What the maker says
What is Flex Seal Liquid made to do?
Flex Seal's own FAQ describes the Liquid as a coating you brush, roll, dip or pour on to form a durable, watertight, flexible barrier. It is a real product with real uses. A roof is a hard place for it.
Flex Seal makes several products, and the details differ by product. On its FAQ page, the maker says Flex Seal Liquid is a coating that seals out air, water and moisture and is chemical and UV resistant. It says the Liquid is best applied to a clean, dry surface without grease or oil. Those are fair claims, and nothing here argues with them.
The same FAQ also sets limits. The maker says it does not recommend Flex Seal Liquid for underfoot applications. It says that if the Liquid is applied in heavy rain, some of the product may wash away, so it suggests applying it 24 hours before any heavy rain, hail or snow if possible. It gives a surface temperature range of 32°F to 120°F for applying it. And it says that once fully cured, most traditional paints will not go over it.
In the Liquid answers we read, roofs and leaks are not named. The maker's how-to page lists guides for epoxy, putty, tape strips and a crawl space, and we did not find a roof leak guide on the first page of it. That does not mean the Liquid cannot slow a drip. It means the maker does not present it as a roof repair method, so treat the roof question as your own judgment call.
The short version, for a quick answer: it may slow a small leak for a while if conditions are right, and conditions on a leaking roof are rarely right. The rest of this page explains why, and what lasts longer.
