Fragile Rooflights: The Hidden Hazard on Older Flat Roofs
A fragile rooflight is any rooflight that will not reliably carry the weight of a person landing or standing on it. The safe working assumption, and the one the Health and Safety Executive asks the construction industry to adopt, is that every rooflight is fragile unless there is evidence proving otherwise. Age makes it worse but age is not the test. A plastic dome fitted last year is fragile. A 30-year-old dome at Kesgrave is fragile and brittle as well. The only rooflight you can treat as load-bearing is one classified non-fragile to ACR[M]001 by drop test, and that classification does not survive decades of weathering.
This page is about that risk and about what a genuinely non-fragile replacement specification looks like. If your dome has already cracked and you have water coming in, that is a different and more immediate problem, and it is covered on what to do about a cracked dome rooflight.
Why plastic rooflights stop holding weight
Polycarbonate and acrylic are tough materials when they leave the factory. Polycarbonate in particular has genuinely high impact resistance, which is why it gets used for machine guards and riot shields. What it does not have is UV stability, and a rooflight sits facing straight up at the sky for its entire life.
Ultraviolet light breaks the polymer chains at the surface. The co-extruded UV layer on the outer skin slows that down and is the reason a decent dome lasts as long as it does, but the layer is microns thick and it erodes. Once it has gone, degradation runs into the body of the sheet. The material yellows, which is the symptom people notice, and it embrittles, which is the symptom that matters and which is invisible.
Embrittlement changes the failure mode. New polycarbonate deflects under load and springs back. Degraded polycarbonate fractures, without warning, at a load it would have shrugged off when it was new. Acrylic is worse again, because acrylic starts brittle and gets more so.
Three other things stack on top of the material itself:
- Fixing points concentrate stress. Hairline cracks radiate from screw holes long before the sheet as a whole looks damaged. Those cracks are the initiation site for a full fracture.
- Thermal cycling works the fixings loose. Plastic expands and contracts far more than the aluminium or timber it is fixed to. Over 30 summers the holes elongate and the clamping pressure that was holding the sheet flat is no longer there.
- The frame degrades alongside the glazing. A dome can be intact and still fall through, because the frame it is clipped into has corroded or the upstand timber under it has gone soft. The glazing is only one part of the load path.
On the estate housing at Chantry, Whitton, Rushmere St Andrew and Kesgrave, the garage and utility roofs went on between about 1965 and 1985 and the domes went on with them. Many are original. A dome that has been up there for 40 years, on a service life that was never more than 20, is not a rooflight any more in the load-bearing sense. It is a translucent lid over a hole.
The HSE position, in plain terms
Falls through fragile surfaces are one of the largest single causes of death and serious injury in construction work in Great Britain, and rooflights are heavily represented in that figure. The industry guidance is deliberately blunt about it. The starting assumption on any roof is that all rooflights are fragile. You do not walk on one, you do not lean on one, you do not put a foot on the frame either, and you do not test it by pressing on it, because a fragile surface gives no useful warning before it goes.
Two details catch people out. A rooflight covered in moss, dirt or paint can be effectively invisible from above, so a roof with old rooflights on it has hazards you cannot see. And domes are often on garages, where the drop is only two or three metres and nobody treats it as a working-at-height risk. A fall of two and a half metres onto a concrete floor, head first, through broken plastic, is more than enough to kill someone.
We will never ask you to go up and check. Not on a flat roof, not on a garage roof, not in dry weather. Every inspection point on this page can be done from the ground, from an upstairs window, or from a photograph taken on a phone held over a windowsill. If we need to see the roof itself, we come and look with proper access and edge protection, and that costs you nothing at survey stage.
Identifying a fragile unit without going near it
Most of what you need is visible from inside the room below, which is the safest vantage point in the building and the one nobody thinks to use.
From directly underneath, looking up
- Colour. Compare the dome to a sheet of white paper held up beside your eyeline. Any yellow, amber or grey cast means UV degradation is well established.
- Crazing. A fine network of surface cracks, like the glaze on old pottery. Easiest to see with a torch held at a shallow angle at night.
