Essential work is to take place on the A5 River Ceiriog Viaduct to replace the major bridge deck movement joint.
The works, which are being carried out in consultation with Highways England, will begin on 22 February and are programmed to take eight days.
Built in 1990, the viaduct spans 500m from the northern side of the Ceiriog Valley in Wales to its southern side in England. After 30 years, the large expansion joint at the southern end of the viaduct is reaching the end of its operational life and must be replaced.
The works will include the demolition and re-construction of the reinforced concrete viaduct where it joins its support on the southern side of the Ceiriog Valley. Planned routine maintenance and bridge investigation works will also take place to make the most of the closure. Highways England will also carry out work on their section of the road between the southern end of the viaduct and the Gledrid roundabout.
The A5 will be closed over a length of 3.6km between the Halton and Gledrid roundabouts for the duration of the work, with traffic diverted through the town of Chirk along the B5070.
Temporary traffic lights will be installed on the B5070 Chirk Road bridge over the River Ceiriog. The bridge must be restricted to single lane working for the duration of the works to ensure that the bridge can carry heavier vehicles without being over-loaded.
To minimise disruption the work will be carried out 24 hours a day at a time of year when traffic flows are historically lower. The work site is small enough to be protected against the worst of the winter weather, reducing the risk of adverse weather delaying the completion of the project.
Road users can find out the latest traffic information and project updates on the Traffic Wales website or Twitter channel.
The planning for this work has been agreed with Wrexham and Shropshire Councils.
Transport Minister Ken Skates said: “It is essential that this work is carried out to maintain the condition of the A5 River Ceiriog Viaduct. Every effort is being made to minimise disruption as much as possible and to complete the work as quickly as possible.
“Planned full closures of carriageways are never taken unless they are absolutely necessary, but the safety of the travelling public is paramount and this work needs to take place to ensure that.
“I would urge the travelling public to follow the diversion route signage while this work is in progress, and I thank them and local residents for their co-operation and patience.”