The selectmen shall personally examine the premises, hear all persons interested, and, after consultation with the department, if they are of the opinion that the mill, milldam or flume ought to be rebuilt or repaired, they shall, by writing under their hands, order the owners to rebuild or repair, specifying the proportion or share of the expense and of the costs of the petition and hearing to be borne by each, and the time, and, if they deem it appropriate, the manner, in which it shall be done. However, no order to rebuild shall be made unless the owners of at least
1/2 of the premises to be rebuilt assent to such order.