Just like the bed, a workbench will need a roof over it in order for it to be used. This can be annoying if you are trying to repair your hammer, which you need to use in order to build. When you notice your hammer's durability is getting low, or before you start building any other structures, you should place two vertical wood beams on top of each other at the sides of the workbench. Then place two double-sided roofs over the beams in order to have a make-shift workbench shed. You may also need to place walls on backside of the shed if you are being prompted that the workbench is still too exposed.