First, fix the owners :
sudo chgrp -R www-data .
Then, fix the access permissions :
For read-only folders :
sudo find . -type f -exec chmod 0460 {} \;
sudo find . -type d -exec chmod 2570 {} \;
For "storage" folders :
sudo find . -type f -exec chmod 0660 {} \;
sudo find . -type d -exec chmod 2770 {} \;
Nginx need to have +x access on all directories leading to the site's root directory:
chmod +x /home/
chmod +x /home/username
chmod +x /home/username/siteroot
Wordpress website :
sudo find . -type f -exec chmod 664 {} +
sudo find . -type d -exec chmod 775 {} +
sudo chmod 660 wp-config.php
Enable the access to nginx from SElinux:
chcon -Rt httpd_sys_content_t /path/to/www