This is my BIL's roof, the house is three stories tall and a century old:





There's a low spot on the cement gutter, maybe two inches below the level of the drain. It's currently watertight, but allowing water to pool there seems unwise.

A proper fix would be elaborate and very expensive, given the details of the existing structure. Is there a sensible material - tar perhaps? - can fill the low spot without popping free after a few freeze-thaw cycles? The surface looks like cement but it's a sort of slightly flexible material that yields under finger pressure.