The family of a 73-year-old grandmother is suing the owner of her Coney ­Island apartment building after she wandered onto the 16th-floor rooftop — where her decomposing body was found a month later.

Crucita Alvarado got onto the roof on Aug. 12, 2014, at 2929 W. 31st St. through a door with a broken alarm, according to the Brooklyn Supreme Court suit.

Owner Sea Park West didn’t return calls for comment.