Because slide-and-pivot doors have no hinges, there is no requirement for a sturdy side frame; its only purpose is to cover the gasket that seals the double glazed unit. This means that the views afforded through the expanse of patio doors have minimal interruptions. At the time of writing, there are two versions of frame-less glass doors available in the UK, both using the slide-and-pivot technique: one manufacturer supplies their frameless glass doors with kite-marked double glazed units which have a visible seal, the other uses an almost transparent method of sealing their double glazing. Contemporary by design, the absence of visible characteristics makes frameless glass doors a viable option for period properties.
Originally, patio doors were all of the French Door variety; that is, a pair of doors that are hinged at the sides and open from the middle, each door pivoting in- or out-wards. French doors are still popular, especially with builders of new homes, as they are comparatively cheap and easy to accommodate in place of windows and offer an alternative evacuation route for families, especially for elderly or disabled people, to meet improved fire safety requirements.
A visible "kitemark" on the glass is the consumers assurance that the double-glazed unit has been manufactured to British standards. The BSI has numerous standards, including: U-value verification, Window Energy Rating and Window Installation. The lower the U-value, the better the thermal performance and most local authorities will require this to be 1.8 or better to meet building regulations.
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