Door Glass Units — UK Standards, Specifications, and Replacement Glazing for Front, Patio, French, and Bi-Fold Doors
Door glass is the most safety-critical glazing in a typical UK building. Every glazed door panel sits in a "critical location" under Approved Document K, which means the glass must carry an impact classification — toughened, laminated, or toughened-laminated to BS EN 12150, BS EN 14449 or BS EN 14179. This page covers the standards that govern door glazing, the typical specifications for composite front doors, French doors, patio sliders and bi-folds, and how to choose between toughened and laminated for each opening type.
Standards, classifications, and critical-location rules for door glass
Door glazing is governed first and foremost by Approved Document K — Protection from falling, collision and impact (consolidated 2013 edition, still current). Section 6 defines a "critical location" as any pane within 1500 mm of finished floor level in or beside a door, or within 300 mm of a door edge below that height. Every glazed door panel — front, back, internal, patio, French, or bi-fold — falls inside this envelope, so the glass must carry an impact classification to BS EN 12600.
The two compliant glass types are thermally toughened (BS EN 12150-1:2015 + A1:2019 — granular fragments, classified 1(C)1) and laminated (BS EN 14449:2005 — PVB interlayer holds fragments, classified 1(B)1). A third specification, heat-soaked toughened to BS EN 14179, is increasingly used for high-value or hard-to-access door glazing because it reduces the risk of nickel sulphide spontaneous fracture — a rare mode where toughened glass shatters without external impact, sometimes years after installation.
Older references in some local Building Control templates still cite Approved Document N (Glazing — safety in relation to impact, opening and cleaning). Approved Document N was formally withdrawn in England on 6 April 2013, its provisions absorbed into Approved Document K. It remains cited in some regional and trade documents, so a specification naming "Approved Document N" is out of date in England but not strictly wrong. Wales, Scotland and Northern Ireland have their own equivalent technical standards.
Safety classification under BS EN 12600:2002 uses a three-part code α(β)φ, e.g. 1(B)1. The first digit is drop height the glass survives without injury-class failure (1 = 1200 mm, most demanding). The letter is fracture mode (B = laminated; C = toughened). The legacy classification BS 6206 (Class A / B / C) is still found on older glass: Class A broadly equivalent to 1(B)1 / 1(C)1. Approved Document K Table 6.1 cross-references both standards.
For new dwellings, Approved Document Q (Security in dwellings) adds a separate obligation: door assemblies must satisfy PAS 24:2022 as a complete frame-glass-locking system, with laminated glazing typically on the attack face. Approved Document Q also governs letterbox cutouts: the plate must restrict reach-through access, sitting at least 400 mm from any thumb-turn lock and fitted with an internal restrictor cowl.
Specification predicates buyers compare on
The attributes below are the buyer-intent predicates we see most often in door-glass enquiries, FENSA replacement orders, and trade specifications. Use them to compare like-for-like across suppliers and to brief Building Control on what you intend to install.
| Predicate | Typical range | Where it matters |
|---|---|---|
| Impact classification (BS EN 12600) | 1(C)1 toughened, 1(B)1 laminated | Mandatory for every glazed door panel (Approved Document K Section 6) |
| Toughening standard | BS EN 12150-1 (standard); BS EN 14179 (heat-soaked) | Heat-soak reduces nickel sulphide fracture risk on high-value doors |
| Lamination standard | BS EN 14449; PVB 0.38 mm – 1.52 mm | Required for overhead glazing; recommended for security-rated doors |
| Composite door insert IGU thickness | Typical 24 mm sealed unit, 4–14–4 or 4–16–4 low-E + argon | Standard composite door slab cassette — verify slab manufacturer's groove depth |
| French / patio door pane size | 500–800 mm × 1800–2100 mm typical leaf glazing | Toughened 4 mm sufficient below 800 mm width; 6 mm above |
| Bi-fold door pane size | 800–1000 mm × 2000–2400 mm per leaf | Toughened-laminated common for large bi-folds (residual barrier on fall risk) |
| U-value (sealed unit) | 1.2 – 1.4 W/m²K (low-E coated, argon, warm-edge) | Building Regs Part L; FENSA replacement compliance |
| Acoustic rating Rw | 30 – 42 dB depending on build-up | Front doors on busy roads; patio doors near garden traffic |
| Letterbox cutout (composite / timber doors) | 250–280 mm × 40–70 mm aperture, ≥400 mm from lock | Approved Document Q for new dwellings; TS 008 retrofit guidance |
| Edge treatment | Ground arrissed; polished for visible-edge bi-folds | Required on toughened for clean cut; polished improves perceived quality |
| Critical-location threshold | 1500 mm from finished floor level | Approved Document K Section 6 — applies to every door pane and side panel |
Door glazing by opening type
Front-door composite replacement
The most common door-glass enquiry Pane Relief handles is a cracked decorative pane in a composite front door. Composite doors (GRP skin, foam core, timber subframe) are supplied with a pre-machined slab and a cassette glazing system that accepts a sealed unit of a defined thickness — almost universally 24 mm for UK composite doors, though some premium slabs use 28 mm or 44 mm triple-glazed cassettes. The IGU is normally a 4–16–4 toughened-low-E-argon sealed unit with a decorative interlayer. Both panes must be impact-classified because Approved Document K does not distinguish between inner and outer face in a critical location.
