A warehouse rooflight that keeps leaking after two or three rounds of sealant is not a stubborn joint. It is telling you the sheet, the laps or the fixings have reached the end of their life, and mastic is being asked to do a structural job it cannot do. The water stops when the failed components are replaced, the laps are re-formed to the right length for the roof pitch, and the fixings are renewed to the correct grade and torque. It does not stop because the next tube of sealant is better than the last.
This page diagnoses persistent seam leaks on large-span profiled roofs and sets out the replacement scope that ends them. It sits under our commercial skylight installation service, and it is written for facilities managers who have a recurring wet patch over the racking and a maintenance line that keeps getting spent on it.
Where profiled roofs actually leak
Water gets into a profiled roof at joints and penetrations, not across the middle of a sound sheet. Around a rooflight there are more joints than anywhere else on the roof, and they join two different materials that move at different rates. That is why rooflight laps are the leakiest detail on most industrial roofs, and why they are usually where a recurring leak is coming from even when the wet patch below appears somewhere else.
Side laps
A side lap runs down the slope where a rooflight sheet overlaps the metal sheet or another rooflight beside it. It seals on a continuous bead of lap sealant held closed by stitching fixings at set centres. Two things fail here. The sealant hardens, shrinks and loses grip after fifteen to twenty years, so the lap breathes open and shut with temperature and lets driven rain track through. And where the original was stitched only at the crowns, or not stitched at all, the lap was never properly closed to begin with.
End laps
An end lap runs across the slope where an upper sheet overlaps a lower one. The lap has to be long enough that water running down the slope cannot climb back up under it by capillary action and wind pressure. Shallow roofs need longer end laps, and industrial roofs are frequently very shallow, down to five or six degrees. An end lap sized for a fifteen degree roof leaks on a six degree one every time the wind drives the rain uphill. This is a design fault built in at construction, and no amount of sealant on the outside fixes an end lap that is too short.
Fixings
Fixings through a rooflight need the right size of sealing washer and the right torque. Over-tightened, they dish the sheet and craze the surface, which starts local degradation and eventually a crack around the fixing. Under-tightened, the washer never seals and the hole leaks. Corroded fixings, common on the coastal belt where chloride attacks standard steel, weep rust-stained water and lose their grip on the sheet. Every fixing is a hole in the roof that is only as good as its washer.
The laminate itself
The failure that sealant campaigns never reach is the sheet. Once a GRP laminate has degraded, water wicks along exposed fibres into and through the body of the sheet rather than running off it. The sheet leaks in the field, not just at the joints, and there is no joint to seal. A leak at a degraded GRP sheet is the laminate failing, and the only repair is replacement. Our GRP rooflight degradation page covers how to read that failure across a roof.
Why the mastic campaign keeps coming back
The recurring-sealant cycle has a logic to it that is worth naming, because it is exactly the pattern the replacement argument is made against.
| What the sealant is asked to do | Why it fails again |
|---|---|
| Bridge a lap that is too short for the pitch | The lap geometry is wrong. Sealant cannot add lap length, and it debonds under the same wind-driven water that beat the lap |
| Reseal over a hardened, contaminated old bead | New sealant will not bond to old cured sealant, dirt and chalking GRP. It looks sealed and lifts within a season |
| Seal a fixing that has lost its washer | The sheet moves on the fixing with every temperature swing and shears the surface seal |
| Stop a leak through a degraded laminate | There is no joint to seal. The water is coming through the body of the sheet |
| Hold on a surface that keeps moving | GRP and steel expand at different rates. A rigid bead across that joint is worked loose every summer |
Each visit buys a season, sometimes two, and then the leak returns a little worse because the underlying components are older. The spend is real, it is recurring, and at the end of it the roof is no drier and no safer. We do not offer resealing or overlay as a service for exactly this reason. It postpones the replacement while adding cost, and on a degraded roof it hides a fragility problem behind a maintained-looking surface.
