Why does a warehouse roof drip condensation?

Before anyone orders sealant or books a gutter clean, work out which problem you have. A leak follows rain. It appears after a downpour, at a particular spot, usually under a lap, a fixing, a rooflight or a gutter, and it stops a while after the rain does.

Condensation follows temperature. It is worst on cold, still mornings and after the heating comes on, it can drip across the whole length of a roof at once, and the water may be dirty or rusty from the underside of the sheets. It can run for days in dry weather.

Many buildings have both. A small leak at a worn fixing can sit beside condensation along the whole run, and treating one does nothing for the other. Our guide to a leaking metal roof covers the leak side. For condensation we look at the underside as well as the top, because the underside often tells the story.

Air always carries some water vapour, and warm air carries more than cold. When warm, damp air reaches a surface cold enough, the vapour turns to liquid. A single-skin steel or fibre-cement sheet with nothing under it is exactly that kind of surface on a cold night, because the air outside cools it and nothing separates it from the room.

The drops form on the underside, grow, and fall. If the sheets are corrugated, the water also runs along the corrugations to the lowest point and drips from a lap or a fixing, so it is often wrongly blamed on a gap in the sheeting.

Insulation helps because it keeps the inner face warmer, so the vapour has no cold surface to meet. Gaps matter as much as the insulation itself, since moist air finds every joint and settles on the first cold bridge it meets. The standard on managing moisture in buildings, BS 5250:2021, is listed by BSI as current but under review.

Roofs rarely create the dampness. They only show it. Common sources in a warehouse or unit include flueless gas heaters, which release water vapour as they burn, wet processes such as washing or damp storage, a kettle-and-canteen area without an extractor, vehicles driving in from the rain, and sometimes the stock itself.

Ventilation decides whether that moisture leaves. A building with closed doors, no extract and little air movement lets the water build up until the cold roof finds it. Opening loading doors during the day helps only partly; the problem returns when they close.

Start by reducing what the building produces. If a heater is part of the problem, an alternative that vents its fumes and moisture outside is often the quickest improvement. A roofer can advise on the roof, but the moisture sources are for the occupier to review with whoever services the heating and ventilation. A short log of when and where the drips appear, with dates and the weather, helps that conversation a great deal.

Fixes tend to stack. First comes less moisture and more ventilation, through vents, extract fans or louvres positioned so the damp air really leaves the building. That is cheap compared with new roofing and sometimes solves a mild case alone, though it rarely cures a severe one on its own.

Next, the roof itself. A lining system with insulation fitted under the sheets lifts the temperature of the inner surface and seals the joints against moist air. Replacing a single-skin roof with a composite insulated panel does the same job in one go and sets a more weathertight roof at the same time.

Anti-condensation coatings and felt-backed sheets exist, and they suit some situations, but they treat the symptom of drips, not the cause. If the air is very damp, the moisture can still settle behind them. A metal roof coating protects the top surface from weathering and does not stop vapour forming below.

Spraying foam under a metal roof is sometimes offered as a one-visit fix. It needs care. The RICS consumer guide on spray foam warns that poor installation can bring dampness, including uncontrolled condensation, and the Energy Saving Trust advises against foaming over a roof that still lets water in. Any roof must be dry and sound before a product goes on it.

Whatever is proposed, ask where the water vapour will go afterwards, how the joints are sealed, and whether the result can be inspected or removed if the roof later needs repair. Concealed fixings and sheets hidden behind a lining are hard to check.

For a building you own or let, we start by surveying the roof from above and from below, photographing the drips and the laps. Our verdict then says whether the roof needs repair, a lining or replacement, or whether better ventilation is the real answer. A single written total follows for whatever work is chosen. See our commercial roofing page for the full range of work.