STAFF at an eighteenth century manor hall have got used to working in the dark. The rules laid down at Ormesby Hall, a sprawling 18-room mansion near Middlesbrough, firmly dictate that less than an hour of daylight can penetrate the centuries-old rooms during the winter months.

"Everything here is conservation-led," explains house steward Garry Savage. "The paintings and fabrics are delicate and can deteriorate so quickly that we only allow 1,000 hours of light into the rooms each year. We try to save most of them for the summer when we have visitors, so if the shutters are opened in winter, the time is always monitored and they are closed again as soon as possible."

Light restriction has been honed to a fine art by staff working to protect the hall's valuable paintings and furniture. Windows are tinted to keep out ultra-violet rays and light readings are regularly taken near the most delicate items in each room.

The entire indoor environment is firmly regulated, with the humidity controlled and each radiator programmed to keep temperatures between 5C and 15C.

"The radiators are always switching on in the middle of summer to control the humidity," said Mr Savage. "A lot of visitors think it's a fault and let us know, especially when it's warm outside, but everything is designed to protect the rooms."

The main part of the three-storey house dates back to 1740 and was built over five years for James and Dorothy Pennyman in the grounds of the estate bought by the family in 1600.

Future generations extended the house and the estate's Old Wing, which was built in 1601, by adding a second storey and building a connecting link between the wing and the house.

The property remained in the Pennyman family for 12 generations, before being left to the National Trust by Col James Pennyman on his death in 1961. His wife, Ruth, continued to live in the house until she died in 1983.

House manager Ian Brockbank said: "Much of the house is just as it was when Ruth Pennyman lived here. The rooms reflect the style of the Fifties and really give an insight into the family's tastes. They weren't terribly wealthy, but there are lots of interesting features. The property has got so many artefacts that it feels like the family has just got up and stepped out."

The house opens to visitors each year from April to November, with staff using the winter months to clean, restore and prepare for the tourist season.

"Most people think that winter is our quiet time," said Mr Brockbank, who has worked at the house for eight years. "But it's at least as busy as the summer, just in different ways."

Work begins each November to put the house into its winter state. Mr Savage is joined in the task by senior housekeeper Brenda Baggot, who has worked in the house for 27 years, and was the housekeeper for Mrs Pennyman.

Her sister-in-law, Jean Baggot, who has worked in the house for 20 years, and Linda Putson, who has been there for ten, complete the group.

The team work along the visitors' route, cleaning each of the hall's 18 rooms from the ceiling down. They do an inventory of the contents of each room, before cleaning and covering each item with a cloth. A patch test is carried out on every fabric before a vacuum is used, and only very mild cleansers are used to help preserve the items.

Mr Brockbank said: "The women have been here a long time and know how things are done, so they get through it like clockwork. The housekeeping team is the one thing we never have to worry about, because we know the job will always be done."

The intricate tasks required have inspired the team to create new ways to cut down on cleaning time.

"We have to wash each light and piece of glass on the chandeliers, which is quite time-consuming," said Mr Savage. "Jean was tired of fiddling around with cloths while balancing on a ladder, and so she devised a new cleaning method which involves wearing a damp cloth glove and just running her fingers over the chandelier."

Throughout the year, the estate employs six full-time members of staff, who are joined over the summer months by eight full-time extras and more than 200 volunteers serving as day leaders, exhibit guides, education leaders and tea room staff, among other things.

During the summer, the estate's 275 acres are used for orienteering challenges, educational tours, weddings and outdoor concerts.

More than 20,000 guests visit the hall each year to learn how the Pennymans lived over the last three centuries, tour the grounds and see the three intricate model railway villages on display.

"A lot of hard work goes into running the hall by the staff and especially by the volunteers," said Mr Brockbank. "There are always things going on, so it's a lot to prepare and organise, but it's worth it because we know that when people come along they always enjoy themselves. It's a lovely place and is staffed, supported and visited by people who all think it is a brilliant piece of history and worth every second of the time and effort that is invested in it."