If you are trying to unmount your own root partition, I think you'll have problems doing that...
You can force fsck to read mounted volumes, just don't let it make any changes (I
think that's the -f and -n flags, but check your man page).
If you want to do a proper fsck of root including repairs, you'll really want to boot off something else (pop a boot disk in the drive and reboot) then run your fsck.
To easily get at /home, just boot up into single user / maintenance mode ('init', 'reboot' or 'halt' commands should do this for you - again, check your man pages for the flags to use to get single user mode).
A slightly more ghetto solution (I have been know to do this on occasion) is to hard power cycle the server (ie pull the cord out of the back rather than press the power button) which will force an fsck on bootup. That said, it might also corrupt the filesystem so, you know, maybe don't do this if the data really really matters.