Ok, here's the code you want:
Code:
{if $location[0].0 eq "Rootname"}
Custom menu display code
{/if}
Replace Rootname with the name of the root category you want it displayed for. If you have three root categories, all of which will show different menus, write it like this:
Code:
{if $location[0].0 eq "Rootname"}
Custom menu display code
{elseif $location[0].0 eq "Rootname2"}
Custom menu display code 2
{elseif $location[0].0 eq "Rootname3"}
Custom menu display code 3
{/if}
Hope this helps!