A Welsh farmhouse that was once in such poor condition that rainwater ran through its rooms is in fact an exceptionally rare 600-year-old medieval hall house, it has been confirmed, after conservation experts used a groundbreaking new dating technique originally developed by climate change scientists.

Llwyn Celyn, which lies in the Black Mountains on the border of England and Wales, was completed in 1420, an analysis of its timbers found, making it one of only a tiny number of domestic buildings to survive from one of the most destructive periods in Welsh history, immediately following the failed revolt of the Welsh prince Owain Glyndŵr.

Conservation experts from the Landmark Trust, who first encountered the building in a perilous state of disrepair but still inhabited by two farmers in 2007, initially believed it dated from much later in the 15th century. But repeated attempts to date its ancient timbers with tree ring analysis failed, in part because the technique is less effective on trees that have grown in a wet climate.

Facebook Twitter Pinterest The house had been found in a state of disrepair. Photograph: Paul Highnam//Royal Commission on the Ancient and Historical Monuments of Wales

Instead, they turned to a technique developed in the geography department at Swansea University. Never before used on an undated historic building, it analyses the oxygen, hydrogen and carbon isotopes preserved within the cellulose of a tree’s rings to determine the climate conditions in which the tree grew.

Each ring has a distinctive isotope signal, which can be used to determine very precisely the age of the timber, even on samples that would be undateable by conventional methods.

The new technique will potentially be “transformative” for the dating of historic buildings and timbers back to the arrival of the Romans, and potentially into the Bronze Age, according to Neil Loader, a professor of geography at the university.

“What is also important,” he says, “is that every timber we analyse and date does not just tell us the age of the sample, important though that is; it also provides a record of the climate experienced by that tree through time, and so in dating a sample we are also enhancing our understanding of the climate of these islands.”

Caroline Stanford, a historian and head of engagement at the Landmark Trust, said the building was “the most important at-risk building in Wales” when the trust began a painstaking process of restoration, partly thanks to a grant from the Heritage Lottery Fund.

“Even in that gloom and dereliction, there were things shining through that said, this is very special, in particular some decorative wooden door heads. But it was also the fact that it didn’t seem to have changed at all since a ceiling was put into the hall, which we thought was some time in the 17th century.”

Facebook Twitter Pinterest The kitchen in the renovated Llwyn Celyn farmhouse. Photograph: John Miller/Landmark Trust

In fact, further investigation of Llwyn Celyn and its timbers revealed that the once open hall had been altered to include an upper floor in 1690, but the fixed bench on which the original lord of the hall would have sat at his high table was still in place, almost six centuries after it was first installed. The building has now been fully restored and is available to rent through the Landmark Trust.

Stanford said the application of the Swansea technique was “a hugely important breakthrough”. She said: “Isotope research is transforming our understanding not only of historic buildings but of archaeology as well. It’s an absolutely fascinating crossover between science and the humanities, and transformative in our understanding [of buildings], but also in our understanding of the planet.

“From the point of view of a buildings historian, it is breaking us out into the sunlight of a much bigger world, in terms of our understanding of how humanity has evolved.”