What causes commercial rooflight leaks?

A commercial rooflight almost never sits flat on the deck. It is mounted on a kerb, sometimes a timber or metal upstand, sometimes a proprietary builder’s kerb, and that kerb is where water gets in. The rooflight can be perfectly sound while the fault sits in the roofing membrane wrapped up and over the upstand around it.

On a flat commercial roof, the membrane has to be turned up the side of the kerb and terminated at the top, under the rooflight’s own flashing or capping. If that upturn is too short, if it was bonded to a dusty surface, or if the termination bar has worked loose, water tracks down behind the membrane and appears inside as a stain some distance from the actual entry point. Water travels along steel purlins and timber before it drops, so the damp stain on the ceiling below rarely sits under the hole.

Movement makes it worse. A steel deck expands and contracts across a day, and a large roof with several rooflights in it moves a surprising amount. Rigid sealant beads that were fine on the day of installation crack along the kerb line after a few seasons, and once there is a hairline gap, capillary action pulls water uphill into it. This is a job for a roofer to assess if the membrane itself has failed. Where the rooflight unit is at the end of its life, taking it out and putting a properly detailed new one back on a sound kerb is the cleaner answer, and that is installation work rather than chasing the joint season after season.

The second place water enters is the rooflight’s own construction. A large commercial unit, whether it is a continuous rooflight run, a modular dome array or a glazed lantern over a reception, is held together by glazing bars, pressure plates, gaskets and wet-applied sealant. Every one of those has a service life shorter than the frame around it.

The parts that fail first are the ones that see the most sun and the most movement:

• Glazing gaskets. The EPDM or silicone rubber that seals each panel into its bar goes hard, shrinks and loses its grip. Once a gasket has taken a permanent set, it no longer presses against the glass, and wind-driven rain finds the gap.
• Glazing bar seals. On aluminium bar-and-cap systems, the gasket under the capping is what keeps water out of the joint. When the cap screws back off or the gasket flattens, the bar becomes the leak path rather than the drain it was meant to be.
• Wet-applied sealant. Any joint finished with a bead of silicone or polyurethane is a maintenance item. It chalks, splits and lifts at the edges, and a splitting bead at a bar junction is a common source of slow ingress.
• End caps and closures. Continuous rooflights have closures at each end where the profile is capped off. These are fiddly to detail and are often where a run first lets water in.

None of this means the rooflight was badly made. It means the sealing components are consumable, and on a unit that is fifteen or twenty years old they have simply reached the end. Replacing the failed unit with a modern one puts fresh gaskets and a current glazing system back over the opening in one go.

Older commercial rooflights are frequently glazed in polycarbonate or glass-reinforced polyester rather than glass, and both weather. GRP in particular loses its gel coat over the years, the surface fibres bloom, and the sheet grows porous and brittle. A GRP rooflight can start to weep across its whole surface rather than at one joint, and it becomes fragile enough that it is a fall-through risk for anyone on the roof.

Multiwall polycarbonate has its own failure. The flutes are sealed at the edges with breather tape and profiles, and when those edge seals fail, water and dirt get inside the flutes and sit there. That reads as a leak and as ugly staining inside the panel that no amount of cleaning shifts. Once a polycarbonate sheet has crazed and yellowed it has also lost much of its impact strength. If your rooflights are original GRP or early polycarbonate, the material is telling you it is time for a planned replacement, which our commercial skylight installation work is built around.

A large share of reported commercial rooflight leaks are not leaks. They are condensation, and the tell is that they appear on cold, still, humid mornings rather than during rain. Warm moist air inside a building, a warehouse, a gym, a kitchen, a swimming pool hall, rises and meets the cold underside of the rooflight, and the water vapour in it condenses on the coldest surface it can find. That is usually a metal glazing bar or the frame, which then drips.

Single-glazed or thermally poor rooflights make this far worse, because the inner surface stays cold. This is where orientation quietly matters even on a leak question. A south-facing commercial roof takes a large solar load through the day, the space below runs hot, and the moisture that heat drives off condenses hard once the sun drops and the glazing cools. Specifying the right glass, with a sensible G-value to hold the summer heat down and a good U-value to keep the inner pane warmer, addresses both the overheating and the condensation that trails it. A well-specified double-glazed replacement with warm-edge spacers cuts condensation dramatically compared with the single-skin domes it usually replaces.

Work through it in order before anyone quotes you. Note when the water appears: only during or after rain points at the roof junction or a seal; on cold still mornings with no rain points at condensation. Look at where it lands and remember the water may have travelled along the steel from somewhere else. Check the age and material of the units, because original GRP or crazed polycarbonate has usually reached the end regardless of any single fault.

If the roofing membrane around the kerb has failed, a commercial roofer is the right first call for that element. If the rooflights themselves are old, leaking through tired seals or a spent material, the sound move is to replace them with a modern, well-detailed, properly glazed unit rather than keep chasing water across the ceiling. Tell us the roof type, the rough sizes and which way it faces, ring 01702 898232 or request a quote, and we will specify replacements that keep the water out and the summer heat down together. Our skylight installation costs page shows where commercial work tends to land.