Horizontal ~upd~ Crack Under Window Sill Review

Worn seals or damaged exterior flashing can allow water to seep behind the sill. This moisture weakens the drywall or plaster, causing it to crack as it dries unevenly.

Go outside and look at the wall directly above that window. Do you see: horizontal crack under window sill

For hairline, non-growing cracks in drywall: Worn seals or damaged exterior flashing can allow