Watch: How do you clean a wall of 900 windows?

It takes window cleaners three days to scrub the outside of the Willis building, Ipswich town centres famous Grade I-listed landmark - and they were hard at work today.

The team have to use specialist brushes with telescopic handles that can reach up to 100 feet when fully extended to get the job done.

This means even the top floor can be cleaned from ground level - a requirement as the 890 smoked glass curtain windows that make up the building’s exterior can only be accessed from the floor or roof.

Dean McKinnon, a window cleaner with The Price Group, explained why the obelisk-esque office could be so difficult to clean.

“The black glass is a nightmare in the summer – it dries so quickly in the heat that the glass is left with streaks.

“We can’t just start and work our way around, we have to plan where we can clean and hope the sun doesn’t come out.”

The recent blistering heat has gone just in time for the cleaners, who can scrub up to 300 windows a day.

Even with the lower temperatures, the team still have to start on the west of the building as the sun rises - then pack up and move around to do the east side later as the sun sets.

Cleaning the outside every three months is one challenge, but the window washers also have to do the inside of the window panes every six months too.

The Willis Towers Watson building was built between 1970 and 1975 and serves as an office for almost 1,300 staff.

It was made a Grade I listed building in 1991 - at the time it was the youngest building to be recognised as a building of exceptional interest.