Matthew 24:17 - Ellicott's Commentary On The Whole Bible

Let him which is on the housetop. — The houses in the streets of Jerusalem were built in a continuous line, and with flat roofs, so that a man might pass from house to house without descending into the street until he came to some point near the wall or gate of the city, and so make his escape. At a moment of danger (in this case that arising from the factions within the city, rather than the invaders without), any delay might prove fatal. Men were to escape as though their life were “given them for a prey” (Jeremiah 45:5), without thinking of their goods or chattels.