OWNERS of historic properties risk being conned out of vast sums of money due to “rogue firms” fitting substandard illegally designed windows, a leading industry figure has warned.

Alex Gray, chairman of Glass and Glazing Federation Scotland and managing director of Blairs Windows, a company specialising in timber windows for heritage properties, sounded the alarm following a “spate of cases” across Scotland and the UK.

Window manufacturers are now calling for a crackdown on the “epidemic”, amid claims local authorities are abetting the problem through the planning process.

Historic property owners, including homeowners in conservation areas, are frequently told replacement windows must look the same as those originally fitted.

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But with many residents wanting to move from single to double glazing, it is claimed some firms are placing non-compliant glazed units inside frames not designed to cope with their size and weight. This then leads to misting and failing in the months after installation.

Mr Gray said there needed to be a “clamp down” across the industry, and insisted Edinburgh is particularly badly hit.

He said: “We have seen this problem going on for too long now and it is an epidemic that needs to be cured by collective action from trading standards, local authorities and the industry.

“Property owners are being unwittingly conned out of huge sums of money and we are seeing examples of lottery money being sunk into restoration projects that are destined to fail.

“Despite firm guidance from Historic Environment Scotland, I know of three major public projects in Scotland that appear to be heading down this non-compliant route despite the advice of industry experts.

“There needs to be a clampdown on what is going on, and consumers need to know where to turn to ensure they are not caught out and lose thousands in the process.”

Mr Gray said he first reported the issue to trading standards at Edinburgh Council in September 2017.

Louisa Gardiner, from West Linton in the Borders, is among those who say they have been affected.

The 54-year-old has been engaged in a five-year battle after ordering 26 windows at a cost of more than £20,000 before installation.

After fitting, Ms Gardiner says a number of the windows started to mist over within weeks, with beads of water visible between layers of glass.

She approached trading standards at Borders Council in May last year but says she is still involved in a protracted fight to get money back.

She said: “We were advised that windows had to be in keeping with what was originally fitted in the property. The problems started soon after installation and the company supplying the windows blamed the installation but it soon became clear that the units themselves were substandard.

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“We were unsuccessful in a legal claim but we continue to pursue the manufacturer regarding a Declaration of Performance certificate that was never issued to us and would confirm whether the windows were compliant with legislation or not.

“Trading standards have been helpful but we remain thousands of pounds out of pocket with a property that now requires replacement windows again. This is in addition to the time and stress spent fighting our case.”

Ms Gardiner said she wanted to prevent anybody else suffering the same problem.

She added: “It’s 2019 now. It’s five years down the line. It’s annoying, that’s for sure. I would love to get windows that we can see through.

“We have beautiful views – but there are some rooms you can’t see the beautiful views.”

Historic Environment Scotland said it publishes guidance which sets out the principles that apply to altering the windows of historic buildings on its website.

It added: “The performance of windows that fail is a matter for individual council’s trading standards.”

Scottish Borders Council said its trading standards team are looking into Ms Gardiner’s case.