I found a succinct article on web site folder permissions.
In a nutshell, for debian servers:
chown -R username /var/www/site-name
chgrp -R www-data /var/www/site-name
chmod -R 750 /var/www/site-name
chmod g+s /var/www/site-name
chmod g+w /var/www/site-name/writable-folder