Stiffening, then lifting
A new shingle flexes. Roof surfaces get very hot in the afternoon and cool overnight, so the material expands and contracts every single day, and a supple shingle absorbs that movement.
Once the oils are gone, the same movement has nowhere to go. The edges lift and stay lifted, and water then has a place to sit.
The other reasons edges lift
Freeze-thaw is a strong second. Water in the shingle or between shingles freezes, expands, and pushes the material apart, so a roof that goes through hard winters curls faster than the same roof in a mild climate.
Wind lifts and flexes edges too. So does a shingle that was never sealed down properly in the first place, which looks similar and is a repair rather than a wear question.
- Dried-out asphalt that no longer flexes back
- Water freezing inside the shingle
- Wind lifting edges season after season
- Shingles that were not sealed at installation
- Missing or slipped shingles above, which let water behind the course below
Why it is treated as an assessment question
Curling is one of the things the score is built from, along with granule loss and the feel of the shingle. A roof that is curling but still has surface left is in a different place from one whose shingles have cracked through.
The treatment that applies depends on where the roof lands, not on the presence of curling alone.
What the treatment does about it
For a roof that has dried out but is still sound, the restorative treatment puts flexibility back into the shingle and makes the surface shed water rather than hold it. That addresses the cause: a dry shingle.
It does not flatten shingles that have already curled and set. On a roof where the shingles are lifting, cracked or missing, that is a repair conversation and possibly a replacement one.
When curling means repair or replacement
Lifted shingles that let water behind the course below are a repair, and a leak is always repaired before any treatment. If the shingles have cracked through or the mat has failed, no surface treatment will hold.
The assessment says which of those you have, and the free estimate says what it costs.