a de - icer formed of a solid block of a pressed , granulated salt has at least a top portion water - proofed to prevent rapid deterioration due to precipitation . an exposed bottom surface is in contact with the upper surface of a flat , sloped roof to provide wicking absorption of water flowing down the roof into the salt . absorption of water by the salt leads to release of the resulting saline solution to the surface of the roof , preventing formation of ice on the roof downstream of the de - icer and in the roof drain . the de - icer may be maintained in place on the surface of the roof by an adhesive tape , or by its own weight .