Upon the application of a member in service or of the chief of the fire department, any member who has had five years or more of creditable service shall be retired by the board of trustees, not less than thirty and not more than ninety days next following the date of filing the application on an ordinary disability retirement allowance, if the medical board after a medical examination of the member shall certify that such member is mentally or physically incapacitated for the further performance of duty, that such incapacity is likely to be permanent, and that the member should be retired.