This is indeed a requirement for multi-site functionality. This works like charm. Another option is to install Members plugin and then add Custom Capabilities in Roles. Capabilities are listed as follows:
delete_documents
delete_others_documents
delete_private_documents
delete_published_documents
edit_documents
edit_others_documents
edit_private_documents
edit_published_documents
override_document_lock
publish_documents
read_documents
read_private_documents
read_document_revisions
