The short answer
Industrial rooflights are replaced by lifting out the old panel and fixing a new one that matches the profile of the surrounding sheets, sealed at the laps and fixed through the correct crests. Old rooflights become brittle in the sun and are a recognised cause of falls through roofs, so the job needs proper working platforms, and safety mesh beneath the new panels is a sensible addition.
How in-plane rooflights on profiled roofs are replaced, why old panels are fragile, and where safety mesh fits into the job.
How an in-plane rooflight is swapped
An in-plane rooflight sits in the same plane as the roof sheets around it. It is usually a translucent panel, made to the same profile as the metal sheet, and it lets daylight into a warehouse or workshop. Replacing one starts with identifying the profile exactly: the depth of the corrugation, the spacing of the crests and the length of the sheet. A panel that nearly matches will not seal, and gaps at the side laps are where leaks start.
The old panel is released by removing its fixings and lifting it clear, working from a crawler board or a platform rather than from the roof surface. The purlins beneath are checked for rust or damage, and the sheets on either side are inspected, because a failing rooflight often has a failing lap next to it. The new panel is laid with the correct side and end laps, fixed through the crests with the right fasteners and sealed with the tapes or mastics the system calls for.
Whenever possible, we replace like for like in the same opening. Changing the number or size of rooflights is a different job, and your building control body should be asked first.
Why old rooflights are dangerous
Plastic rooflights age. Sun and weather make the surface chalky and brittle, and a panel that looks solid from a few metres away can break under the weight of a person. They are often hard to see from above, because moss, dirt and sun glare make them blend into the roof. That is why falls through rooflights are a real hazard when anyone goes on an industrial roof, including cleaners, engineers and surveyors.
Treat every rooflight as fragile, whatever its age, and treat every sheet as fragile until someone has checked it. Roofs made from asbestos cement are fragile as well; HSE guidance says such roofs are always fragile. On a unit dating from before 2000, HSE says to assume asbestos is there, so nothing that disturbs old sheets should start until that is checked.
No rooflight should be stepped on to reach another. Walkways, edge protection and boards are laid first. Our page on fragile roof safety sets out how this is done, and our answer on roof edge protection requirements covers the perimeter.
Safety mesh and nets under the panels
Mesh is a second line of defence. A steel or high-strength wire mesh is fitted below the rooflights, either as a retrofit to existing panels or at the same time as new ones go in. If someone does step on a weak panel, the mesh is designed to catch them before they reach the floor. Safety nets slung beneath the roof can do a similar job during a larger project, protecting both the roofers and the people working in the building.
Mesh does not make a rooflight safe to walk on. It lessens the consequences of a fall, and the rule remains that nobody stands on the panel. Ask for mesh that suits the rooflight spacing and the purlin layout, and for the fixing method to be agreed before work begins. Poorly fixed mesh can be pulled loose by the very weight it is meant to hold.
Warning signs and markers at each rooflight remind later visitors where danger lies. Where a building has many panels, painted or taped markings help, though they should not be trusted as the only defence. A record of where mesh has been fitted belongs in your maintenance file.
Choosing replacement panels and planning the work
Ask for panels that suit the roof they sit in. A rooflight needs to be compatible with the sheet profile, suitable for the pitch of the roof and rated for the loads it will meet. Choose a product that is made to be fixed to the sheet system you have, rather than a generic panel cut to size. If the roof sheets are tired, the right answer may be to replace a bay of sheeting and rooflights together, as our answer on replacing metal roof sheets explains.
Planning matters just as much as product. A site visit comes first, with access arranged and a clear plan for working over occupied space. Floors beneath are protected, machinery is covered or moved and exclusion zones are marked. The CDM regulations for roof work apply to this job as to any construction work, whatever its size.
To get a written price, tell us the building address, how many rooflights there are and the sheet type if you know it. The survey then checks the purlins and laps around each panel. See also our commercial roofing page.
More on this in Commercial Roofing in Crawley & Gatwick
Two rooflight questions
Can I replace just one cracked rooflight?
Yes, if the panels around it and the purlins beneath are sound and a matching profile can be found. A single swap is common on units where only one panel has failed. If the neighbours are chalky and brittle too, replacing a whole line at once usually saves a second visit and a second set of access costs.
Do rooflights need building control approval?
A like-for-like swap in the same opening usually raises no new question, but adding rooflights or enlarging openings changes the building and can affect energy and fire requirements. Because every building differs, the safe step is to check with your building control body before any change to the number or size of rooflights.