The most distinctive feature of Harappan cities was the carefully planned drainage system. In the Lower Town the roads and streets were laid out along an approximate “grid” pattern, intersecting at right angles. The streets with dratins were laid out first and then houses built along them. If domestic waste water had to flow into street drains, every house needed to have at least one wass along a street.
Further Mackay noted that the drainage system was the most complete ancient system. According to him every house was connected to the street drains. The main channels were made of bricks set in mortar and were covered with loose bricks that could be removed for cleaning. In some cases covers were made of lime stone. House drains were first emptied into a sump or cess pit into which solid matter settled while waste water folwed out into the streets drains. Very long drainage channels were provided at intervals with sumps for cleaning. Drainage systems were not unique to the larger cities but were found in smaller, settlements as well. For example at Lothal while houses were made of mud bricks, drains were made of burnt bricks.