FileAppender should be able to automatically create parent directories as needed when writing the log file.

For instance, if configured with <File>/var/tmp/project_name/some_file.log</File>, and the new option set to true (suggest <CreateParentDirectories>true</CreateParentDirectories> maybe), then in FileAppender.setFile(), it should do

new File( fileName ).getParentFile().mkdirs()

workaround is to create the directory before initializing logback. Possible but far less than ideal, and rather complicated too