The mechanism, plainly
Water expands when it freezes. If that water is sitting in a shingle, or in the space between two shingles, the expansion pushes the material apart where it is weakest.
Then it thaws, the water drains or is absorbed deeper, and the next cold night repeats it. A roof in a hard freeze-thaw climate can go through that dozens of times a winter.
Why a dried roof suffers more
A supple shingle can flex with the movement. A shingle whose oils have left is brittle, so the same stress tends to crack rather than bend, and cracks let more water in.
That is the loop: drying makes the surface hold water, held water freezes, freezing opens the material, the opening holds more water.
Where the damage shows first
At the edges and in the valleys, where water lingers, and on the slopes that stay shaded longest. Also at the points where the roof is interrupted, which is where water concentrates.
On a southern roof the pattern is often seasonal: a hard winter shows up as granules in the gutters in spring, and as edges that no longer sit flat.
- Edges lifting and staying lifted
- Granule loss accelerating after a hard winter
- Cracking along the shingle or at the mat
- Water marks in the valleys
- Growth appearing on the shaded slopes
What the treatment does about it
The range is built against two forces, and freeze-thaw is one of them alongside UV. The logic is that keeping water out of the shingle leaves freeze-thaw less to work with, and restoring flexibility gives the material something to flex with.
What it cannot do is repair a shingle that has already cracked through. At that point the answer is a repair, and possibly a replacement.
What to do before winter
Have the roof scored while the weather is dry enough to work, because the application needs 24 to 48 hours of dry weather to cure.
If the score says the roof has dried out, treating it before the freeze-thaw season is doing the work in the right order.
The season it shows up
Freeze-thaw damage is usually noticed in spring rather than in the cold: granules in the gutters, edges that no longer sit flat, and water marks where ice sat through the winter.