Replacing rooflights on industrial roofs
Updated 23 October 2026 · SEO Dons Editorial
Rooflights are a part of an industrial roof that is expected to be replaced within the life of the building, and an old one is both a leak risk and a fall risk. Replacement is therefore less a product swap than a safety decision: what protects people until the new units are in, what evidence shows the new assembly is non-fragile, and how it is fixed and sealed. This guide covers GRP and polycarbonate in-plane rooflights on profiled metal and fibre cement roofs. Domestic skylights, roof windows and lanterns are out of scope.
How do industrial rooflights age?
MCRMA's durability guidance lists rooflights among components that may be classed as replaceable on long-life buildings, and its archived design guide (Technical Paper 6, June 2004) says rooflights are subject to gradual deterioration that causes fading and embrittlement, that PVC is particularly susceptible, and that replacement may be anticipated during the life of the building. LRWA's guidance on coating profiled roofs sets the point at which a sheet must go: where it has deteriorated until the glass fibres have come loose, or has lost its structural integrity, it should be renewed. The Rooflight Association describes GRP sheet as the material of choice for factory and warehouse rooflights.
So the signs to look for are fading, exposed glass fibres, brittleness, and fixing points that have stripped or opened at the laps. The safety consequence is why the topic matters. HSE lists rooflights as likely to be fragile, particularly those in the roof plane that can be difficult to see in certain light conditions or when hidden by paint, and the fragile roofs and rooflights guide covers what owners must do while they remain.
What are the options?
| Option | What it involves | Points to check |
|---|---|---|
| Cover or guard the existing rooflights | Permanently fixed covers or framed wire mesh over in-plane rooflights, as a stopgap | The Rooflight Association's Safety Bulletin 2026, produced with NFRC, relates to all in-plane rooflights and sets out considerations before using covers. HSE says covers must be secured and labelled with a warning |
| Replace like for like | New in-plane sheets in a matching profile | The Rooflight Association says current profiles can be matched, as can most discontinued ones. Test evidence, fixing and laps are covered below |
| Replace within an over-sheeting scheme | New rooflights set into a new outer sheet on spacers | MCRMA says purlin spacings must suit the new sheets and rooflights, and existing rooflights need temporary protection during the works |
| Replace the whole covering | New roof with rooflights designed in | The same evidence applies to the new assembly |
Rooflights are one of the five places an industrial roof fails, so the choice usually sits inside a wider industrial roof refurbishment decision, where the over-sheeting guide explains the method. On a roof built before 2000 the sheets beside the rooflights may be asbestos cement, and the guide to telling fibre cement from asbestos cement covers the presumption that applies before anything is drilled.
What does non-fragile mean for a replacement?
The Advisory Committee for Roofsafety's Red Book, ACR(M)001, is the test for non-fragility of large element roofing assemblies. The ACR lists the sixth edition, published on 31 December 2019 and amended in May 2020, as the latest; MCRMA guidance published earlier in 2019 cites the 2014 fifth edition. The ACR says that when a new edition is issued, manufacturers must check that the interpretation used at testing is still valid, so ask which edition a supplier's evidence relies on. An assembly that passes is classified non-fragile Class A, B or C, and one that fails must be classified as fragile.
The classification belongs to the assembly rather than the sheet alone. NARM, the name the Rooflight Association traded under until its 2023 rebrand, said that GRP rooflights can safely be specified as non-fragile if they have been manufactured to industry standards, to the appropriate weight, fixed to the manufacturer's specification and used with structurally compatible roofing components.
Non-fragile also does not mean walkable. The Red Book describes its test as showing whether an assembly can support the instantaneous loads from a person stumbling or falling on it, and both MCRMA's advice note AN 06 and the Rooflight Association's NTD11.1 are devoted to the difference between non-fragile and walkable or walk-on rooflights.
What should be asked before ordering replacements?
- The test evidence. The ACR says anyone claiming compliance with the Red Book must be able to state the classification of the assembly specified and, on request, provide all the test data behind it.
- The sheet itself. Before its rebrand NARM warned that some imported GRP sheets had a glass content well below the 33% minimum in the UK annex to BS EN 1013 and in its technical document NTD03, and said documented evidence of CE marking, weight, glass content and ACR testing should always be sought from the supplier. The Rooflight Association now lists that document as NTD03.1, so check the current figures.
