Albert Barnes Commentary Matthew 24:17

"let him that is on the housetop not go down to take out things that are in his house:" — Matthew 24:17 (ASV)

Him which is on the housetop. The roofs of the houses in eastern countries were made flat, so that they were favourable places for walking and retirement. (See Barnes' notes on Matthew 9:2).

The meaning here is that he who should be on the housetop when this calamity came upon the city should flee without delay; he should not even take time to secure any article of apparel from his house. So sudden would be the calamity that by doing it he would endanger his life.