Get your staff to assess and re-organise their own job descriptions with some room for a "wish list". It saves you time in initial assessment, and encourages buy-in from staff.
Have each staff member keep a list of the jobs they do in a day/week/month, including those odd one-off tasks that don't seem to fit with any one person. Have them include a wishlist for things like: "I did this job today, but it made me uncomfortable - I wish I didn't have to," or "I did this, but it would have been faster and easier if I'd been able to do that at the same time". Once you have their completed lists, you can see whether or not there are multiple staff members picking up pieces of the same job, and discuss with them where they feel they are performing tasks outside their job description, or not performing some tasks. Their wishlist might include ideas that would make a task easier, completely outside of the job description.
It's easier to establish role boundaries with input from the people who do the job. Staff might say, "I do that part of my job description - but it doesn't come naturally, and I would feel safer if someone else did it," or, "I'd like to do more of..." or, "so-and-so often does that for me and enjoys it. It could be moved to their task list."
Of course, there will be those jobs that absolutely no one wants to do, but there may be someone more willing or better suited to perform the tedious jobs. The wish list also allows for training considerations - if someone wants to try a job they don't yet have the skill set for, consider training them. You'll have willing back-up when the regular person is off sick, and staff who feel they are nurtured in their roles.
