Throughout biblical times the house roofs of the common folk were often made of layers of plaster and mud that were laid down over reed mats that in turn were placed over branches and logs.
Because of the mud roof, grass would grow there during the winter months.
These roofs were often rolled to help make them moisture resistant.
This grass, because of a meager root system, was the first to wither and dry up. Note what is said of the wicked in Psalm 129:6:
May they be like grass on the roof,
which withers before it can grow.
Stigers, H. G. “Roof.” Page 204 in volume 5 of The Zondervan Encyclopedia of the Bible. Edited by Moisés Silva. Grand Rapids, MI: Zondervan, 2009.