Question answered
Replacing industrial rooflights
The short answer
Replace industrial rooflights when they are cracked, brittle, leaking at the laps or no longer fixed securely, and treat every one as fragile until proven otherwise. The new unit must match the profile of the sheets, sit on a sound upstand and be sealed at the laps. Access is planned first, before anyone sets foot on the roof.
Rooflights on an industrial unit tire sooner than the metal sheets beside them and can turn into the most dangerous part of the roof. Replacing them properly is a matter of fixing, sealing and safe access.
Why old rooflights fail before the roof does
A rooflight on a factory or warehouse is usually a translucent sheet that follows the profile of the metal sheets either side of it. Years of sun, weather and cleaning leave the plastic discoloured, crazed and brittle, and the fixings around it work loose as the sheet expands and contracts. A unit that was strong when installed can give way under a person's weight, or let water in long before the surrounding steel needs attention.
Leaks usually start at the laps and the fixings, not in the middle of the panel, and a thin hairline crack can pass unnoticed for a whole winter. The water then runs along the purlins, the horizontal beams under the sheets, and appears well away from the entry point. If a leak sits near a rooflight line, our page on a roof leaking in a shop or office explains what to do inside while the cause is traced, and what to record for the insurer.
Fragility is the first hazard
Treat every rooflight as fragile, new or old, and as hidden in the sheet when moss or dirt covers it. HSE's guidance on asbestos cement roofs states that they are always fragile, and plastic rooflights set into any other roof deserve identical caution, because a unit can break without warning the moment a person steps on it. Moss and dirt can hide the line entirely.
That is why the working method is decided before anyone climbs up. Access is planned across the roof, with the fragile areas identified, covered or protected, and the edge secured. The fixings are checked from below where that is possible. Where an older roof mixes several sheet types, read about planning safe work on fragile roofs. Staff should never be sent onto the roof to clear gutters or look at a leak, whatever the urgency, because a ladder and a hurried visit is exactly how people fall through.
Matching the profile and the laps
A replacement rooflight has to match the profile of the sheets around it, with the same rib shape and corrugation pitch, or the laps will not close. Side laps are the overlaps along the length of the sheet, and end laps are the overlaps up the slope. Both need the sealant, stitching fixings and overlap length that the system calls for, so that rain running down the roof meets a continuous barrier all the way to the gutter.
The same job is a chance to inspect the sheets next to the rooflight. Corroded edges or loose fixings beside a failing unit often mean the rest of the line needs work, which is the ground covered by replacing a damaged roof sheet. Fixing into sound steel with the right fastener for the roof's exposure matters more than the rooflight itself, so those fixings get the closest look.
Upstands, kerbs and the Part L figures
Not every rooflight sits in the profile. Some are glazed units set on a kerb or upstand. Approved Document L separates these rooflights, which stand proud of the roof, from roof windows that sit in its plane. Its dwellings volume sets 2.2 W/m²K as the limiting U-value for rooflights that are new or replaced, with 0.35 as the maximum for an upstand built on site.
That volume is written for dwellings. A commercial building is dealt with by separate guidance, so ask your building control body which standard applies to your roof and whether a replacement needs approval. Whatever the answer, the upstand has to be sound, tall enough to clear the roof covering and sealed to it. A weak kerb brings back the very leak the new unit was bought to cure, and it is far harder to put right once the unit is fixed down.
What the survey covers and what you receive
The inspection takes in each rooflight line from inside and out, with pictures of every unit, its laps and fixings and the sheets either side. The quote that follows is written and gives a single total, which also reflects any scaffold or platform the access needs. The work is planned around access, the number of units and any need to keep the building working during the job.
Where the roof is asbestos cement, which a building from before 2000 may have, HSE says to assume asbestos is present. Cement sheeting that is weathered but not substantially damaged is non-licensed work if it is removed carefully, and breakage makes the work notifiable. Our guide to types of commercial roof helps you identify what you have, and our commercial roofing pages show how the work fits together.
Rooflight replacement: two more questions
Can I replace only the cracked rooflights and leave the rest?
Often, yes, if the neighbouring sheets and fixings are sound. A survey tells you. Where the fixings along the line have worked loose or the edges are corroded, new units fixed into weak steel will not last. The pictures from the visit show which case applies, and the work is sized to match.
Can anyone step onto a rooflight that looks new?
No. Treat it as fragile whatever its age, unless the manufacturer states a load rating and it has been installed to match. Even then, plan the route of anyone who goes on the roof, and keep people off the sheets and rooflight lines whenever the job can be done from a platform or lift.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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