In the mid-20th century, sliding doors became very popular - two or three panels of glass that slide along grooves in the floor. To distinguish them from traditional French doors, they were marketed with the thoroughly modern name of Patio Doors and this is often the image people have today when that term is used. Easily installed in place of a window, the immediate advantages were additional natural light and access to the garden. They also became a popular option to use where a pivot door opening space was limited or where the aperture was wider than a pair of French doors. Older installations were typically single-glazed, prone to warping and usually became difficult to slide open and closed. Still available today but in a developed form with double glazing and rollers for easier sliding, the popularity of sliding doors during this century has declined as bifolding doors gained market share.
Sight lines is the term used to describe the interruptions in the view through the doors; in other words, the width of the vertical opaque areas between the glass when the doors are closed. On hinged doors, such as French doors and bi folding doors, timber and pvc frames generally have broad sight lines because, unlike aluminium, narrower frames would not be strong enough to be fit for purpose. As a guide to the width of two frames together, cheaper pvc door frames can exceed 200mm, aluminium frames are typically between 135mm and 160mm and frameless patio doors sight lines are under 40mm. Depending on the width of each door and number of doors to be installed, the difference in glass to frame ratio could be significant.
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.
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