If rat or mice are in the shed, then it would be easier to resolve the problem. But if they are outside in the garden, I would find it tricky to deal with the problem due to the other animals presence such as hedge hogs, cats or even dogs, which might be passing the area and get in contact with the poison / wax blocks or traps.
A few year ago when there were signs of mice or rat in my old wooden shed, I used to run petrol lawn mower or petrol hedge cutter engine running in the shed for half hour with the door closed. A few times of the session, the rat / mice seem all moved away from the shed. And recently, I put on the air compressor or shop VAC in the shed often, and use air nail guns to patch up small holes in the shed, or vacuum the shed. The loud noise (98dB) from these devices must have them moved away from the shed, so far no sign of rat or mice in the shed, touch wood.