The viaduct is so big and so high that contractor Birse Rail has to bring in a combination of abseilers and a fleet of large powered access kit.
The 27-arch viaduct spans 550m across the river Mersey, a motorway and several smaller roads. Built in 1840, it stands 34m high.
Stockport viaduct was renovated in 1989 at a cost of £3m. “Since then buddleia has taken hold, and while it may be very good for butterflies, it can cause us real problems,” said Network Rail route director Jo Kaye.
“Its prominent position in the town centre and under the flight path to Manchester Airport means it is seen by millions of people, so we want to clean it up and give a good impression of both the railway and Stockport as a whole.”
During the project, vegetation will be removed and treated to prevent its re-growth, and the brick and stone work repaired.
Work on the project starts on Monday 8 August and is expected to take 14 weeks.