Programmatically create custom Role Definitions with Powershell

Here’s some information on SharePoint Role Definitions

Role Definitions can be created quite easily

$spRoleDef = New-Object Microsoft.SharePoint.SPRoleDefinition
 $spRoleDef.Name = "Custom Permission Level"
 $spRoleDef.Description = "This is the description of a custom Permission Level"
 $spRoleDef.BasePermissions = ("ViewListItems","AddListItems","EditListItems")
$spWeb.RoleDefinitions.Add($spRoleDef);

However, I kept getting SharePoint errors when I ran this script

Exception calling "Add" with "1" argument(s): "You cannot customize permission levels in a 
web site with inherited permission levels."
At line:38 char:17
+ $Web.RoleDefinitions.Add($spRoleDef);
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 + CategoryInfo : NotSpecified: (:) [], MethodInvocationException
 + FullyQualifiedErrorId : ArgumentException

Again, this seemed like one of those common issues because there were a lot of posts online about how to fix it. You have to break inheritance with the site’s parent.

$spWeb.BreakRoleInheritance($true)

This makes perfect sense. If you’re inheriting from the parent then obviously you can’t start adding your own custom Permission Levels.
The problem was that this didn’t work for me. I kept getting the exact same error as before

What I misunderstood was that there are different types of inheritance:

• There are the Users and Groups that have been assigned to the List
• and then there are the Permission Levels that can be assigned to the Users and Groups

It is the Permission Level inheritance that needs to be changed.

$spWeb.RoleDefinitions.BreakInheritance($true, $true)

Once that was done, then the Permission Levels could be created