Why a steel warehouse rains indoors in a humid climate
Every few months I get a message that reads something like this: “the roof is dripping, the goods underneath are wet, and there is no hole in the sheet.” Sometimes there is a photograph of a dry roof from outside and a wet floor from inside, which is exactly the contradiction that makes people suspect the supplier.
There is no contradiction. The building is not leaking. It is condensing, and that is a different problem with a different solution.
What is actually happening
Air holds water vapour in proportion to its temperature. Warm air holds a lot; cool air holds less. When air is cooled past the point where it can hold what it is carrying, the surplus comes out as liquid. That point is the dew point, and the surface it comes out on is whatever surface is coldest.
In a single-skin steel warehouse, the coldest surface in the building is the underside of the roof sheet. It is thin, it has almost no mass, and at night it is directly exposed to the sky.
Three things drive it below the dew point:
- Night sky radiation. On a clear night, the roof sheet radiates heat upward into the sky and can end up several degrees colder than the air around it, on both sides. This is why the worst nights are the clear, still ones — the weather nobody worries about. Cloud cover acts as a blanket and suppresses the effect entirely, which is why the problem seems erratic and therefore imaginary.
- Warm, moist air arriving from outside. An open door in the evening, an unsealed wall junction, or a wall that stops short of the sheet lets humid outside air into contact with a surface that has already cooled. The moisture in that air comes out immediately.
- Water that was built in. A concrete slab releases a large quantity of water as it cures. If the building is closed up and the roof goes on before the slab has dried, that water is trapped inside with nowhere to go. It leaves through the roof underside, and it looks exactly like a leak for the first several weeks.
Note that two of those three have nothing to do with rain, and one of them has nothing to do with the climate at all. That is why blaming the weather rarely fixes it.
Why a single skin is the worst case
A single-skin roof has one layer of steel between the inside air and the outside temperature. There is no mass to buffer the swing and no second surface to interrupt the moisture. The sheet is thermally tied to both sides at once, so it follows the outside temperature closely — and in a humid climate, closely is enough.
This is also why the problem is more common on the roof than on the walls, and worse in a tall building than a low one. The roof sees the sky; the walls mostly see the ground.
What does not work
Worth listing, because these are the things people try first:
- Painting the underside. It changes the appearance of the condensation, not the amount of it. The surface still reaches the dew point.
- Thermal break washers at the purlin fixings. These address cold bridging between the sheet and the purlin — a real issue, and a different one. They do nothing for moisture condensing on the broad face of the sheet.
- Better gutters or more downpipes. Relevant to a leak. Irrelevant here.
- Waiting for it to dry out. It will, in the morning. Meanwhile the drips landed on the goods, and in a humid climate the cycle repeats the next night.
What works, in order of importance
1. A continuous vapour barrier on the correct side
This is the rule that matters more than any other, and the one most often broken. Insulation alone does not stop condensation. It controls where the condensation happens, which is only useful if you also decide where the moisture is allowed to stop.
The principle is that vapour moves from the side with the higher vapour pressure toward the side with the lower one. In a warehouse in a humid climate, that is almost always from inside to outside. So the barrier goes on the inside face, behind the lining, and it must be continuous — lapped and sealed at every joint, at every penetration, and at the wall-to-roof junction.
A barrier with a gap in it is worse than no barrier, because it lets moisture into the insulation where it can no longer dry out. It then sits in the insulation, or comes through the liner in a different place, and the problem moves rather than disappearing.
2. Insulation with a proper facing
Insulation gives the inner surface a temperature closer to the room air, so it stops reaching the dew point. The facing is what makes it a system rather than a blanket — a foil or laminate facing acts as the vapour control layer when its joints are sealed.
For a warehouse that is not air-conditioned, a moderate thickness is usually enough to break the cycle. The point is not thermal comfort; it is keeping the inner surface above the dew point. Those are different targets and they lead to different thicknesses, which is why copying a cold-store specification is a waste of money.
3. An anti-condensation membrane, where the building is unheated and ventilated
A fleece-backed membrane has a fibrous underside that absorbs moisture as it condenses and releases it again when the air warms. It prevents drips without insulating.
It is genuinely useful in unheated, well-ventilated buildings, and it is often the cheapest honest answer for a store or a shed where nobody works at night. It has a finite absorption capacity, so it is not a solution for a building with a high internal moisture load, and it is not a substitute for insulation in a space that is cooled. Be clear about which of those you have before choosing it.
4. Ventilation that removes the moisture, not just the heat
Ridge ventilators with matched low-level inlets let humid air out and drier air in, reducing the supply of moisture to the cold surface. This is not a substitute for the barrier, but it changes the balance and it is often appropriate in a store where the alternative is a full insulated build-up.
Ventilation has to be designed as a pair. Ridge vents without low-level inlets do very little, because there is no replacement air to push the moist air out.
5. Take the moisture sources away
Let the slab dry before the building is closed. Do not store wet material, green timber or damp packaging. Keep doors closed at night where the operation allows it. None of this is engineering, and all of it is cheaper than any of the four items above.
The one structural interaction worth knowing
Because this is a roof build-up decision, it belongs to the adjustable category rather than the five numbers that fix the frame. That is good news: you can change it later.
There is one exception. If you want a lined roof with a proper vapour barrier, the purlins have to carry the lining system and its fixings, and the purlin spacing has to suit the liner spans. That is a frame detail fixed at design stage. Deciding on a lining system after the purlins are rolled is possible but it constrains your options, and the constraint is invisible until someone tries to specify the lining.
So: if you expect to line the roof, say so when you ask for the frame. It costs nothing to declare and it keeps the choice open. The wider logic of which decisions are reversible is set out in the five numbers that decide the frame.
Where this comes up most
The pattern to look for is not average temperature. It is high daytime humidity plus a genuine night-time drop, which is why the enquiries I see come from coastal West Africa, the East African highlands, and the northern coast of South America — places where people are often surprised by it, because the daytime weather feels nothing like the problem.
If your site has clear nights, humid air, and a single-skin roof over anything that must stay dry, assume this will happen and specify for it. The alternative is finding out after the first consignment is loaded.
Two related notes: on ventilation in a moisture-sensitive store, a cocoa bean store works through the same trade-off from the product side. On the cost side, what really drives the cost of a steel building explains why a roof build-up is a small share of the total and a poor place to economise.
Written by Johnny, who advises owners and project managers in Africa and South America on steel buildings. If your roof is dripping and the sheet is sound, send me a photo of the inside of the roof and the roof build-up you specified, and I will tell you which of the five causes above it most likely is.
Related: warehousing notes · all notes · the five numbers
