Can I require my employees to wear masks?
Unless an employee has a legitimate medical reason why they cannot wear a mask you can require their use. In this environment there is really no difference between a face mask and safety boots or a hard hat. Employees can be disciplined and ultimately terminated if they refuse to comply. If you have a health and safety committee hopefully the requirement of wearing a mask will come with their endorsement.
The more sensible you are about where and when masks have to be worn the more you are going to get compliance without disciplining anyone.
If there are tasks that can be performed at a distance for an extended period of time, requiring a mask would be unreasonable. It will take a while, however, for everyone to remember to put it back on where appropriate. Hopefully your employees have a place where they can get outside during their breaks and take off their masks.
It is up to you to provide the masks. When you contact employees for their recall to work you should set out in writing the new working conditions and procedures so they know what to expect before they even walk in the door. Most of them will be comforted that you are proactively dealing with the issue