The short answer
Industrial rooflights should be replaced when they have yellowed, crazed or cracked, when the fixings and seals leak, or when they can no longer be trusted to take a person's weight. The sheet around them may be sound, so the work is usually done light by light or in bays. Age alone is not the test, but a fragile or leaking rooflight should not wait.
How rooflights age
Most rooflights on factories and warehouses are translucent sheets set into the same profile as the roof, in GRP, polycarbonate or PVC, fixed with the same sheeting screws and sealed at the laps. Daylight through them cuts lighting costs and makes the building pleasant to work in, so the loss of that light is often the first sign of trouble.
Sunlight is the main enemy. UV breaks down the surface of GRP, exposing the glass fibres, so the sheet turns rough, chalky and yellow-brown, and it transmits less light. Polycarbonate can craze, which means a network of fine surface cracks, or turn cloudy. Dirt and moss collect in the surface texture and make it worse.
The sheets also become brittle. A rooflight that was strong when new can crack under snow, a dropped tool or a person's weight once it has aged. Cold weather makes it worse. That is why a rooflight is often the most dangerous part of a roof to step on, even when it looks fine. See our guide to fragile roof safety for how we protect people working on such roofs.
Signs it is time to replace
No single date sets the moment for replacement. These are the signs we look for.
- Discolouration. Yellow or brown tint and a chalky surface mean the sheet has lost light transmission and strength.
- Visible cracks or crazing. Surface cracking, white stress marks around fixings and splits at the laps all point to a brittle sheet.
- Leaks at laps and fixings. Water stains below a rooflight on a dry day after rain, or drip marks on racking, often come from failed seals or enlarged fixing holes.
- Sagging or deflection. A rooflight that bows between the purlins has lost stiffness.
- Corroded fixings. Rust streaks running from screws show the fixing and sealing washer have given up.
- Poor light. If staff have switched on the lights during the day because the roof is dull, the rooflights have worn out their useful life.
One or two weathered sheets in a run can be swapped while the rest stay. A whole roof with aged rooflights is better handled as a package, with the rooflights, the screws and the sealing strips all renewed together.
Replacement options
The usual choice is like for like, with a new rooflight of the same profile fixed into the existing sheeting. That is the simplest, and it works well where the surrounding roof sheets are in good order. Care is needed in matching the profile, because the laps must close correctly with the adjacent sheets, and a mismatch lets wind-driven rain in.
Where the roof covering is also ageing, rooflights are replaced as part of a bigger scheme. Overcladding with a new insulated system, for example, usually includes new rooflights that match the new profile. Our page on commercial roof systems describes the main covering types and how rooflights fit into each.
Newer rooflights offer better strength and often carry a stated load rating, but a rating is not a licence to walk on them. Ask for the maker's information about fixing, load and expected life, and remember that the figures assume correct installation. Building control may take an interest where the rooflight area or thermal performance changes, so ask your building control body whether your scheme needs a notification before work begins.
Doing the work safely
Replacing rooflights means working beside, and sometimes on, the most fragile parts of the roof. Safe working starts before anyone climbs up: the rooflights are located and marked, crawling boards or platforms spread the load, and edge protection or a harness line is in place. Netting under the rooflights is used where it can be fitted. Nobody stands on an old rooflight to remove it.
Each old sheet is unscrewed rather than broken, and the sealing strips are renewed. Where purlins beneath the opening show rot or rust, they are dealt with before the new sheet goes down, because a new rooflight on a weak support will not stay tight. New fixings are installed at the spacing the maker specifies, with the right washers. The roof is left watertight at the end of each day. If your unit is occupied, stock and machinery under the opening are covered or moved first.
If any roof sheets turn out to be asbestos cement, the job changes. The material must be identified and handled under the HSE guidance, as we explain in our answer on asbestos on an industrial roof. This is one more reason a survey comes before a quotation.
Leaks that start at the rooflight
A rooflight leak is not always the rooflight. Water can travel along a profile and appear metres from where it entered, and a failing valley gutter beside a rooflight run is a common source. Our answer on why valley gutters on industrial units fail covers that. Condensation also drips from cold translucent sheets in humid buildings such as food units and wash bays, and it is often mistaken for a leak.
We trace the water before we recommend replacement, working along the lap lines, the ridge and the gutter edges rather than stopping at the first stain. Sometimes new sealing strips and screws are enough. Sometimes the whole sheet must go. The survey tells you which, and you see pictures of your own rooflights.
Commercial roofs are priced on survey, with the fixed figure given to you in writing after our visit. A replaced rooflight comes under the 10-year written workmanship guarantee on our work. To see how repair is handled on a smaller scale, read our answer on shop roof repair, or return to the commercial roofing hub.
