The short answer
- The drips are condensation, not always a leak. Warm, moist air inside the building touches the underside of a cold metal sheet, cools below its dew point and turns to water.
- It is worst on clear, cold nights, when the sheet loses heat to the sky and gets colder than the air, and in buildings with heaters, vehicles, people or wet goods adding moisture.
- Ventilation and dehumidification help, but they fight the symptom. Board linings help too, if every joint and fixing is sealed.
- Closed-cell spray foam bonded to the underside of the sheet stops warm air reaching the cold metal at all, insulates the roof and seals the laps, in one layer with no brackets.
Dew point explained in two minutes
Air always holds some water vapour. Warm air can hold more than cold air. The dew point is the temperature at which a particular parcel of air can no longer hold its vapour, so the surplus turns to liquid water.
A single-skin metal roof is a very good condenser. The sheet is thin, conducts heat quickly and has almost no insulating value. So:
- the air inside a working warehouse is warmed by heaters, machinery, lighting, vehicles and people;
- outside, the sheet cools, and on a clear night it radiates heat to the sky and can drop below the outdoor air temperature;
- indoor air touching the underside of the sheet is chilled below its dew point;
- water forms on the sheet, runs down the profile, collects at purlins and fixings, and drips.
That is why a roof can "rain" indoors on a dry, cold morning, when nothing is getting in from outside. It is also why a roof that drips in patches often turns out to be dripping at the fixings and purlins, where the metal is coldest and water gathers.
Quick test: if the drips appear after a cold, clear night and dry off as the day warms up, suspect condensation first. If they follow rain, suspect the laps, fixings, rooflights or gutters. Many roofs have both problems.
Why Irish buildings are prone to it
Ireland has a wet, mild maritime climate. Met Éireann puts the national annual average rainfall at 1,288 mm for 1991–2020. Most of the eastern half of the country gets 750–1,000 mm a year, the west generally 1,000–1,400 mm, and many mountainous districts more than 2,000 mm.
Rain is not condensation. What matters for condensation is how much moisture ends up in the air inside the building, and the Irish climate rarely gives that air a chance to dry out. The main sources inside a working building are usually:
- Heating. Direct-fired gas and paraffin heaters without a flue release water vapour into the building as they burn.
- Vehicles. Gas and diesel forklifts, delivery vehicles running with doors open.
- Goods and processes. Wet deliveries, washing down, food production, drying crops, stored timber.
- People. Breathing and activity in a busy picking or production area.
- Open doors. Loading doors let damp outside air in, especially in mild, humid weather.
Farm buildings add livestock and silage to the list. Our farm building condensation guide covers those, including why livestock buildings must stay ventilated.
Signs and costs of a condensation problem
Condensation costs more than people expect, because the damage spreads slowly and is often blamed on something else.
- Damaged stock and packaging. Cardboard, paper, textiles, electrical goods and food packaging marked or ruined by drips.
- Corrosion from the inside. Rust on the underside of the sheets, at fixings and on purlins, sometimes long before the outside of the roof looks tired.
- Slip hazards. Wet patches on warehouse floors, loading bays and walkways, often in exactly the same spots every morning.
- Mould and staining. On walls, linings, racking and anything stored close to the roof.
- Wasted repairs. Roofers called back again and again to "fix a leak" that is not a leak.
- Heating waste. A roof cold enough to condense is also losing heat, which is paid for all winter.
If the building also leaks when it rains, deal with both. A warehouse roof repair decision and a condensation fix are often best planned together.
Roof dripping on the stock?
Tell us the building, the roof type and when the drips appear. We will pass it to an approved installer for a free survey.
The fixes compared
| Fix | How it works | Strengths | Limits |
|---|---|---|---|
| More ventilation | Replaces moist indoor air with outdoor air | Low cost where louvres or ridge vents can be added | Cold, damp outdoor air does not always help; heat goes out with it; sheet stays cold |
| Dehumidification | Removes water vapour from the indoor air | Useful in closed stores and small buildings | Running cost; hard to keep up in large or open-door buildings |
| Removing moisture sources | Flued heaters, electric forklifts, closed doors | Tackles the cause | Not always practical in a working building |
| Anti-condensation felt or fleece | Factory-applied layer on the underside of new sheets | Holds small amounts of moisture | Only for new or replacement sheets; no insulation value |
| Insulated board or liner | Board or liner system fixed under the roof | Adds insulation | Needs fixings and brackets; air behind unsealed joints still reaches the cold sheet |
| Insulated overclad or replacement | New insulated roof over or instead of the old one | Solves condensation and leaks together | Highest cost, most disruption, roof structure checks |
| Closed-cell spray foam | Foam bonded directly to the underside of the sheet | Stops air reaching the sheet, insulates, seals laps, no brackets | Needs a clean, dry sheet, a fire strategy and an approved installer |
General comparison. The right answer depends on the building, its use and its moisture sources.
Several of these work well together. Removing moisture sources and keeping sensible ventilation always helps. The difference with a bonded, closed-cell foam is that it deals with the cold surface itself, rather than trying to keep the air dry enough never to touch it.
How closed-cell spray foam stops the drips
Closed-cell spray foam is sprayed onto the underside of the roof sheets as a liquid. It expands, bonds to the metal and cures into a rigid, continuous layer. Three things then happen at once:
- Warm indoor air can no longer touch the cold sheet. The foam's surface is on the warm side, so it stays well above the dew point in normal use.
- The roof is insulated. Enverge Nexseal LE, for example, has a declared thermal conductivity of 0.027 W/m·K and a closed-cell content above 90%, according to the Nexseal LE product data.
- Laps, gaps and fixings are sealed. The foam follows every corrugation and trough, with no brackets or joints for air to find.
Nexseal LE carries a 25-year manufacturer-backed product warranty. It is one of the closed-cell foams Duratite supplies through approved spray foam installers. See anti-condensation spray foam and corrugated roof insulation for the application in more detail.
Upgrading the insulation at the same time
If the work counts as a material alteration or works to a thermal element, TGD L 2022 Table 10 sets 0.25 W/m²K for a pitched roof insulated on the slope, and 0.25 W/m²K for a flat roof. Your designer or assigned certifier confirms whether it applies. Our warehouse insulation guide covers the thickness and U-value side.
Asbestos cement sheets
Many older Irish warehouses and farm buildings have asbestos cement roofs. Underside spray foam bonds to the sheet without drilling or fixing through it. The work still follows the asbestos rules, which changed in 2025. Read asbestos regulations in Ireland before you plan it.
The retrofit sequence in a working building
- Survey. Confirm the roof type, any asbestos, the condition of the sheets and fixings, the rooflights, and where the moisture is coming from. Moisture readings and photographs go in the report.
- Fix the leaks first. Foam on the underside of a leaking roof hides the leak. Laps, fixings, gutters and rooflights are repaired, or the roof is coated externally, before the underside is sprayed. See silicone roof coating.
- Agree the moisture plan. Flued heaters, sensible ventilation and door management still matter. Livestock buildings keep their ventilation.
- Agree the fire strategy. Your designer confirms the requirement under TGD B in the Republic or Technical Booklet E in Northern Ireland. Where a protective coating is specified, DC315 intumescent coating can go over the foam.
- Plan the zones. The installer works in sections, protects stock and equipment, and keeps people out of the spray area until it is safe to return.
- Apply and record. The sheet must be clean and dry. Thickness is checked as the work goes, and photographs and readings go in the handover file with the product warranty documents.
Cold stores and chillers have their own design issues. See cold store insulation.