Non-fragility is a tested classification of a complete rooflight assembly, not a permanent property of the sheet, and it degrades with age. A rooflight is classified by dropping a weighted bag onto the whole assembly, installed as it would be on a real roof, and seeing whether it holds. That result belongs to that sheet, at that thickness and profile, over that span, with that fixing pattern, into that structure. Change any of them and the classification no longer describes what is on the roof, and a rooflight that passed when installed will fail the same test decades later as the resin breaks down. Specifying non-fragile rooflights well means specifying the whole assembly, recording it against the real building, and knowing the clock is running from day one.
This page sets out how to specify non-fragile rooflights and what documentation to demand. It sits under our commercial skylight installation service, and it is written for facilities managers, building owners and duty holders who carry the responsibility for fragile roofs. It is not a substitute for a competent fragility assessment of your actual roof.
What ACR[M]001 actually tests
The industry test for non-fragility is ACR[M]001, published by the Advisory Committee for Roofsafety. It is a drop test. A weighted bag is dropped onto a complete rooflight assembly, installed as it would be on a real roof, at specified positions including the worst case, which is usually the centre of the span and the edge. The assembly passes if it does not let the bag through.
Assemblies are classified by outcome. Class B is the standard commonly specified for new industrial rooflights and describes an assembly that arrests the drop. Class C describes an assembly that does not fail immediately but is not relied upon as a working surface. The classification wording matters at specification stage, and it belongs on the submittal and the handover documents, not in a sales conversation.
Why the assembly, not the sheet, carries the classification
This is the single most misunderstood point about non-fragility, and getting it wrong is how a building ends up with a fragile roof and a document that says otherwise.
The test does not certify a sheet. It certifies a sheet, at a given thickness and profile, spanning a given distance, fixed with a given fixing pattern into a given supporting structure. All of those are part of what was tested. A sheet that arrests the bag on 1.8 metre spans may not arrest it on 2.4 metre spans, because the longer span deflects more and the sheet works harder. A sheet fixed at the specified centres may fail if it is fixed at wider centres, because the fixings are part of what holds it. Change the purlin spacing and the classification does not carry over. Change the fixing pattern and it does not carry over.
The trap in a single sentence. A supplier can truthfully tell you a sheet is “non-fragile to ACR[M]001”, and you can still end up with a fragile roof, because that classification travelled with the test configuration and your building is not that configuration. The sheet is never non-fragile on its own. Insist that the classification is recorded against the span and fixing pattern actually installed on your roof.
Non-fragility degrades with age
A rooflight that genuinely passed the drop test when installed does not stay non-fragile. The sheet arrests a fall because the resin holds the glass fibres in tension and lets the sheet flex and absorb the impact. Ultraviolet light breaks that resin down over decades. As it goes, the sheet gets brittle and loses the property that made it non-fragile, and it does so invisibly, well before the surface looks obviously failed.
The practical consequences for a duty holder are firm.
- A twenty-five year old GRP rooflight installed as non-fragile should be assumed fragile now, whatever the original certificate says.
- An undated rooflight of unknown origin is a fragile surface for planning purposes, because you cannot show it is not.
- The working position across the industry is that any rooflight is treated as fragile unless there is documentation showing it is not, and that documentation applies to that assembly at that age.
This is why the installation date matters as much as the classification. Recording it gives the degradation clock a documented start rather than a guess. Our when to replace industrial rooflights answer gives the age bands, and what non-fragile actually means puts the definition in plain terms.
The questions to put to a supplier
When you specify non-fragile rooflights, these are the questions that separate a real classification from a marketing line.
| Ask | Why it matters |
|---|---|
| What span was the classification tested at? | The classification only holds up to that span. Your purlin spacing must be at or below it |
| What fixing pattern was tested? | The fixings are part of the tested assembly. They must be installed as tested |
| What class, B or C? | Class B arrests the drop. Class C is not a working surface. The specification names which |
| Is the classification current and for this build-up? | Single-skin and twin-skin liner assemblies are tested and classified separately |
| What is the expected service life of the surface film? | The classification degrades with the sheet. Film grade sets how long it holds |
| Will you record the classification against my installed configuration? | This is what makes the document describe your roof rather than a test rig |
The documentation to demand
The output that lets a duty holder answer “which parts of this roof are fragile” without hedging is a documentation set, not a single certificate. For each rooflight position it should give:
- The assembly specification and its non-fragility classification as installed.
- The span and fixing pattern the classification was achieved against, matched to what is on your roof.
- The installation date, so the degradation clock has a documented start.
- The manufacturer declarations for the sheet material, filed rather than mentioned.
- A roof plan marking every position, so the record is spatial and can be handed to anyone going on the roof.
That is the set we hand over on every commercial replacement, and it is what a facilities manager needs to manage the roof as a duty holder. Most of the buildings we survey cannot produce any of it for their existing rooflights, which is itself the answer to whether those rooflights should be treated as fragile.
CDM and safe-access documentation
Specifying non-fragile rooflights sits inside a wider set of duties for work on and around a roof. Under the Construction (Design and Management) Regulations 2015, and the Work at Height Regulations 2005, the duty holder has to manage the risk of falls through fragile surfaces: knowing which surfaces are fragile, recording it, communicating it to anyone who goes up, and either preventing access or protecting against a fall.
A replacement scheme produces the documents that support that duty: a construction phase plan, risk assessments and method statements that treat the existing sheets as fragile during removal, a roof access and rescue plan, and information for the health and safety file including the as-installed rooflight schedule and its classification. The new non-fragile rooflights do not remove the duty to manage the roof, but a documented non-fragile roof is a far more defensible position than an undocumented one, and it is the record that lets the next survey start from fact rather than assumption.
A specification note, not a safety clearance. Nothing on this page replaces a competent assessment of your actual roof. Non-fragility as installed is one input to managing a fragile-roof risk, alongside access control, edge protection and safe systems of work. If a rooflight is suspect, restrict access to it and get it assessed. Never treat a rooflight of unknown condition as a walking surface.
Is a rooflight non-fragile because the sheet is described as non-fragile?
No. The sheet is never non-fragile on its own. ACR[M]001 tests a complete assembly: the sheet at a given thickness and profile, over a given span, with a given fixing pattern, into a given structure. A supplier can truthfully call a sheet non-fragile while your roof, with different spans or fixings, is fragile. Insist the classification is recorded against the configuration actually installed.
Does a non-fragile rooflight stay non-fragile?
No. It is a tested classification that degrades with age. Ultraviolet light breaks down the resin that lets the sheet arrest a fall, and the sheet gets brittle over decades, invisibly and ahead of what the surface shows. A twenty-five year old rooflight installed as non-fragile should be assumed fragile now, whatever the original certificate said. Recording the installation date is what gives the degradation clock a documented start.
What documents should I get on handover?
For every position: the assembly specification and its classification as installed, the span and fixing pattern it was achieved against, the installation date, the manufacturer declarations for the sheet, and a roof plan marking every position. That set lets you state which parts of the roof are documented non-fragile and manage the roof as a duty holder, and it lets the next survey start from record rather than assumption.
Specify it properly, once
Non-fragility done properly is specified as an assembly, recorded against your real spans and fixings, and dated so the clock is documented. We specify and record it that way on every commercial replacement, and we hand over the plan and the declarations that let you manage the roof afterwards.
Request a survey and a specification for your roof. Read across to GRP rooflight degradation and replacing rooflights in an occupied building, and see the short how-to on specifying non-fragile rooflights.
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.