Historic windows at some of the country's most important sites – including one in Tynedale – are under threat from heat, rain and a skills shortage, English Heritage has warned.

The charity looks after about 13,000 windows at locations including the Roman town of Corbridge in Northumberland, the site of the Battle of Hastings in East Sussex and the home of Charles Darwin in Bromley.

More than £1 million is needed for repairs over the next five years, English Heritage said, as it launched an appeal to support conservation of the windows.

It said the varied craftwork involved - from medieval stone and timber, Tudor casements and Georgian sashes, to Victorian windows with elaborate ironmongery - is a time consuming task requiring highly specialised experts.

But a heritage skills shortage poses a threat to the windows, according to the charity.

The weather is also having a detrimental effect, English Heritage said.

More heat can cause window frames to blister and crack while more rain can also damage buildings and fittings.

And working with modern materials such as eco-friendly paints and weaker woods can also damage older windows, the charity said.

Nicola Duncan-Finn, English Heritage's senior estate manager, said: "When thinking about caring for an historic building, the protection of windows may not be at the forefront of many people's minds, but these features are important artefacts in themselves.

"Windows are the eyes of a building and can tell you a huge amount, not only about the status of its owners, trends of the time and the materials available, but uniquely they also bear the hand of their original carpenter and so are irreplaceable.

"We can't just replace them with a modern alternative, so if they are to survive for future generations to enjoy, we need the public's support today in order to repair their wooden frames, stabilise their glass and lead fittings, and protect them from harsh weather and potential rot before time runs out."

To support the appeal, visit www.english-heritage.org.uk/windows