Append your id_rsa.pub to the authorized_keys of the root user via sudo.

ssh-copy-id-root.sh

echo "Remote Server (e.g. example.com)"
read remote_server

# test if login already possible
ssh -q -o ConnectTimeout=3 -o PasswordAuthentication=no root@$remote_server echo "" > /dev/null
if [ $? -eq 0 ] 
then
	echo "You can already login!"
	exit
fi

pub_key=`cat ~/.ssh/id_rsa.pub`

command='sudo mkdir -p /root/.ssh ; echo '$pub_key' | sudo tee --append /root/.ssh/authorized_keys > /dev/null'

ssh -t $remote_server "$command"