A cracked roof lantern pane is usually safer than it looks, because of how overhead glazing is built. The outer pane is toughened, so if it breaks it holds together as a network of blunt granules rather than falling as shards. The inner pane is laminated, two sheets bonded to a plastic interlayer, so even cracked it stays stuck to the interlayer and holds in the frame rather than dropping into the room. Clear the room and act urgently only if the inner laminated pane is bulging, sagging or shedding fragments, which means both panes have failed. A single cracked pane needs prompt replacement, but rarely an evacuation.
That is the reassurance and the caveat together. Below is how to tell which situation you are in, and how a damaged pane is held safe until the replacement glass is fitted. This sits under our main roof lantern installation page.
How each pane behaves when it cracks
A roof lantern is glazed as a sealed double unit, and the two panes are deliberately different because they do different safety jobs over your head.
| What has cracked | How it behaves | What to do |
|---|---|---|
| Outer toughened pane | Shatters into blunt granules held in place by the inner pane and gaskets | Not an emergency; book replacement promptly, keep an eye on it |
| Inner laminated pane, hairline crack | Holds on its interlayer; fragments stay bonded | Prompt replacement; room usually stays usable |
| Inner laminated pane, bulging or sagging | Both panes have failed and the interlayer is loaded | Clear the room below, keep out, act urgently |
| Both panes shattered | Interlayer is doing all the work | Clear and cordon the area, urgent survey and replacement |
The point of the toughened-outer, laminated-inner build-up is exactly this moment. The outer pane takes the impact or thermal stress and breaks safely; the inner pane keeps everything in the frame so nothing lands on whatever is below. A budget lantern glazed toughened-toughened has no interlayer and no such safety net, which is why we never accept that specification, as our page on choosing roof lantern glazing explains.
When to clear the room
Clear the space below if the inner pane is bulging, sagging or dropping fragments. That means both panes have gone and only the interlayer is holding the glass up. Move people, pets and anything valuable out from directly underneath, keep the room out of use, and get it surveyed urgently. A single cracked pane with everything still flat and held does not need this, but if in doubt, clear the area and wait for us rather than standing under it.
You do not need to do anything on the roof, and you should not. A cracked pane is made safe and replaced from proper access in dry conditions by someone insured to be up there. We never advise anyone onto a roof to deal with glass, and there is no safe or useful task for a homeowner up there.
What usually causes it
Cracked lantern glass is not always an impact. The common causes are:
- Thermal stress. A lantern faces the sky and swings through a wide temperature range, and a unit under strain, badly bedded, or with a shaded corner against a sunlit centre, can crack from thermal movement alone. This is more common on older units nearing end of life.
- Failed edge seal and age. A twenty-year-old sealed unit with a perished edge seal is under more stress and cracks more readily. Misting between the panes often precedes a crack.
- Impact. A fallen branch, hail, or debris in a storm. On exposed coastal sites at Felixstowe, Harwich and Frinton, wind-driven debris is a genuine cause.
- Frame or fixing movement. A frame that has been allowed to distort, often on a poorly built kerb, loads the glass unevenly.
How a damaged pane is made safe pending replacement
The glass in a lantern is a sealed unit made to the surveyed size, so it cannot be fitted the same day; it is manufactured to order. In the meantime the pane is made safe rather than left. Where the inner pane is holding, that usually means leaving the safe build-up to do its job and monitoring it, with the room clear if there is any doubt. Where it is more advanced, we can fit temporary protection to contain the pane and keep the unit weathertight until the replacement glass arrives. We do not attempt to repair, patch or bond a cracked pane back together; a damaged sealed unit is replaced, because a cracked overhead pane has lost the safety margin it was built with and no repair restores it. The whole lantern is assessed at the same time, because a crack on an old unit is often the first sign the rest is close behind, which our page on failed roof lantern glazing covers.
Related questions
- Roof lantern leaking in heavy rain: what to do now? if it is letting water in as well.
- How much does it cost to replace an existing roof lantern? for the cost of a full replacement.
- What time of year is best for fitting a roof lantern? because replacement glazing can be fitted any season.
For the safety build-up in full, see choosing roof lantern glazing, and for end-of-life sealed units, failed roof lantern glazing.
Get cracked glass surveyed quickly
Send us a photograph from inside and from ground level, and tell us whether the inner pane is flat and holding or bulging. We will tell you how urgent it is and arrange a survey to replace the unit safely. Request a fixed quote for roof lantern glazing replacement in Ipswich, Suffolk or north Essex.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.