A leaking factory roof gets repaired, and the same leak returns the next monsoon. The client concludes the contractor was careless. Usually nobody was careless. The repair was made at the wrong place.
Water does not fall where it lands
The single most expensive misunderstanding in industrial roofing is that the wet patch on the floor marks the hole. It almost never does. Water enters higher up, travels along a purlin, a liner sheet, a cleat or the underside of a sheet run, and falls where that path ends — a purlin hanger, a light fitting, a column head. By the time it becomes a drip you are seeing the exit, not the entry.
So a repair party that goes up and seals everything above the wet patch is sealing the wrong discipline. Sometimes they get lucky. Often they seal a joint that was fine and leave the actual opening untouched, and the leak survives to be called a comeback.
Before anything is sealed, the entry has to be found. That means going onto the roof in daylight, looking for the four things that actually open up:
- Fastener back-out. Screws walk out of purlins under thermal cycling — every hot day expands the sheet, every cool night contracts it, and the fixing works loose. The hole is the width of a screw and it does not look like a hole.
- End laps and side laps. The overlap between two sheets is a capillary gap. If the lap is under-driven, over-driven, or the sealant has aged out, water crosses it by capillary action in a way that defies gravity and inspection from the ground.
- Penetrations. Every vent, cable tray, flue and monsoon drain is a hole in a sheet with a sealant ring around it. Sealant has a service life. Nobody replaces it until it fails.
- The ridge and the eave. Ridge caps and eave gutters are where the wind drives water upward and sideways. A ridge that is fine in still air leaks in a crosswind.
The repair that lasts is the repair that removes the movement
Even a correctly located repair will fail again if the detail that caused it is still moving. Two things cause most repeat failures:
Sealant over a moving joint. Elastomeric sealants are good, but they are not a structural fix. If two sheets are moving relative to each other because a fixing is missing, sealing the gap simply moves the stress to the next sheet along. The correct repair restores the fixing first, then seals.
Screwing into the old hole. A backscrew — driving the same fastener back into an enlarged hole — holds nothing. The standard is a larger fastener with a sealing washer, or the hole relocated, or a lap stitch. If a repair crew's van contains only one size of screw, that tells you what kind of repair you are buying.
What a diagnosis should cost you
A useful roof survey is not an inspection from the yard. It is:
- A walk of every sheet run, from above, in daylight — not a drone flight, because a drone cannot show you a lifted fastener or an under-driven lap.
- A marked-up roof plan with each defect numbered and photographed in place, so the repair scope can be argued about without anyone climbing back up.
- A distinction between defects that are leaking and defects that will leak. The second list is the one that saves money, and it is the one a repair-only contractor has no interest in writing.
That survey is the deliverable worth paying for. A quotation that begins with "we will seal all joints above the affected area" is not a diagnosis, it is a guess with a price attached.
When re-sheeting beats repair
Repair stops being economical when the defects are widespread rather than local — when fastener back-out is general across a run, or the sheet coating has failed to the point where the substrate is rusting through at the laps. At that point you are paying repeatedly to chase leaks across a roof that has reached the end of its life.
The decision is roughly:
| Condition | Right answer |
|---|---|
| Isolated defects, sheet serviceable | Targeted repair, with a marked-up roof plan |
| Repeated defects in one area | Investigate the fixing pattern and the frame, not the sheet |
| General fastener back-out across runs | Re-fix the run; re-sheet if the laps are also failing |
| Coating failure and through-corrosion at laps | Re-sheet. Repair is now a subscription. |
| Frame or purlin section loss | Structural repair before any sheet goes back on |
That last row matters and is frequently skipped: putting new sheets on corroded purlins buys you a new roof over a failing structure, and the new sheets will be blamed when it starts to move.
Doing it without stopping production
The reason factory roofs are not fixed properly is usually that the plant cannot afford to stop. That is a real constraint, not an excuse, and it has a real answer: work in sectioned bays with temporary weatherproofing, sequence the re-sheeting over shutdown windows, and keep a documented handover per bay so the client knows exactly which part of the roof is now new.
The one thing that cannot be compromised is access. Nobody should be standing on a bare sheet run. Roof work is done from crawling boards or staging on the sheets, or from a boom lift at the eave — with harnesses anchored to a designated point. We withdrew photographs of our own that showed otherwise, and explain why on the safety page. If a roofing crew arrives without fall protection, that is the survey you should do before they start.
If you have a roof that has been repaired twice and still leaks, we would rather look at it and tell you why than quote for a third repair. Send us the details or request a site visit.