Quick answer
A cracked composite door glass unit is a safety and security issue, not just cosmetic, since a compromised pane weakens the door's resistance to forced entry. Most composite doors use toughened glass to BS EN 12600, so a crack usually means impact damage. Replacement of the sealed unit, not the whole door, resolves it.
Overview
A cracked pane needs assessing - laminated glass can hold together safely for weeks, toughened glass typically shatters completely. Either way, the sealed unit will start misting and needs replacement.
Symptoms
- Visible crack.
- Chip.
- Corner damage.
Common causes
- Impact.
- Thermal stress (rare).
Why toughened glass cracks the way it does
Composite door glazing is almost always toughened safety glass conforming to BS EN 12600, chosen specifically because it resists impact far better than standard annealed glass and, if it does eventually fail, breaks into small blunt fragments rather than sharp shards. This is a legal and safety requirement for glazing near doors, and every reputable composite door manufacturer specifies it as standard within the sealed unit.
A crack in toughened glass typically starts at a small edge or corner chip that's gone unnoticed - a knock from a delivery trolley, a stone thrown up by a strimmer, or a slammed door catching the frame edge. Once the toughened surface is breached at a stress point, a crack can propagate slowly over days or appear suddenly as a spiderweb pattern across the whole pane, sometimes with no obvious new impact at all.
It's worth noting that a full spiderweb fracture in toughened glass, while alarming to look at, is the glass behaving exactly as designed - it has absorbed an impact and failed safely rather than shattering into dangerous shards. The unit still needs replacing, but there's no need to treat it as an emergency hazard beyond boarding it if fully fractured.
Distinguishing a crack from a blown unit
It's easy to confuse a cracked pane with a blown double-glazed unit, but they're different faults with different causes. A crack is physical damage to the glass itself, visible as a line or spiderweb pattern you can trace with a finger on the surface. A blown unit, by contrast, shows as misting or a hazy patch between the panes with no visible line on either glass surface - that's a seal failure letting moist air in, not a break in the glass.
Occasionally both occur together, particularly after a significant impact strong enough to crack the outer pane and disturb the unit's edge seal at the same time. In that case the whole sealed unit needs replacing regardless, since a cracked pane can't be repaired on its own within a sealed double-glazed cassette.
Is a cracked pane a security risk?
Yes, and this matters more for composite doors than most people assume. Many composite doorsets, particularly those specified to PAS 24 for enhanced security, rely on the glazing being intact and correctly retained within its beading as part of the overall resistance to forced entry - a cracked or missing pane creates a weak point that undermines that rating even if the door slab and lock are otherwise sound.
For front doors this is worth acting on promptly rather than living with a cracked pane through a full glazing lead time, particularly on ground-floor doors or doors on holiday lets where the property may be unoccupied for periods between guest changeovers.
What we check before quoting a replacement
Before ordering a replacement unit we confirm the exact glazing aperture size, the bead profile fitted around it, whether the original unit was flat glass or a decorative/leaded design, and the glass specification - thickness, gas fill, and any obscured or Low-E coating. Getting any of these wrong means a unit that either won't fit the aperture or looks visibly different from the original once fitted.
We also check whether the crack has caused any distortion to the internal beading or allowed water ingress into the door core around the aperture, since a long-cracked pane left unaddressed through a wet Cornish winter can occasionally lead to secondary problems beyond the glass itself.
Sourcing a like-for-like replacement
Decorative composite door glazing comes in a wide range of designs, from simple obscured or frosted patterns through to leaded, bevelled and coloured lead detail units, and matching the existing design is important for a front door where the glass is a visible feature of the property's kerb appeal. Where the original design is a stocked pattern from a major glass manufacturer, matching is usually straightforward.
Older or discontinued decorative patterns can sometimes need sourcing from a specialist glass supplier rather than a generic stock unit, which can add a little to the lead time. We'll always confirm design matching against a photograph of the existing pane before ordering rather than risk a visible mismatch on install day.
The replacement process
The internal beading around the glazing aperture is removed first - a straightforward job on nearly all composite doors since beading is designed to be removable for exactly this kind of service work, rather than glazed in permanently. The damaged unit is then lifted out, the aperture and rebate are cleaned of old glazing tape and debris, and the new sealed unit is set on fresh setting blocks before the beading is refitted and, where needed, resealed.
