I am having a small extension at the back of my house, extending the dining room another six foot. Am i able to use a second hand double glazed rooflight. I am loathed to use new when perfectly serviceable used items are available at the fraction of the cost. I realise that they need to meet certain building regulations on the alledged "U" value but how do the inspectors know what the "U" value is when the window is fitted and there is no evidence of any markings on the glass.
Your help would be appreciated.