What is a non-fragile rooflight?

The recognised standard in the UK is ACR[M]001, the test for the fragility of roof assemblies published by the Advisory Committee for Roofsafety. In plain terms, a weighted bag of around 45 kilograms is dropped onto the rooflight assembly from a set height to represent a person landing on it. If the bag passes through, the assembly is fragile. If it is held, the assembly is classed as non-fragile.

The wording that matters there is “assembly”. A pane of glass or a plastic sheet on its own tells you very little. What is being judged is the sheet in its frame, fixed the way it would be on a real roof, because a rooflight fails at its weakest link. That link is often the fixing or the edge of the frame rather than the middle of the sheet. This is why a supplier who quotes a non-fragile rating for the glazing but says nothing about the frame or the fixing pattern has not answered the question. Ask for the classification of the complete assembly and the conditions it was tested under.

A non-fragile result is also read alongside a second point: whether the assembly held the load without the tester passing through, and whether it did so cleanly or by tearing and only just catching. A good specification will state the class achieved rather than the bare word “non-fragile”, because the classes describe how comfortably the assembly held.

Falls through fragile roofs and rooflights are one of the steadiest causes of serious injury and death in construction and building maintenance. The Health and Safety Executive treats rooflights as a particular danger because they are easy to miss. A weathered plastic dome sitting flush in a large flat roof can look much like the roof around it, especially under dirt, moss or a dusting of frost, and someone crossing that roof to reach a plant unit or a gutter can put a foot straight onto it.

Under the Work at Height Regulations, anyone responsible for a roof that people access has to manage that risk. Specifying non-fragile rooflights is one of the cleanest ways to do it, because it designs the danger out rather than relying on a warning sign or a harness that someone might not clip on. It is standard thinking on warehouses, factory units, schools, retail sheds and any commercial roof that will be walked for cleaning, inspection or servicing. If you are re-roofing or fitting out a commercial building, non-fragile is usually the sensible default for every rooflight, and it is a core part of our commercial skylight installation work.

Domestic roofs sit under different rules, but the principle still helps. A loft conversion or a rear extension rooflight that a window cleaner or an aerial fitter might lean a ladder against, or that a solar panel installer might work around, is safer as a non-fragile unit. The cost difference on a single dwelling is small set against what it prevents.

There is no single material that earns the rating. It is the build-up as a whole. A few routes get there:

• Multi-skin polycarbonate. Thick multiwall or solid polycarbonate sheet is tough and flexes rather than shatters, which suits the drop test well. It is common on industrial rooflights and on plastic and polycarbonate skylight installations where a large clear area is needed at a sensible weight.
• Reinforced GRP. Glass-reinforced polyester sheet, often in a sandwich panel, is a long-standing choice for factory and warehouse rooflights and can be specified to hold the load.
• Laminated glass. In a glazed rooflight or lantern, a laminated inner pane keeps the assembly together even when the glass cracks, because the interlayer holds the fragments. This is how a glass unit can be both good to look through and safe to fall on.
• Frame and fixings. The kerb or upstand the rooflight sits on, the frame section and the number and spacing of the fixings all feed into the result. Under-fixing a strong sheet can still give a fragile assembly.

So the honest answer to “which material is non-fragile” is that the question is aimed at the wrong part. You specify a rooflight, framed and fixed a particular way, and that whole thing carries the classification.

One more point sits behind all of this: a rooflight can pass the test when it is new and drift towards fragile as it ages. Plastic sheet in particular is worked on by ultraviolet light year after year. It can yellow, go brittle and lose the flex that let it hold the load. A dome that was safe when the roof was laid may not be safe two decades later, even though nothing about it looks obviously wrong from below.

Because of that, roofs are assessed on the working assumption that any older rooflight of unknown history should be treated as fragile until proven otherwise. If you are taking on a building with existing rooflights and no paperwork, that is the safe starting point. When those units come out and new ones go in, the replacement is the moment to bring the roof up to a known, current non-fragile specification across the board, so the whole roof reads the same rather than being safe in patches.

Safety is one axis. Heat is the other, and on a large commercial roof the two decisions are made at the same time. A non-fragile rooflight is still a large area of glazing letting the sun in, and here on the estuary that matters more than most places. Southend-on-Sea faces south across the water, so a south-facing commercial roof collects sun from mid-morning to late afternoon through the whole warm season, and reflected light off the estuary adds to it.

Put a run of rooflights across the south slope of a warehouse or a south-facing shopfront and you can solve the fall risk while creating an overheating one. The fix is to look at the glass specification at the same time as the fragility rating: the G-value, which sets how much solar heat comes through, and the glare on the floor below. A laminated glass unit can carry a solar-control coating and still be non-fragile, so you are not forced to choose between a safe roof and a comfortable one. This is the same thinking that runs through our energy-efficient skylight installation work, moved onto a commercial roof. Get the orientation read first, then the glass, then the unit.

If you are specifying rooflights for a commercial building, ask every supplier for the non-fragile classification of the complete assembly to ACR[M]001, not just the glazing, and keep that paperwork with the building records. If you are inheriting a roof with old rooflights and no history, treat them as fragile and plan the access around that until they are replaced.

When you are ready to price a job, tell us the roof orientation, the rough size and number of openings and whether the roof gets walked for maintenance, and we will come back with a non-fragile specification and the glass to go with it. Ring 01702 898232 or request a quote, and we will look at the safety rating and the solar performance together rather than one at a time.