A non-fragile rooflight is one that has been tested by dropping a weighted body onto it and has not let that body through. It is a classification earned by a complete assembly, unit, fixings and supporting structure together, not a property of the glass or the plastic on its own. It does not mean the rooflight can be walked on. It means that if somebody falls onto it, it should hold them long enough for them to survive.
The test behind the label
The classification comes from a drop test using a soft body impactor, a bag loaded to represent a person, released from a set height onto the assembly. The unit passes if the impactor does not penetrate and the assembly does not fail. That is the whole idea. Nobody is testing whether the rooflight is still usable afterwards, and in most cases it will not be.
Two things follow from that. First, a non-fragile rooflight that has taken a fall is finished and needs replacing, even if it looks intact from below. Second, the rating describes a survivable accident, not a working surface. Those are different promises and they get confused constantly on site.
Why old GRP sheets became the problem they are
Glass reinforced polyester was the standard in-plane rooflight sheet on industrial roofs for decades, laid straight into the profile of the metal deck so that from below it reads as a translucent strip in the roof line. New, it has reasonable impact resistance. After twenty or thirty years of ultraviolet exposure the resin has degraded, the surface has gone chalky and the glass fibres show through, and the sheet has very little strength left.
The dangerous part is that it still looks like a roof. From above, a weathered GRP sheet under a film of moss and dust is close to indistinguishable from the metal beside it. Someone crossing the roof to reach an aerial or a condenser puts a foot on it and goes through. This is the single most common cause of serious falls on industrial roofs in this country, and it is why the classification exists at all.
Non-fragile and walk-on are not the same word
A walk-on rooflight is designed to be stood on routinely as part of the building’s use: a terrace light, a pavement light, glazing in a roof garden. It carries an imposed load specification and a slip-resistant surface, and it is expected to be undamaged after use.
A non-fragile rooflight is designed for a single accidental event. Standing on one deliberately, day after day, is not what it was tested for. If people genuinely need to cross a roof over glazing as part of maintenance, the answer is either a specified walk-on unit or a designated walkway alongside the glazing that keeps feet off it entirely. Our page on whether commercial rooflights are walked on for maintenance takes that further.
The assembly holds the rating, not the pane
This is the part that catches people out on a replacement. A rooflight sheet or unit rated non-fragile achieved that rating in a specific configuration: a stated fixing pattern, stated fixing type, stated support spacing, and often a specific under-sheet or spacer system. Fit the same sheet with fewer fasteners, or across purlin centres wider than the tested arrangement, and the rating does not travel with it.
So the honest position on any installation is that non-fragility is something you build, not something you buy. We fit to the manufacturer’s tested detail or we do not claim the rating. If a roof has purlin spacings that fall outside a product’s tested range, we say so and we specify something that suits the roof rather than forcing the product that was in the enquiry.
Non-fragility has a date on it
A plastic rooflight is non-fragile when it is installed and becomes progressively less so from that morning onwards. Manufacturers publish a service life for the non-fragile classification, commonly expressed as a period in years for a given sheet build-up, sometimes with a heavier grade offering a longer one. After that period, the assumption should be that the rating has lapsed.
Glass behaves differently. A laminated glass unit in a rated assembly does not weather the same way and does not chalk, but the interlayer, the frame and the fixings still age, and a unit that has been on a roof for thirty years should not be assumed to perform as it did. The sensible approach on an older roof is to treat age as the primary evidence and to plan renewal rather than wait for a fall.
Reading what is already on your own roof
From the ground you can usually establish the type but not the rating. In-plane translucent strips flush with the profile of a metal deck are almost always GRP or polycarbonate sheet. Domes on square kerbs are typically acrylic or polycarbonate multiwall. Flat glass units on a kerb are the newest arrangement of the three.
What you cannot see from the ground is the fixing pattern, the support spacing, or whether an under-sheet is present. From inside, looking up on a bright day, you can often see the fasteners and read the spacing. If there is a health and safety file for the building, or an operation and maintenance manual from the original construction, the product data may be in it. Failing all that, the surveyor’s default is to treat it as fragile, which is also what the roof signage should say.
Where this bites on an Oxford industrial estate
The stock around Cowley, Osney Mead, Kidlington and the older parts of Witney and Bicester is largely 1970s and 1980s steel portal frame with in-plane translucent sheets. That means a great many roofs locally are carrying sheets well past their rated life, on buildings whose owners have never been told there is a rated life at all.
The practical consequence is that any contractor going onto that roof for any reason has to plan for fragile surfaces, which adds cost to every trade that touches the building. Renewing the sheets with a rated assembly removes that constraint permanently and usually improves the daylight considerably, because a degraded sheet passes far less light than a new one. There is more detail in our page on non-fragile rooflights and the standards that apply.
Swapping fragile sheets for rated units
This is work we do. It is a replacement, in-plane sheet out, rated assembly in, with the fixings, spacings and any under-sheet set to the tested detail. On a large roof it is usually phased so the building stays weathertight and in use, and the roof is never left open at the end of a day.
What we will not do is fit a rated sheet into an arrangement that does not match its tested configuration and let you believe you have bought non-fragility. If the roof structure will not accept the product, we specify a different one. Call 01865 704245 with the building type, the age and roughly how many sheets are in the roof, and we will give you a range by email and a fixed price after survey. Our workmanship carries a ten-year guarantee.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
Tell us about your roof
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