- The fixing method. MCRMA says it is very easy to strip a threaded fastener in GRP, so a grommet-type fastener through pre-drilled holes is recommended, and that PVC and polycarbonate need predrilled oversize holes to allow for thermal movement.
- The laps. MCRMA says rooflights are inherently more flexible than metal panels and leave larger, more variable gaps, so the sealing detail should follow the sheet maker's instructions. The fixings and lap seals guide explains why laps leak.
- The fire classification. Approved Document B volume 2 specifies rooflight performance in a similar way to roof coverings and sets limits on plastic rooflights, including boundary distances, in its Table 14.2 and Diagram 14.1. The BROOF(t4) guide explains the classification.
- The warranty. What it covers, including loss of strength and discolouration, and what it requires in return.
How do daylight and heat loss trade off?
The Rooflight Association says a good specification balances daylight, thermal performance and solar gains. Polycarbonate comes in multi-wall sheets designed for increased thermal insulation, but the Association notes that its longevity cannot compete with glass. Impact resistance is not the same as non-fragile classification, so the ACR evidence is still needed.
In England, Approved Document L volume 2 (2021 edition incorporating 2023 amendments, in force from 15 June 2022) says that if an entire rooflight unit is replaced it should be draught-proofed and meet the minimum standards in its Table 4.1, and that enlarging a rooflight or creating a new one should stay within the maximum areas in Table 10.1 or be offset by compensating measures. Its glossary defines a rooflight as a glazed unit installed out of plane with the roof on a kerb or upstand, which describes kerb-mounted units rather than in-plane profiled sheets, so ask building control how a like-for-like in-plane replacement is treated. A 2026 edition takes effect on 24 March 2027 for work not connected with higher-risk buildings, and the Rooflight Association publishes separate quick guides for the other nations.
If you want contractors to price rooflight replacement against evidence of this kind, you can send an enquiry through the site's form.
Sources
- ACR, the Red Book and Red Book FAQs (Advisory Committee for Roofsafety, checked 22 October 2026)
- The Rooflight Association, safety on roofs, updated safety guidance (20 March 2026), imported GRP rooflights notice (NARM, 22 August 2019), GRP, thermoplastic and energy saving pages, and getting rooflight projects right (14 October 2026) (checked 22 October 2026)
- HSE, fragile surfaces and roof work (HSE, checked 22 October 2026)
- MCRMA, Technical Paper 6 (June 2004, archived), GD33 (May 2020), GD19 (September 2019), GD38 (January 2022), GD31 (August 2019), GD39 section 4 and advice notes (MCRMA, checked 22 October 2026)
- LRWA, Guidance Note 10 (LRWA, issued February 2014, checked 22 October 2026)
- MHCLG, Approved Document B volume 2, Approved Document L volume 2 (2021 edition) and Approved Document L volume 2 (2026 edition), with the GOV.UK Approved Document L page (GOV.UK, checked 22 October 2026)
Common questions
Can the old rooflights just be covered instead of replaced?
As a stopgap, yes. HSE says fragile rooflights must be protected with barriers or covers that are secured and labelled with a warning, and the Rooflight Association has published safety guidance on permanently fixed covers and framed wire mesh. A cover protects against a fall onto it; it does not make the rooflight underneath non-fragile or stop it leaking.
Do replacement sheets have to match the old profile?
They have to fit it. The Rooflight Association says GRP rooflights are made in profiles to match steel and asbestos cement sheets, that all current roofing profiles can be matched, and that most discontinued profiles can be matched for roof refurbishment. Fit still has to be backed by test evidence for the assembly.
How long should a replacement GRP rooflight last?
There is no single figure. NARM's technical document NTD03 was written to give a sufficient safety margin to maintain non-fragility for 25 years or more, but MCRMA says rooflight replacement should be anticipated within a building's life. Ask for the supplier's evidence and warranty terms for the exact product.
Does replacing rooflights on an asbestos cement roof change the work?
Yes. The sheets around the rooflights may be asbestos cement, which brings the duty to manage, survey and method requirements before anything is drilled or removed. The guide on telling fibre cement from asbestos cement explains the presumption and the next steps.