A sliding window moves horizontally within its frame instead of swinging out. Nothing projects into the space outside — which is the whole point where a window opens onto a footpath, a balcony, a light well or a neighbour's boundary.
If an outward-opening casement would swing over a public footpath, it is a hazard and in many cases a planning problem. If it would foul a balcony rail, a fire escape route or a passing pram, the same applies. A horizontal slider removes the issue entirely.
Sliders are also the sensible answer for very wide, low openings — a 2.4m wide, 1.1m high opening looks and works far better as a two- or three-pane slider than as a row of casements.
Historically, sliding windows sealed worse than casements, because a casement compresses its gasket when it locks and a slider only brushes past it. Modern aluminium sliders solve most of this with interlocking meeting stiles, multi-fin brush seals and, on premium systems, a lift-and-slide mechanism that drops the sash onto a compression seal when locked.
Expect a slightly higher air permeability figure than an equivalent casement. On an exposed or coastal elevation, that difference is worth thinking about before you specify.
| Job | Typical cost |
|---|---|
| Casement window, approx. 1800 × 1200mm supplied & fitted | £980 – £1,450 |
| Sliding patio door, 2 panes, approx. 3m supplied & fitted | £3,000 – £4,500 |
Ranges include VAT and installation. Final price depends on size, spec, access and site condition — a survey gives you a fixed number.
A modern slider with interlocking meeting stiles and multi-fin brush seals performs well, but it will typically have slightly higher air permeability than a casement, which compresses a gasket when it locks. On very exposed elevations, specify a lift-and-slide mechanism, which pulls the sash onto a compression seal.
Yes — restrictor locks let you hold the sash open by a set amount and lock it there, which is the usual solution for ground-floor rooms.
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