The A817 Haul Road through Glen Fruin will be closed for five weeks, starting on August 5, it was confirmed today.
Argyll and Bute Council is carrying out repairs to Ballevoulin Bridge, with a £77,000 contract awarded to Diack and Macaulay.
The work will involve replacing the existing bridge waterproofing and surfacing.
It aims to extend the life of the bridge and reduce the risk of deterioration of the running surface.
The road will remain open for local access on each side of the bridge, but other traffic will be diverted via the A814, Rhu Road Higher, West Montrose Street, Sinclair Street, the A818 to Crosskeys roundabout and the A82 at Arden roundabout.
A council statement said this route had been agreed after consultation with police and emergency services, and taking into account feedback from local residents and councillors.
The statement added: “Using the alternative route should not conflict with traffic using the diversion already in place while improvement works are carried out on Helensburgh seafront.
“Every effort will be made to complete the repairs to Ballevoulin Bridge as soon as possible and minimise disruption, and the co-operation of motorists during the repair period is greatly appreciated.”