In theory, the calcium chloride will melt a trough down and over the frozen gutter. The water has enough of the chemical to keep the opening from re-freezing and prevents a build up which causes the leaks.
You will want to hose the calcium chloride off the roof in the spring, but it will not damage the roof between now and then. It certainly outweighs the damage of leaks to the interior.... but do not use rock salt.
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