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Follow the drying edge
Water moving through masonry can carry dissolved salts to the face. As it evaporates, the salts remain. This deposit is called efflorescence. It can appear on brick or mortar. A photo can show its outline, but cannot identify the salt, distinguish every white stain, or locate where the water first entered.
A powder and a missing face tell different stories
Use a ground-level view or a window that already provides a clear view. Look for a film over an intact brick outline versus a missing corner, pieces on the ground, or a joint that has washed away. Report falling fragments immediately and keep the area below clear. Avoid scraping the deposit or leaning a ladder against the stack to collect a sample.
Bring two views of the same patch
Save one wider picture that includes the roof junction and one closer view from the same safe position. Date both. Add any recent roof or masonry work and whether there is also an indoor stain. A reviewer can use that sequence to decide whether to examine a single local moisture path or a wider area before discussing cleaning.