The short answer
An in-plane rooflight on an industrial roof is replaced by lifting out the old translucent panel and fitting a new one in the same profile, with the laps, fixings and sealing redone to suit. The risk is the work itself, since old panels are often fragile. Safe access, edge protection and a plan for the order of work come before the first fixing is undone.
What an in-plane rooflight is and why it fails
An in-plane rooflight is a translucent panel, usually GRP (glass-reinforced plastic) or polycarbonate, rolled to the same profile as the metal or fibre-cement sheets either side of it. It sits in line with the roof and lets daylight into a warehouse or workshop. Because it lies in the roof plane, it is lapped and fastened just as the sheets are, which makes replacement a like-for-like job in principle.
Panels fail from sun, weather and age. Ultraviolet light breaks down the surface of older GRP, which turns chalky or yellow, cuts light and exposes glass fibres. Polycarbonate crazes and cracks. Fixings loosen as the panel expands and contracts, and the sealed laps open. A tired panel can look perfectly good from the floor and still fail the moment someone steps on it. A leak around a rooflight can also be a failed lap or a rusted fixing in the sheet next to it, which is why we look at the whole run, not just the panel.
How the replacement is carried out
The work follows a set order. Access and edge protection come first. Then the fixings are released, starting from the ridge end of the run, and the old panel is lifted clear. Debris is kept out of the building. The new panel is lapped over the sheet below and under the sheet above, in line with the profile, with fresh sealant at the side and end laps, and fixed through the crowns of the profile with new fasteners, washers and caps.
Matching the profile matters. A panel in the wrong profile will not lap tightly, leaving a gap that no mastic will hold for long. Fixing patterns, lap lengths and sealing details come from the sheet maker's guidance, and where the existing sheets are tired, fixings through them may need replacing or the neighbouring sheet changing at the same time.
Sheets and rooflights should be considered together. If the sheets themselves are close to done, replacing rooflights alone only restores the lights; our guide to metal roof sheets for commercial units explains when the sheets deserve attention. Rusting at the cut edges of the sheets around the panel is the first thing we look for.
Safety: why rooflight work comes first on the risk list
An old rooflight panel can look like part of the roof and carry no load at all. People fall through them, because from above a weathered panel resembles the sheet beside it. Moss, dirt and fading make it worse. So before the roof is walked, the panels are identified and covered or marked, and the crew works from crawling boards or a platform spanning the sheets, not from the panels themselves.
Edge protection goes on at the eaves and gables. Where the work is above a trading floor, a safety net, catch deck or temporary ceiling below the panels protects the people and stock beneath. Falling items are another hazard, so the area outside is cleared and barriered. Our guide to fragile roofs and safe access on Stroud commercial sites sets out how access is organised, including for engineers and cleaners who visit afterwards.
Where the building stays in use, our page on repairing a commercial roof while trading explains the protection below, and the protection is costed on its own in the quote.
Choosing the new panels: light, insulation and condensation
Replacement is a chance to improve on the original. Better UV protection on the panel surface slows the discolouring that spoils the old ones. Double-skin or insulated rooflights reduce heat loss and condensation compared with a single skin, though they can cost more, and in a refurbishment they need to suit the sheets around them. If the roof is single-skin, the rooflights stay single-skin so the profile still matches.
Energy rules apply to replacement. For non-domestic work, the Building Regulations cover new and replacement elements, and the lighting area and insulation values depend on the building, so check with your building control body before specifying panels. Part L values for homes are different, and do not carry over to an industrial building.
The number and position of panels affect the daylight inside. Moving rooflights, adding more or taking some out changes the structure and the lighting design, and should be agreed with whoever manages the building. Where we find panels in several different ages and makes, matching them gives a more even finish and a roof that ages in one way.
Timing, cost and the rest of the roof
Rooflight work is tied to weather. Sealants and some adhesives need dry surfaces and a minimum temperature to cure, and a roof open to rain cannot be left. For the timing questions, our page on commercial roof replacement in winter is the place to look. A planned, dry-weather slot is usually quieter and cheaper than an emergency call after a panel fails.
Costs depend on the number of panels, their size, the access needed and the height of the roof. Access can be the biggest item on the page, since a mobile platform, scaffold or tower has to reach the roof edge. Hence the written quote that follows the survey, covering access, removal of the old panels, new panels, fixings, sealing and waste.
A rooflight swap is also a moment to look at the gutters beside them. A box gutter running along the foot of a rooflight run takes the water that the panel sheds, so our guide to box gutter leaks and linings may apply. Our workmanship guarantee runs for 10 years in writing. The commercial roofing page has the wider service.