Given that logging in as root is not a good idea, have a look at sshd manpage:
Specifies whether root can log in using ssh(1). The
argument must be
forced-commands-only'' orno''. The default is ``yes''.
If this option is set to ``without-password'' password authenti-
cation is disabled for root.
If this option is set to ``forced-commands-only'' root login with
public key authentication will be allowed, but only if the command
option has been specified (which may be useful for taking remote
backups even if root login is normally not allowed). All other
authentication methods are disabled for root.
If this option is set to ``no'' root is not allowed to log in.
So you could use
PermitRootLogin without-password for allowing private/public key authentication while disallowing password authentication; or maybe
PermitRootLogin forced-commands-only to let you login as root but without interactive access.
The latter case requires you to edit
authorized_keys file, to specify which command is enabled for the logging is user, like this:
command="rdiff-backup --server" ssh-rsa AAAAB3NzaC1y... (rest of key)
or even better, allow forced-commands-only root login only from specific ip address:
from="10.1.1.1",command="/home/user/command/to/execute" ssh-rsa AAAAB3NzaC1y... (rest of key)