An employer is liable for the negligence of a contractor:

(1) When the work is wrongful in itself or, if done in the ordinary manner, would result in a nuisance;

(2) If, according to the employer's previous knowledge and experience, the work to be done is in its nature dangerous to others however carefully performed;

(3) If the wrongful act is the violation of a duty imposed by express contract upon the employer;

(4) If the wrongful act is the violation of a duty imposed by statute;

(5) If the employer retains the right to direct or control the time and manner of executing the work or interferes and assumes control so as to create the relation of master and servant or so that an injury results which is traceable to his interference; or

(6) If the employer ratifies the unauthorized wrong of the independent contractor.