Specifying external sliding doors

Latest developments in window technology have radically improved the design, performance, and security of fully-glazed external sliding doors for residential use. However, as sliding doors become increasingly sophisticated, specification also becomes more complex. Andy Trewick, VELFAC direct sales manager, comments

Site suitability

To create the large opening required by many sliding door designs, the structure above the door must not transfer any weight or load onto the door when installed. Although most wall types provide adequate fixing grounds, given the area of glazing involved take advice on the type and number of fixings required for the door being installed. A solid base is also crucial, as unstable grounds can lead to issues with door alignment and operation, prompting repeated adjustment post-installation. Load and other calculations must also ensure minimal glass deflection, as excessive movement under environmental loadings can be disconcerting for occupants.

Threshold options

Different door systems feature different threshold or bottom track options. A flush track allows a step-free passage through the door but requires a pocket sunk into the structure for the track, and provision made for water drainage on and below the external floor level. In exposed locations, flush track options can be vulnerable to driven rain and high winds. An alternative is to fit the lower sliding gear in a profile or cill member which sits on top of the structure; this provides a more robust weather solution and is also easier to install. Doors featuring ‘step and slide’ designs (when the door moves inward or outward before sliding across) offer even greater protection as the door usually ‘clamps’ onto gaskets and weatherseals, rather than depending on simple brush seals.

Choosing the ideal design

Three main designs dominate the market – sliding, bi-parting (two panes sliding apart to create a large opening without any central obstruction), and concertina folding. Sliding and bi-parting designs are ideal for most domestic installations, especially those where space is limited (such as a balcony) as very little space is required to house the door when open; folding door designs need adequate room to allow the panes to flex and fold, and to store when open. Sliding doors are available in a variety of frame constructions including composite aluminium / wood frames, which give property owners greater flexibility when specifying internal and external frame finishes.

Improved energy efficiency

The greatly improved energy performance of modern window design means that an external sliding door should no longer be a source of heat loss. However, a slim frame design further improves energy performance as most heat is lost through the window frame, with different framing materials and profile designs having different thermal conductivity values. Slimmer profiles constructed from low conductivity materials are now available to meet increasingly demanding Building Regulations. Low energy glass coatings can also significantly reduce excessive solar gain and improve energy conservation in winter, minimising ‘cold spots’ and enabling full use of floorspace near glazing. To further improve energy efficiency Building Regulations require increasing levels of air tightness – but this shouldn’t compromise ventilation. Trickle vents fitted into the frame eliminate the need to house ventilators in additional profiles as these add more framing, reduce daylight levels and increase thermal losses.

Security and safety

Traditionally considered a weak point in household security, the best sliding doors now feature espagnolette locking mechanisms, locking handles, internal glazing beads (to prevent external glass removal) and anti-lift devices. If the door opens onto a balcony or steps, then childproof locks may be required, while glass thickness must also be suitable for the desired door size, and must break safely in order to meet Building Regulations.

Do your research

Sliding doors are a valuable addition to a property but are also a significant investment. When specifying, look for well-made, solid construction, a smooth and efficient sliding mechanism, and a design that can be adapted to specific performance requirements. Maintenance should also be considered – tracking systems should be easy to keep clean and sliding mechanisms protected from the weather, while frame and finish should not need a comprehensive maintenance routine to ensure long lasting performance.