Industrial rooflights
How are rooflights replaced on an industrial roof?
The short answer
An industrial rooflight is replaced by identifying its type, making the access safe, lifting out the old sheet or unit, then fitting a new one that matches the roof profile or the kerb and sealing it into the surrounding covering. Plastic sheet rooflights in a metal roof are swapped much like any sheet. Glazed units on a kerb need the kerb checked first. Access is the real hazard, because rooflights are among the most fragile parts of any roof.
How translucent sheets and kerb-mounted units are taken out and replaced on warehouses and workshops, and what makes the job a safety job first.
Which kind of rooflight is it?
Two families turn up on industrial roofs. The first is a translucent sheet made to the same profile as the metal sheets beside it, so it lies in the roof like any other panel. The second is a glazed or domed unit that sits on a raised upstand, called a kerb, which is built up from the roof surface and sealed around.
The family decides the method. A sheet rooflight is a straight swap in the sheeting line. A kerb unit is a small building job, because the upstand, its flashing and the weathering around it all have to be sound before a new unit goes on.
We identify the type, the profile, the fixings and the size by looking at the roof outside and the underside within, and photographs of each rooflight go with the survey. That tells you if this is a simple swap or a repair to the surrounding roof as well. Our commercial roofing page lists the other work we take on.
Safety before a single sheet moves
HSE lists rooflights among the fragile parts of a roof. They can look solid and still give way under a person, and moss, dirt or paint can hide them from above. Someone who does not know a panel is there can go straight through it, so the first job is to find every rooflight, mark it and cover it.
Work is done from crawling boards or a mobile platform that spreads the weight, with edge protection where the roof edge is close and nets or covers below where the building allows it. Our guide to fragile roof safety sets out the thinking, and the answer on roof edge protection covers the perimeter.
The same care applies inside. Anyone working or walking beneath the rooflight when it is released needs to be kept clear, which is a planning point for a site that is still operating. A forklift lane or a workbench directly under the opening can be moved for the morning, and that costs far less than a falling sheet.
Swapping a rooflight in profiled sheeting
Once the area is protected, the fixings come out and the old translucent sheet is lifted clear. The sheets above and below are checked at the laps, because a failing rooflight often has damp or loose fixings around it that the owner has put up with for years.
The new sheet must match the profile and the length so the laps close properly. It is laid with the lap sealant specified for the system and fixed with sealing washers into the crowns or the troughs as the profile requires. Rooflights and metal sheets are often made by different companies, so a few millimetres of difference in rib shape can leave a gap that lets rain in.
The rooflight is usually fixed to the same purlins as the steel sheets either side. We check those purlins for rust while the old sheet is out, so a rotten rail is found and dealt with rather than buried under a bright new panel that will only hide the problem.
Kerb-mounted units and the upstand
A kerb unit lives or dies by its upstand. If the old unit has leaked for a while, the kerb timber or metal may be wet, the flashing may have lifted, and the waterproofing at the base may have split. A new glazed unit placed on a failed kerb will carry on dripping exactly like its predecessor.
So the kerb is inspected first and repaired or rebuilt where needed, the flashing is reformed, and only then is the new unit bedded and fixed. The unit has to fit the opening, so we measure the kerb on site and not from drawings alone, because buildings seldom match their plans after years of repair.
Where the roof is a flat membrane, the upstand is tied into the covering with the membrane maker's own method. Our guide to commercial flat roof systems shows how those details are built, and why the kerb junction is the first place we look when a flat roof leaks near a rooflight.
Why old rooflights fail, and what to ask for
Plastic sheet ages in sunlight. It can yellow, craze and lose light, and a brittle sheet is more likely to crack at a fixing or give way underfoot. Dirt builds up in the troughs, and the sealant at the laps dries out. Condensation on the underside can also be mistaken for a leak, which our answer on warehouse roof condensation explains.
When you order replacements, ask the supplier for the data sheet. It should state the build, the way the rooflight is meant to be fixed and any strength or fire details. Replacing rooflights can raise Building Regulations questions about thermal performance and about access, and your building control body can say what applies to your building.
The matching, the access and the sealing are all covered in the written quote, which gives you a single price after the survey.
Related questions
Rooflight questions
Can new rooflights be added to a roof that has none?
Sometimes. Cutting a new opening removes part of the sheeting's support, so the purlin layout has to be checked and the edges properly framed and sealed. It is a design matter and a roofing one, and building control may need to be told. The survey tells you what is feasible.
Why does a roof leak around a rooflight?
The usual causes are dried lap sealant, loose fixings, a cracked sheet, and a kerb flashing that has lifted. Run-off from higher up the slope can also overload a rooflight lap. We trace the water to its source before anything is replaced.