Avoid having to manually change the permissions after adding files as root or another user. These commands have to be ran as root. After running these commands the files and directories inside www will inherit the ownership and permission given.

better-server-permissions.md

# add user to www-data
gpasswd -a user www-data

# change ownership
chown -R user:www-data /var/www/

# find all files, set permissions to rw-r-----
find /var/www -type f -exec chmod 0640 {} \;

# find all folders, set group id and permissions to rwxrwx---
find /var/www -type d -exec chmod 2770 {} \;

Source: Lekensteyn