Verse 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. See Barnes "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.