Do I need to cover independent sub-contractors for workers compensation?

No, you are not required to cover independent sub-contractors for Worker's Compensation.  But, if the independent sub-contractor does not have a Worker's Compensation policy in place to cover their business they would be automatically covered under the hiring company's Worker's Compensation policy.  It is important to request a certificate of insurance showing a Worker's Compensation policy is in force from all independent sub-contractors prior to allowing them to perform work.   

Independent sub-contractors who have no employees or who are not required to be insured may buy a policy to cover themselves.