Proper owner of var/www/html

Some CMSes and Wordpress is especially bad about that because it's actually in the code to use the web user.

BTW you should never need to use root for ftp. www-data the default apache user on ubuntu should own your web files/directory to work properly with many cmses.

So this is what has worked before and what we did for clients with the same issue. chown the whole web root as www-data for both user and group.

So if your document root is /var/www/html, cd or change directory to /var/www and run this to change ownership on all files and directories.

chown -R www-data: html/

while still in the /var/www directory add write permissions to the group for files and directories by running this command.

find html -type f -exec chmod 664 {} + -o -type d -exec chmod 775 {} +

Finally add your FTP user to the www-data group.

usermod -a -G www-data username

Replace usename with your FTP client username

Now this setup should allow you to use manage files and still allow the CMS ftp backend to still function and write to the direc. Let me know how that works for you.