Jamieson, Fausset and Brown on Deuteronomy 22:8

8. thou shalt make a battlement for thy roof, that thou bring not blood upon thine house, if any man fall from thence--The tops of houses in ancient Judea, as in the East still, were flat, being composed of branches or twigs laid across large beams, and covered with a cement of clay or strong plaster. They were surrounded by a parapet breast high. In summer the roof is a favorite resort for coolness, and accidents would frequently happen from persons incautiously approaching the edge and falling into the street or court; hence it was a wise and prudent precaution in the Jewish legislator to provide that a stone balustrade or timber railing round the roof should form an essential part of every new house.