How to Fix a Damming Effect on a Flat Roof

Climb onto the roof and locate any drains. If using a ladder to reach the roof, make sure that it is firmly planted on a level surface and, if possible, use a spotter.

Shovel off any loose snow and remove any organic debris like leaves that is preventing water from draining.

Melt ice and packed snow around drains. Do not use an ice chipper, hammer, shovel or any other tool to remove the ice, as this could harm the drain and lead to leaks. Instead, pour hot water around the drain or apply a suitable deicing compound.

Lay electric heating cables on the roof. Stretch the cables from near but not directly in the drain to the corners of the roof in order to cut drainage channels. If there is standing water in a portion of the roof, run at least one heating cable through the ice or snow that is blocking its prompt drainage.

Apply a chemical deicer as an alternative to or in combination with electric heating cables. Sprinkle paths of deicer from near the drain to the corners of the roof. Choose a noncorrosive deicing compound that will not damage roofing materials. Avoid sodium chloride, common rock salt, and instead use products that contain agents like calcium magnesium acetate or urea. Deicer applications may need to be repeated occasionally to maintain open drainage channels.