature) all the components return to their "normal" lengths and the bottom chords return to their normal position.

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"Baseball is like church. Many attend but few understand." Leo Durocher
Bruce Breedlove
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