Every state has different requirements for who has to have workers’ compensation insurance. In California, all employers must have workers’ compensation insurance coverage, even if there is only one employee.
Sole proprietors are not generally required to have workers’ compensation insurance coverage. If the sole proprietor is a roofer, then they are required to carry workers’ comp coverage for themselves, even if they have no other employees.
Workers’ compensation is also required for part-time employees. How many hours a person works has no bearing on the employer’s workers’ comp requirements.