Albert Barnes on Matthew 24:17

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. Mt 9:2. The meaning here is, that he who should be on the house-top 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.