The short answer
Rooflights on an industrial unit should be replaced when they are cracked, yellowed, brittle or poorly fixed, and always treated as fragile until they are changed. The new panel must match the profile of the surrounding sheets, be fixed and sealed to suit them, and give the daylight the building needs. Work is planned with edge protection and safe access.
Why old rooflights become the weak point
A rooflight is a translucent panel set into the same profile as the metal or fibre cement sheets around it. Most older ones are GRP or PVC. Sun and weather work on them for years: the surface chalks, the colour yellows, the plastic loses flexibility and the panel becomes brittle.
A brittle rooflight can look perfectly intact from a distance. It may be coated in moss or dirt, or look level with the sheets either side, but a person's weight can put it through the roof. For that reason, anyone working on an old roof should treat every rooflight as fragile, whatever it looks like. Our guide to fragile roofs and safety goes through the rules for owners and contractors.
Leaks are the other symptom. Cracks, split laps and rusted fixings let water in around the panel, and this is a frequent source of drips over stock and machinery.
Matching the profile and the fixings
A replacement rooflight has to fit the roof it sits in. Metal sheeting comes in many profiles, with different rib spacing and depth, and a panel that does not match will leave gaps at the laps. Step one is to identify the profile, measure the sheet pitch and check whether the roof is single skin or a built-up system with insulation underneath.
Then the fixings. Each panel has to be secured to the purlins through the right part of the profile, with washers and sealing caps that suit the material. Fix too tight and the panel cracks; too loose and it lifts in wind.
Laps matter just as much. Side laps and end laps need sealant and the correct overlap, so that water runs off the panel and onto the sheet below it, not under it. Rushed work at the laps is where a new rooflight starts to leak.
Keeping the light the building needs
Warehouses, workshops and factories rely on rooflights for daylight, and they cut the cost of electric lighting. Replace them with solid sheets by mistake and the floor goes dark. Replace them with the wrong type and you can get glare, heat build-up or an uneven spread of light.
Panels differ in how much light they transmit and how they diffuse it. If you are changing the position or number of rooflights, or switching to a different type, tell us what happens under the roof: picking, assembly, offices, cold storage. We can then advise on a layout that fits.
If the roof is being insulated or over-clad at the same time, the thermal performance of the rooflights and the compliance route are matters to settle with your building control body before ordering. We can explain what the survey finds and what the quote covers, but the formal answer comes from them.
Doing the work safely
Replacing a rooflight means working on or next to the weakest part of the roof, so the method is planned before anyone steps onto it. Edge protection, safety netting beneath the roof where it is possible to fit it, and boards laid across the ribs to spread a person's weight are all standard parts of a plan like this.
Access from below matters as well. Where people or vehicles work beneath the roof, the area is cordoned off during the change, so that nothing falls on them. If the building must stay open, our page on repairs while a business stays open shows how a roof can be worked on in stages.
Where a panel has already failed, we make the opening safe first, with a temporary cover, and then plan the permanent replacement at a convenient time.
Asbestos cement sheets and rooflights together
Many older industrial roofs are cement sheets with GRP rooflights set in them. HSE advice is to presume asbestos in any building put up or refurbished before 2000. Where asbestos cement is carefully removed without breaking it up, the work is non-licensed; where the sheets are badly damaged or break up during removal, it becomes notifiable non-licensed work, with notification and medical examination duties.
The rooflights themselves are rarely the issue. The sheets around them are, because lifting a panel can crack the cement at its edges. If your roof is of that age, read our page on asbestos cement roofs on commercial buildings before arranging any work, so the survey can check the material first.
Where sheeting is already due for replacement, the rooflights can usually be dealt with on the same visit.
Replace the panels or replace the roof?
Sometimes new rooflights in a tired roof are money badly spent. If the sheets around them are rusted, their fixings are failing and the purlins are corroded, new panels will not cure the leaks. Our survey looks at the whole roof and tells you which it is.
Metal sheets with surface rust or damaged coatings may need more than a rooflight swap, and our guide to metal roof sheeting for commercial buildings covers the options. Drainage may also be part of the problem, and the gutter, not the panel, can be the real culprit. See our page on industrial gutter lining for those cases.
Our survey produces pictures of every rooflight and the sheets around it, plus a written quote. Whatever we change carries a 10-year written guarantee on the workmanship. The rest of our industrial and retail work is on the commercial roofing page.