When replacing, capture from the customer: (1) cassette thickness measured edge-to-edge; (2) sight-size width × height (visible glass area, not aperture cutout); (3) decorative pattern and obscure level (1–5 BS scale); (4) any directional features (bevels, leaded designs) with a "right way up"; (5) the door slab manufacturer if known, because some slab profiles have proprietary cassette grooves. The glass calculator walks customers through this; the door-glass-units collection lists the most-ordered cassette sizes.
Patio sliding doors and French doors
Patio (sliding) and French (hinged, double-leaf) doors have different failure modes. Sliding patio doors carry glass weight on a bottom roller and bumper; the spec must leave adequate edge cover (typically 12 mm minimum) so the gasket seats without point-loading the toughened edge. Toughened 4 mm sealed units (4–16–4 or 4–20–4) cover most domestic patio leaves up to about 1100 mm × 2100 mm; above that, 6 mm toughened manages deflection and wind load. French door leaves are typically narrower (500–800 mm), so 4 mm toughened remains adequate, but the leaf height (up to 2100 mm) is a heavy unit on a single hinge stack — hinge rating matters as much as glass thickness.
For acoustic-sensitive patio installations, an acoustic-laminated build-up — 6.4 mm acoustic-PVB laminated outer + 4 mm toughened inner — delivers Rw 36–38 dB versus 30 dB for a standard toughened 4–16–4. See acoustic glass for Rw vs build-up data.
Bi-fold doors and commercial-opening glazing
Bi-fold door leaves are typically the largest single-pane glazing in a domestic specification — 800–1000 mm wide × up to 2400 mm tall. Two issues emerge at that scale. First, fall-from-height: a tall pane over a low cill can act as a falling-impact barrier, so BS 6180 requires the glass to retain barrier function after fracture — pushing the spec toward toughened-laminated rather than toughened alone. Second, thermal stress: large south-facing panes pick up solar gain on the unshaded half, and that thermal differential is a known nickel-sulphide fracture trigger — making heat-soaked toughened to BS EN 14179 the right choice for premium bi-folds.
Commercial bi-fold openings (cafés, hospitality, school halls) add: BS 6262-4 manifestation markers at 850–1000 mm and 1400–1600 mm, the Equality Act 2010 walking-into-glass obligation, and on public-facing installs a P1A or P2A security-rated outer pane to deter casual vandalism. See safety and security glass.
Toughened vs laminated vs toughened-laminated — for door glass
For door glazing specifically, three options dominate the spec sheet: toughened, laminated, and toughened-laminated. The choice is rarely between toughened and laminated in absolute terms — it is about which failure mode you want at the specific door location.
| Type | Classification | Failure mode | When to specify for doors |
|---|---|---|---|
| Toughened | BS EN 12150-1; 1(C)1 | Granular pieces; no residual barrier | Default for composite front-door cassettes, French door leaves under 1.8 m, internal half-glazed doors |
| Heat-soaked toughened | BS EN 14179; 1(C)1 | Granular; reduced nickel-sulphide risk | Premium bi-folds, south-facing large panes, hard-to-access overhead door glazing |
| Laminated | BS EN 14449; 1(B)1 | Cracks but held by PVB interlayer; residual barrier | Overhead door glazing, listed-property door restorations, PAS 24 attack face |
| Toughened-laminated | BS EN 12150 + 14449; 1(B)1 | Granular fracture + PVB retention | Large bi-folds where both fall-protection AND blunt failure required; commercial high-traffic doors |
The critical-location rule (Approved Document K, 1500 mm from FFL) means none of these is "more compliant" than the others — all four meet the impact classification. The decision is driven by the secondary risk: do you need a residual barrier after fracture (laminated or toughened-laminated)? Or is granular blunt failure acceptable and cost-down the priority (toughened)? For a typical composite front door at street level, toughened is correct and adequate. For an upstairs Juliet bi-fold over a balcony, laminated or toughened-laminated is correct.
Standards bodies and regulatory references
This page references the following UK standards bodies, regulators, and trade associations. Their published documents are the authoritative source for any door-glazing specification decision.
- Approved Document K — Protection from falling, collision and impact — the primary statutory reference for door-glass impact safety in England. Section 6 defines the critical-location envelope (1500 mm from FFL, 300 mm from a door edge).
- BSI Group — publisher of BS EN 12150 (toughened), BS EN 14449 (laminated), BS EN 14179 (heat-soaked toughened), BS EN 12600 (impact classification), and the legacy BS 6206 classification. Available individually or via BSI Knowledge subscription.
- FENSA — Fenestration Self-Assessment Scheme. Installation by a FENSA-registered firm self-certifies compliance with Approved Documents K and L for replacement door and window glazing in dwellings. Pane Relief supplies through FENSA-registered installers.
- British Board of Agrément (BBA) — third-party certification for proprietary door systems and PAS 24 frame-glass-locking assemblies. BBA-certified composite door slabs include the IGU cassette compatibility in their certificate.
- Glass and Glazing Federation (GGF) — UK trade body. The GGF Glazing Manual is the working reference for door-glass thickness selection, edge cover, and gasket specification.
- Planning Portal — consolidated guidance on Approved Document Q (Security in dwellings — letterbox restrictions, PAS 24 requirements) and the equivalent technical standards for Wales, Scotland, and Northern Ireland.