The department of transportation is hereby authorized to assist in the rehabilitation of existing wooden covered bridges upon the state secondary and town road systems in the proportions set forth under RSA 234:10 and 11, for the following purposes:
(a) Replacing of floor beams and reflooring.
(b) Reroofing.
(c) Repair or replacement of truss members or wooden arch members.
(d) Replacement or repair of piers, abutments and wing walls.