A recurring leak is a diagnostic signal, not a nuisance. Three rounds of sealant on the same lap is the roof telling you the components there have failed. The useful response is a survey that finds the real source, which is often several metres up-slope of the wet patch below, and a replacement scope that renews it, rather than a fourth tube of mastic.
Finding the real source
Water rarely drips straight down from where it entered. It comes through a lap, runs along the underside of the sheet or the top of the liner, and drops several metres away at the first obstruction. A survey traces it back. We check the laps and fixings up-slope of the internal wet patch, look for staining tracks on the liner, and where the picture is unclear a controlled hose test run from the bottom of the slope upwards isolates the entry point. Guessing at the source from the position of the drip below is how the wrong lap gets sealed and the leak carries on.
The replacement scope that stops the leaks
A scope that actually ends a recurring seam leak addresses the whole failed detail, not the visible symptom.
- Replace the failed rooflight sheets in the affected bay, rather than resealing them. Old sheet out, new sheet in, so the laminate leak and the fragility risk go together.
- Re-form the laps to the correct specification for the actual roof pitch. Correct end lap length, continuous fresh lap sealant, and stitching fixings at the specified centres rather than at the crowns and hoped for.
- Renew the fixings to the correct grade, with correctly sized sealing washers set to a controlled torque. On coastal and port-side roofs that means 316L stainless, because standard fixings pit in chloride air.
- Sort the liner and the cavity on twin-skin roofs, so the corresponding liner rooflight is sound and the cavity is not trapping condensation that reads as a leak.
- Check the surrounding metal sheeting at the same time. The roof is open and access is paid for, so cut edges and corroded laps nearby get treated in the same visit rather than becoming next year’s leak.
Because most of the cost of the job is safe access rather than the sheet, it is almost always right to replace a whole affected bay or slope rather than a single leaking sheet. The neighbouring sheets are the same age and a season behind, and doing them now shares the access cost and avoids a return visit. Our warehouse rooflight replacement cost answer sets out how that prices per sheet and per bay.
What replacement gives you that sealant cannot
- A dry roof that stays dry, because the failed components are gone rather than coated.
- A documented non-fragility position for the replaced area, recorded against the actual span and fixing pattern installed.
- Recovered daylight, because the new sheets transmit what the old bloomed ones no longer did.
- An end to the recurring maintenance line, which over a few years usually exceeds the cost of replacement.
Why does my rooflight leak in a different place from where the water shows inside?
Because water enters at a lap or fixing up the slope, then runs along the underside of the sheet or the top of the liner and drops at the first obstruction, often several metres away. This is why sealing the spot directly above the internal drip usually fails. A survey traces the water back to the real entry point before anything is specified.
Can you just reseal the leaking laps instead of replacing?
We do not offer resealing or overlay as a service, because on a roof old enough to be leaking at the laps the sealant is being asked to fix failures it cannot reach: laps that are too short for the pitch, fixings that have lost their washers, and laminates that leak through the body of the sheet. Resealing buys a season and hides a fragility problem. Our replace rather than patch answer sets out the reasoning.
Do I have to replace the whole slope, or just the leaking bay?
Not necessarily the whole slope, but usually more than the single leaking sheet. The cost is dominated by safe access rather than the sheet, and the neighbouring sheets are the same age and close behind. Replacing the affected bay in one visit shares the access cost and avoids returning within a year or two for the next failure. The survey and grading decide the sensible unit.
Get the leak found, not just sealed
If a warehouse rooflight has taken more than one round of sealant and is still leaking, the components there have failed and the water will keep coming. We survey the roof, trace the leak to its real source, and come back with a replacement scope priced by bay that ends it rather than postpones it.
Request a leak survey for your roof. Read across to GRP rooflight degradation and in-plane rooflight replacement on profiled metal roofs for how the replacement is detailed, and see the costs page for budget rates.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.