The allowable amount of glazing (window) in a fire rated door also gets discussed often in the field. Many Owners want to make changes in the door glazing during construction. So you should know the rules of thumb on fire door glazing. 3 hour fire doors can't have glazing, 1 1/2 hour fire doors can have a maximum of 100 square inches of glazing and 3/4 hour fire doors can have a maximum of 1,296 square inches of glazing. Again, there are code variations for certain circumstances, but these are the normal case rules of thumb.