When faxing documents should there be a cover letter?

There are many types of documents that one would fax to someone. An actual cover letter is necessary only in the most formal circumstances. For example, if the recipient doesn't know the document is coming or the reason you are sending it.

Most places that communicate by fax have a standardized fax cover form; this is the most common cover for a document sent by fax. The cover form will include lines or boxes for the date, the recipient's name, perhaps title and organization, the senders name and title, if applicable (cover forms should be on letterhead making it unnecessary to write the company or organization of the sender), a subject line and an area to write a message. See examples on the link below.

For very informal circumstances, for example, you and the recipient are on the phone with each other while the fax is being sent, then just the recipient's name written at the top of the page is sufficient. Some companies that fax between departments or locations regularly throughout a business day buy 'post-it' notes with pre-printed 'To-From' boxes, just fill in and stick on the document to be faxed.