Root Asus ZenPad S 8.0 (Z580CA) How can I root ZenPad Z580CA - 8'' IPS - 64 GB - Wifi? Are there any ROMs available for it?
I successfully rooted ZenPad Z580CA by following these two links:
Link1: http://www.asus-zenfone.com/2015/11/how-to-root-asus-zenpad-s-80-z580c.html 
Link2: http://dottech.org/190386/how-to-root-asus-zenpad-s-8-0-z580c-on-android-5-0-guide/ 

Thanks to the authors of the above links, but some steps mentioned in this link need more clarification, so I am adding a detailed procedure here:

Tools Required
 1. Windows PC
   2. Download ADB Fastboot tool (Download)
   3. Download the Intel USB driver (Download) 
   4. Download IntelAndroid­FBRL­05­16­2015­PTR (Download)
   5. Download SuperSu (Download) 
&nbsp;&nbsp;&nbsp;&nbsp; 

STEPS
1. Install ADB fastboot tool, Intel USB driver on your Windows machine. Check the link mentioned in Link2:Step4 for installation and checking whether ADB fastboot tool is working. I followed METHOD 1 mentioned in that link.
2. Activate  USB  Debugging  at ZenPad (Settings->Developer  Options,  if  no
developer options, then go to Settings->About->Software information->tap build number 7 times)
3. Connect ZenPad to your Windows machine (laptop/PC). 
4. Copy the SuperSU package (the zip file) to the internal SD storage of the ZenPad device.
5. Type: adb devices in windows cmd (press Windows key + R -> type in cmd -> OK) to check whether your device is connected properly or not. If connected properly you can see some key number as the output for this command in cmd. If your device is connected properly proceed to next step. 
6. Extract the IntelAndroid­FBRL­05­16­2015­PTR package on your Windows machine. Click the launcher.bat file that is found inside the same extracted folder. Run it in cmd. Wait until it says your device is detected. Now select T2 and Enter. Wait for the device to boot into the custom recovery after about 2-5 minutes.
7. Once the main page of recovery menu has appeared on your tab, use the Volume keys to navigate through the recovery menu and power button for selection. Choose the Install option to browse the SD card and then install SuperSU zip file (file copied in step 4).
8. Once the installation is done, go back to main menu of recovery mode and reboot.
9. Done 

You can see that the SuperSu app is installed in your ZenPad now. You can double check the root status by some root-checker apps available in the Play Store.