The crossing has been shut for refurbishment since October last year and was originally due to re-open in April but was delayed twice.
Workmen have been reinforcing the underside with steel girders so it can cope with modern traffic levels.
Engineers are now laying a new carriageway and damp-proofing the structure and the bridge can be used again once utility companies have relaid pipes and cables under the road.
Geoff Weir, secretary of the Whitchurch Bridge Company, said: “We are pleased that our contractors have finished the structural work. We are now waiting for the utility companies to finish their work. This is out of our hands but it’s safe to say it will re-open by September 26.”