New tiles being fitted to the roof of a 15th-century moated manor house have been specially adapted to stop bats slipping off.
Conservationists selected tiles to look the same as those used at the National Trust’s Oxburgh Hall in Norfolk more than two centuries ago, which have become weatherworn, cracked and damaged.
But they discovered that bats slipped off the black glaze on the new pantiles.
Experts found, through testing, that a coating of paint mixed with sand of different sizes enabled the nocturnal creatures to grip with the tiny claws on their thumbs and feet.
Builders are using this non-slip coating on tiles around 32 new bat openings on the roof, which will allow bats to climb into roosts, including in the attics and roof voids.
While bats would not have come to harm from the slippery tiles, they would have lost their safe roosting space in the roof, a National Trust spokesman said.
It is a condition of planning permission for the £6 million re-roofing project that the National Trust protects the bats.
Project manager David White said: “Our survey carried out by local bat experts found numerous signs of brown long-eared bats in the attics and roof spaces right across the hall.
“There were signs of a maternity roost in the past but the current use has been as day and night roosts for numbers of brown long-eared and common pipistrelle bats.
“We have worked with bat experts to create a new roost in the nearby Bell Tower and installed bat boxes in the trees on the north terrace to provide alternative roosting places whilst the roof works takes place.
“Currently the bell in the Bell Tower can’t be rung, so as not to disturb the bats.”
Surveys found a total of six bat species flying close to the moated manor house but it is brown long-eared, common pipistrelle and soprano pipistrelle bats which roost in the building.
The brown long-eared bats use attics and roof spaces, and all three species use crevices under roof tiles, ridge tiles and lead flashing.
The new bat openings around the roof include some under the ridge tiles, some lower on the roof under pantiles, and others on the dormer windows.
Several of the roosts have carefully designed gaps in the roof lining to allow brown long-eared bats to get into the attics and roof voids.
A total of 14,000 new pantiles are being used for the re-roofing project, fewer than the 50,000 pantiles and 800 ridge tiles used in the last re-roofing in the 1770s as the new tiles are larger.
It is only the second roof restoration in Oxburgh Hall’s lifetime.
The project to repair the roof started when a 150-year-old dormer window unexpectedly collapsed in 2016, exposing a structural weakness in the roof of the hall.
As well as resolving the structural problems, the roof has been repaired, all 14 dormers that were added in the 19th century have been dismantled and rebuilt, and the 27 distinctive chimneys, made with moulded hand-made bricks, have been rebuilt.
The project has been made possible thanks to National Trust supporters and grants from the Heritage Stimulus Fund, which is part of the Culture Recovery Fund, the Wolfson Foundation and The National Lottery Heritage Fund.
Oxburgh Hall is currently open to visitors and the restoration work is due to be completed early next year.