folder-permissions

// https://askubuntu.com/a/51953

// If you want all new files in a particular directory to be owned by a particular group, just apply the setgid bit on it:

chmod -R 755 /some/dir

chgrp www-data /some/dir
chmod g+s /some/dir

// If you have an existing tree of directories that you want to apply this behaviour to, you can do so with find:

find /some/dir -type d -exec chgrp www-data {} +
find /some/dir -type d -exec chmod g+s {} +

// (if the directories are already owned by the relevant group, then you can omit the first command in both of these examples).