You can setup cron job for root user using shell
List corn of root user
# crontab -l -u root
or,
$ sudo crontab -l -u root
Setup cron job from shell/terminal
# crontab -l | { cat; echo "@monthly /usr/bin/php5 /location-to-execute"; } | crontab -
Run/setup Cron job for root from local user
#su - root -c "crontab -l"