On our systems, the root account is set to be a role, so you can't login as root, just su to root from an user which has the root role.
Also, the root user has the bourne shell (/bin/sh), but working with /bin/bash is much easier.
Using this command, you can directly log in as root with a bash shell.
ssh demo.lab.example.com -t "su - root -c /bin/bash"'
I set it up as an alias in .bashrc:
alias demo_root='ssh demo.lab.example.com -t "su - root -c /bin/bash"'
This enables me logging in with just:
$ demo_root Password: #
The Password prompt on the second line is from the su, for the root password.