- Radial cracks at the fixings. Look specifically at the screw positions around the perimeter. Short cracks running away from the holes are the classic pre-failure signature.
- Debris inside the flutes. Twin and triple-wall sheets have hollow channels. If those channels have gone green or black with algae, moisture has been sitting inside the sheet for years.
- Daylight through the joint. A visible line of daylight anywhere between the dome and its frame means the clamping is gone.
From an upstairs window or the ground with a zoom lens
- Chalking on the upper surface, a dull powdery bloom that a nearby wall does not have.
- Standing water sitting against the base of the unit two days after rain.
- Moss growth on the dome itself, which both hides damage and holds moisture against the plastic.
- Rust staining running down the upstand from the fixing heads.
- Any patch, tape, mastic smear or offcut of sheet laid over the top by a previous owner. That is somebody’s temporary answer to a hole, and it is still a hole.
| Unit type and vintage | Typical condition now | Treat as | Sensible action |
|---|---|---|---|
| Triple-skin polycarbonate dome, 1985 to 2000 | Yellowed, crazed, brittle, fixings weeping | Fragile, and at end of life | Replace. Expect the upstand below to be wet |
| Acrylic dome, any age over 10 years | Brittle from new, worse with age | Fragile | Replace on any habitable room |
| Corrugated GRP roof sheet or rooflight panel | Fibres exposed, surface furred, resin eroded | Fragile, often severely | Replace, and never load it in the meantime |
| Wired or patterned glass in an old metal frame | Frame corroded, glass may be single glazed | Fragile | Replace. Wired glass is not safety glass |
| Modern polycarbonate dome, under 10 years | Sound, but never load rated | Fragile unless certified | Leave alone. Do not stand on it |
| Modern flush glass unit, toughened outer, laminated inner | Sound | Fragile for access purposes | Not a walking surface. Only a rated walk-on unit is |
| Unit classified non-fragile to ACR[M]001, in date | Depends entirely on age since test | Non-fragile while the classification holds | Confirm the assembly and its age before relying on it |
Fragility is a property of the whole assembly, not the glazing alone. A sound sheet in a corroded frame on rotten timber is a fragile rooflight.
What non-fragile actually means
Non-fragile is not a marketing adjective and it is not something a supplier can assert about a product on a datasheet without evidence. It is the outcome of a physical test, ACR[M]001, published by the Advisory Committee for Roofsafety and universally known in the trade as the red book test.
The test drops a weighted bag from a defined height onto a full assembly, at the centre and near the fixings, and the assembly passes only if it retains the bag rather than letting it through. Three points follow from that, and they are the three that get lost when the phrase is repeated in a sales conversation:
- The classification applies to the tested assembly, not to the sheet. Same sheet, different frame, different fixing centres, different span, and the result no longer applies.
- The classification is a condition when new. It says nothing about the same assembly after 25 years of UV and thermal cycling. This is precisely why the industry falls back to treating everything as fragile.
- Class B and Class C exist as well as Class A. Grades below the top one carry restrictions on how the roof may be accessed. A blanket claim of non-fragile without a class is not information.
For a domestic flat roof in Suffolk the honest position is this. If a rooflight needs to be walked on as part of how the building is used, a terrace or a light well people cross, then it must be a purpose-made structural walk-on unit designed for imposed foot loads, not a dome with a certificate. That is a different product entirely, set out on our walk-on rooflight installation page. If the unit only needs to survive an accident during maintenance, non-fragile classification is the relevant idea, and it is worth asking for on commercial and outbuilding work.
Replacement specification: what we fit instead
Replacing a fragile rooflight is a specification exercise, not a like-for-like swap, because the reason the old one became a hazard is usually still present in the detail underneath it.
The glazing
Our default replacement on a habitable room is a thermally broken flush glass unit with a toughened outer pane and a laminated inner pane. The laminated inner is the safety-critical layer. If anything strikes the unit from above, hail or a branch or a dropped tool, the outer pane can break and the laminated inner holds the fragments in its interlayer rather than dropping glass into the kitchen. Laminated glass protects the room below. A walk-on build-up protects the person above. They are not the same thing.
