FENSA-registered glazing experts. Made-to-measure sealed units, shipped UK-wide, backed by a 10-year warranty.
Toughened double glazing — Class 1(C)1 impact rating, critical-location compliant by default
If the pane is in a door, beside a door (within 300mm of the door edge, up to 1500mm high), or in a window with the lowest edge below 800mm from the floor — Building Regs Approved Document N requires impact-rated safety glass. Toughened double-glazed sealed units are the most common, most cost-effective answer. Toughened to BS EN 12150, classified Class 1(C)1 impact, around five times stronger than annealed glass, and they break into harmless granules rather than dagger-shaped shards. Pane Relief manufactures replacement toughened DG units in stock and bespoke sizes, with optional low-E coatings, argon fills, warm-edge spacers, and acoustic or laminated counter-panes.
Replacing a critical-location DG unit
Cracked toughened in a door, sidelight, low window, or stairwell? You can't legally replace it with non-safety glass. We ship UKCA-marked toughened DG units with the BS EN 12150 kitemark laser-etched in the corner, ready to drop straight in. Free UK mainland delivery, 10-year edge-seal warranty, helpline 08:00-18:00 Mon-Fri.
Configure my unitInstaller, builder, FENSA registered
Net 30 trade accounts, volume tiers on 5+/20+/50+ units, scheduled site delivery, and a full BS EN 12150 + BS EN 1279 Declaration of Performance on every order — what you need for Building Control sign-off without paperwork chasing.
Trade pricing & bulkPick your toughened DG configuration
Six common toughened DG specifications. Pick the closest match; the configurator handles any combination.
Both panes toughened
Standard 4-20-4 or 4-16-4 toughened-toughened DG. Most common spec for doors and sidelights.
Toughened + low-E
One toughened pane + one low-E coated toughened pane. Energy-efficient critical-location unit, A/B rated.
Toughened-laminated
Outer toughened + inner laminated. Belt-and-braces safety. Schools, hospitals, balustrades, glazed entrances.
Oversized toughened DG
Panels up to 2400×1200mm stock. Patio doors, bi-folds, large picture windows. Toughened-toughened only beyond 1.6m².
Obscure toughened
Toughened obscure (Pilkington Texture range) for bathrooms, en-suites, stairwells. P5+ privacy ratings.
Heat-soaked toughened
BS EN 14179 heat-soak treatment. Overhead, balustrade, frameless, and any critical-safety application.
Heat-soak testing and the nickel-sulphide question
Toughened has one well-known failure mode: rare nickel-sulphide inclusions causing spontaneous breakage years after installation. Here's when to specify the heat-soak upgrade.
What heat-soaking does
The toughened pane is held at 290°C for two hours in a calibrated oven per BS EN 14179. Any pane with an unstable nickel-sulphide inclusion fails in the factory rather than in your window five years later. Heat-soaked toughened carries a lifetime no-spontaneous-breakage warranty.
Cost premium: typically 15-20% over standard toughened. Lead time adds 3-5 working days.
When to specify heat-soaking
- Mandatory: overhead glazing, balustrades, structural glazing, frameless glass walls.
- Strongly recommended: any glazing above ground floor where a spontaneous fall would endanger pedestrians.
- Recommended: high-value or hard-to-access installations where replacement labour cost dwarfs the heat-soak premium.
- Optional: standard residential ground-floor windows and doors — the spontaneous-breakage probability is around 8 per million panes per year, so most homeowners don't specify it.
Toughened double glazing — common questions answered
Q: Can I drill, cut, or sand toughened glass after manufacture?
No — and this is the single most-broken rule on every job. Toughening is the final step in manufacture: the entire pane is in a state of compressive stress on the surfaces, balanced by tensile stress in the core. Any breach of that surface — drilling, scoring, edge-grinding, deep scratching, even some types of permanent marker pen if alcohol-based — releases the stress and the whole pane explodes into granules. All cut-outs, holes, edge polishing, bevels, and shaped profiles must be specified before the pane enters the toughening furnace. Send a CAD or paper template at order; lead time on shaped toughened DG is 10-14 working days.
Q: How do I tell if my existing glass is toughened?
Look for the kitemark — typically laser-etched or sand-stamped in the bottom-right corner of the pane, reading "BS EN 12150" or older "BS 6206-A" with a triangle BSI logo. If there's no mark, look for the polariser test: stand a few metres back with polarised sunglasses on; toughened glass shows distinctive iridescent leopard-spot patterns where the heat-treatment rollers contacted the glass during cooling. If both methods are inconclusive, send us a photo of the corner and we'll identify it from a glance.
Q: Is toughened DG the same as laminated DG for safety purposes?
Both meet the safety classification (toughened = Class 1(C)1, laminated typically = Class 2(B)2 or 1(B)1) but they behave very differently on impact. Toughened breaks into thousands of small harmless granules — perfect for low-level glazing, doors, sidelights. Laminated breaks but holds — the shards stay glued to the PVB film, the pane stays in the frame. Laminated is the only legal spec overhead (toughened granules from above are still a hazard). For everything else, toughened is the default and cheaper choice; step up to laminated if retention matters (security, balustrade, school, hospital).
Q: Can the inner pane be annealed if the outer is toughened?
Only if the inner pane is not in a critical location of its own — and the outer pane meeting the safety requirement is enough to comply. In practice, for door glass and low windows we strongly recommend both panes toughened (the configurator defaults to it): the cost delta is small, the hazard from a broken inner pane is the same as a broken outer pane, and any insurance assessor or Building Control inspector will look for matching safety marks on both faces. The exception: toughened-laminated combo, where the inner is intentionally laminated for retention.
Q: Are toughened DG units heavier than standard annealed DG?
No — toughening is heat treatment, not added mass. A 4-20-4 toughened DG unit weighs exactly the same as a 4-20-4 annealed DG unit (around 20 kg/m²). Where weight does go up is on toughened-laminated combos (laminated layer adds PVB mass) and on triple-glazed builds. We flag pane weight on every product spec sheet and the configurator output.
Q: Will toughened DG impact U-value or solar performance?
No. The toughening process doesn't change the glass's thermal or optical properties — a 4mm toughened pane has identical U-value, light transmittance, and solar gain to a 4mm annealed pane. The U-value of the sealed unit is driven by the spacer, gas fill, and low-E coating, not by whether the panes are toughened. So you can spec a toughened low-E argon-filled unit (A-rated) for a critical-location window and lose nothing on energy performance vs the non-safety equivalent.
Q: What's the lead time and warranty on toughened DG units?
Stock sizes: 7-10 working days. Bespoke rectangular: 10-12 working days. Shaped, oversized (>1500mm any side), or heat-soaked: 14-18 working days. Every unit ships with a 10-year edge-seal warranty (BS EN 1279-2 weathered durability), a 5-year warranty on low-E coatings, and on heat-soaked toughened, a lifetime no-spontaneous-breakage warranty. UKCA, kitemark, and Declaration of Performance available on request.
Q: Can I replace a toughened unit with non-toughened if the building was built before current regs?
Legally — replacement glazing is treated as new work under Approved Document N, so the current safety rules apply regardless of when the building was constructed. Practically — a competent glazier will always replace like-for-like or upgrade. If the existing pane has a kitemark and is in a critical location, the replacement must also be safety-rated. Replacing toughened with annealed in a doorway is a Building Control infraction and a hard-to-defend claim if anyone is injured.
Related guides and specifications
Compliant by default — toughened DG ships kitemarked
Need a non-standard size, shape, or heat-soak spec? Call 0117 330 3057 (08:00-18:00 Mon-Fri) or send drawings.