The process involves wrapping large sections of the cables in air tight seals. Warm air is then injected along the sections to dry out the cables and remove any moisture which might be accelerating the corrosion process.
Faber Maunsell is carrying out the work, which it is hoped will prevent further corrosion to the main cables.
The consultant’s technical director Charles Cocksedge said: "The sheer scale of the Forth Road Bridge and the size of the cables involved has meant we have had to spend a long time at the design and preparation stage.”
The work has been commissioned by the Forth Estuary Transport Authority following the discovery of corrosion and broken wires when the main cables of the bridge were internally inspected back in 2004.
The two main cables are more than 400 m long. The entire dehumidification project is expected to take up to 18 months to complete.