A cracked pane replacement on a standard aperture typically takes under an hour once the correct unit is on site, with no disturbance to the door's hinges, lock or frame.
Boarding up in the meantime
If a pane has fully fractured and is at risk of falling out, or a hole has appeared, a temporary board-up keeps the property secure and weatherproof while the replacement unit is sourced - this is particularly worth doing overnight or if the door won't be attended for a day or two. Board-ups are a short-term measure only and shouldn't be left in place through a prolonged lead time on decorative or unusual glass.
We can usually turn around a standard clear or obscured toughened unit within a few working days, with decorative or specialist designs taking somewhat longer depending on supplier stock.
Insurance and cracked glass
Where a crack has resulted from an identifiable one-off impact - a garden tool, a delivery mishap, storm debris - it may be worth checking with your buildings insurer before booking a replacement, since accidental glass damage is sometimes covered depending on policy terms and excess. We're happy to provide a written quote and description of the damage to support a claim if needed.
Cracks that appear with no identifiable cause are more likely to be a stress fracture from thermal cycling or a manufacturing flaw at the toughening stage, and these are less likely to be covered as accidental damage - though it's still worth checking, as some policies cover glazing failure more broadly.
Preventing repeat damage
Once a replacement is fitted, the same impact risks that caused the original crack are usually still present, so it's worth thinking about what caused it in the first place - a strimmer used too close to a ground-floor glazed panel, a gate or bin repeatedly knocking the door in wind, or a habit of the door being slammed hard enough to flex the frame around the glazing rebate. A small change in routine often prevents a second, avoidable replacement.
For doors in genuinely exposed positions - close to a driveway, gate, or play area - a slightly thicker or laminated inner pane can sometimes be specified on replacement for extra impact resistance, worth discussing at the time of quoting if repeat damage is a concern.
Working with beading types and older doors
Internal beading comes in a few different profiles across manufacturers - some clip in with sprung tabs, others are screw-fixed, and a small number of older doors use a glazing tape and adhesive system that needs a little more care to remove cleanly without damaging the slab's finish around the aperture. Identifying the beading type before starting avoids unnecessary marking or cracking of the surrounding decorative skin, particularly on woodgrain-effect finishes where any scuffing is more visible than on a smooth painted surface.
On doors older than around fifteen years, beading can occasionally have become brittle or slightly misshapen from repeated expansion and contraction, and in a small number of cases it's more practical to replace the beading section at the same time as the glass rather than risk it cracking on removal. We check this on site and factor it into the quote rather than as an unexpected extra once the job is underway.
Matching the beading finish to the rest of the door - particularly on woodgrain foiled doors where colour can vary slightly between manufacturing batches - is something we account for when sourcing replacement sections, since a visibly mismatched bead line around an otherwise perfect glass replacement undermines the whole job's appearance.
Cracked glass on side panels and sidelights
Many composite door installations include a glazed sidelight panel alongside the main door, often sharing a similar or matching decorative glass design. These sidelight units are glazed and beaded in essentially the same way as the door's own glazing, and a cracked sidelight pane is dealt with identically - full sealed unit replacement rather than any partial repair.
Because sidelights are typically fixed rather than opening, access for replacement is sometimes marginally easier than the door leaf itself, though matching the decorative pattern across both the door and sidelight glazing becomes more important visually, since any mismatch between the two is far more noticeable than a single door pane viewed on its own.
Where a whole glazed suite - door plus one or two sidelights - has aged together, a cracked pane in one section is sometimes the trigger to consider whether the remaining older units are worth upgrading to current specification glass at the same time, particularly if they're approaching the age where seal failure becomes more likely anyway.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Standard toughened unit replacement (clear, small aperture) | £120-£190 | Most common single-panel front door glazing |
| Decorative/obscured pattern unit replacement | £160-£280 | Depends on pattern availability and size |
| Leaded or bevelled design replacement | £220-£380 | Specialist glass, longer lead time |
| Temporary board-up (if required) | £75-£120 | Short-term security measure pending replacement |
| Beading replacement if damaged during removal | £40-£80 | Occasionally needed on older or brittle beading |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Any crack in a security door is worth replacing promptly.
Prevention
- Specify laminated glass on new orders.
