Lucas 550 EZ Seal Emergency Roof Patch 3.5 Gallon:

Lucas #550 is a fast and effective product for temporarily stopping leaks in all types of low-slope roofs. Manufactured from gel forming bentonite* clay, #550 forms a watertight barrier when exposed to water. Formed into free-flowing micro-pellets, #550 pours easily from the spout on its reusable waterproof plastic container. The small pellet size exposes a greater surface area resulting in a faster gel and water stopping seal.

Fun Fact: "Bentonite" is named after Fort Benton, Montana, a frontier town on the Missouri River where this unusual, useful clay was first found.