Where a glass unit is not proportionate, on a garage or a detached outbuilding, a new dome is a reasonable answer and we will fit one. It is still a fragile surface. It will just be a fragile surface with 20 years of life in it instead of minus 20.
The upstand and the frame
On a unit this old the upstand is almost always bare uninsulated timber, and it has almost always been damp for years. We survey on the assumption it may need rebuilding, we quote it as a provisional item, and we photograph what we find once the old unit is off and show you before going further. Fixings go in at 316L stainless on anything within about a mile of open sea at Felixstowe, Harwich or the Clacton and Frinton belt, because a corroding fixing is one of the ways a rooflight assembly loses its integrity. The full kerb build is covered on kerb construction for flat roof rooflights.
Why we do not patch a fragile unit
The trade instinct with a cracked or crazed dome is a sheet of tape, a bead of mastic or a plastic offcut screwed over the top. It is cheap and it stops the drip for a winter. What it does not do is restore any load-carrying capacity, and it makes the hazard worse in one specific way: it hides the damage. Somebody who goes up there next year sees a patched panel instead of an obviously cracked one and makes a worse judgement about whether to lean on it. We do not offer patching or resealing on any rooflight, and on a fragile unit the reasoning goes beyond the economics. The comparison between a temporary fix and a replacement is set out on reseal or replace a dome rooflight.
If a rooflight has already broken, treat the opening as live. Keep people and pets out of the room below where anything could still fall, and do not send anyone up to cover it. A hole in a flat roof needs proper access and edge protection, which is a survey visit, not a ladder and an optimistic afternoon.
Common questions about fragile rooflights
Can I walk on my flat roof rooflight to clean it?
No. Treat every rooflight as fragile unless you have documentary evidence that the assembly was classified non-fragile and you know how old it is. That applies to modern glass units as much as to old domes, because a flush glass rooflight is designed for wind and snow loads spread across the pane, not for a person’s weight concentrated through a heel. Flat glass units with a self-cleaning coating and a built-in fall largely keep themselves clear in Suffolk rainfall, and where they do need a rinse it is done from a ladder or a pole, not from on top.
How can I tell if my dome is brittle without touching it?
You cannot tell for certain, which is why the assumption is fragile. What you can do from the room below is look for yellowing against a white reference, fine crazing across the surface, short cracks radiating from the perimeter fixings, and algae inside the hollow flutes of a twin or triple-wall sheet. Any one of those means UV degradation is advanced. A torch held at a shallow angle after dark shows crazing far better than daylight does.
Is a laminated glass rooflight non-fragile?
Not automatically, and the two ideas are separate. Laminated glass keeps broken fragments bonded to the interlayer so nothing falls into the room, which is why we specify it as the inner pane on every flush unit. Non-fragile classification to ACR[M]001 is about whether the whole assembly will arrest a falling person. A unit can be laminated and still not rated to carry anyone. If you need a surface people actually cross, that is a walk-on unit.
My garage domes are painted over. Is that a problem?
Yes, in two ways. Paint or heavy moss makes the rooflight indistinguishable from the surrounding roof, so anyone working up there has no visual warning that there is a hole under their foot. It also traps moisture against the plastic and accelerates the degradation it is hiding. Painted or obscured rooflights on garage roofs across Ipswich are a common find and worth dealing with rather than leaving as a concealed trap.
Get the fragile units on your roof dealt with
If you have old domes or plastic panels on a flat roof, garage or outbuilding anywhere in Ipswich, Suffolk or north Essex, send a photograph taken from a window or the ground and tell us roughly how old the extension is. Then we survey it with our own access, so nobody has to guess by standing on it. Every installation carries a 10-year workmanship guarantee and the quote is fixed after survey. For how these roofs are built, start at our flat roof skylight installation page, and if the unit is discoloured rather than dangerous, yellowed dome replacement covers that.
Request a fixed quote for replacing fragile rooflights, or read when to replace a dome rooflight if you are trying to work out whether yours has reached that point.
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.