A roof that drips in cold weather but stays dry in the rain is not leaking. It is condensing, and the fix is completely different from the fix for a leak. Understanding why it happens makes the solution obvious.

The mechanism

Air holds water vapour, and warmer air holds more of it. When moist air touches a surface that is colder than its dew point, the vapour turns back into liquid on that surface. A steel roof sheet is very good at getting cold, because thin metal loses heat quickly to a clear night sky. So the underside of the sheet becomes the coldest surface in the building, and that is where the water appears.

This is why the problem is worst on clear, still nights in autumn and spring: the roof radiates heat away, the temperature difference is at its greatest, and there is no wind to move the moist air on.

Where the moisture comes from

More sources than people expect:

• Livestock — animals produce a great deal of water vapour simply by breathing
• Wet vehicles and machinery brought in out of the rain
• Stored crop, which continues to respire
• Bare ground or an unsealed slab, which wicks moisture up from below
• People, in any occupied workshop

A building with none of these rarely has a condensation problem. A building with two or three of them almost always will, unless the roof is designed for it.

Why ventilation alone often disappoints

The instinctive answer is to ventilate: move the moist air out before it can condense. This does work, and on some buildings it is enough.

The trouble is that ventilation and the purpose of the building are often in conflict. Livestock housing needs to stay draught-free at stock level. A workshop you are heating loses the heat along with the moisture. And ventilation depends on air actually moving, which on a still night is precisely when it does not.

So ventilation is worth getting right, but relying on it alone is what leads to buildings that are ventilated and still dripping.

What actually works

Anti-condensation backing

A fleece membrane bonded to the underside of the sheet absorbs moisture as it forms and holds it there, then releases it back into the air as conditions change and the building warms. The water never gets the chance to form a droplet and fall.

This is a neat solution because it treats the symptom exactly where it occurs, needs no ongoing intervention, and does not fight the building. Our anti-condensation roof sheets are box profile with the backing already applied.

The important limitation to understand: the fleece has a capacity. In a building generating very large amounts of vapour continuously, backing alone may not be sufficient, and you are into insulation.

Insulation

Insulating keeps the inner surface warmer than the dew point, so condensation does not form in the first place. That makes it the more complete answer, particularly in heated or occupied buildings. Insulated roofing sheets combine the outer sheet and the insulation in one component.

The pitfall is a partial job. Insulation with gaps creates cold bridges, and condensation concentrates at exactly those points. Patchy insulation can leave you worse off than none, because the water that used to form as a thin film across the whole roof now runs off a few cold spots.

Getting the details right

Whichever route you take, cut edges and laps deserve attention. A backing that stops short of the sheet edge leaves an untreated strip, and that strip will drip.

Diagnosing it on an existing building

Three checks distinguish condensation from a leak. Does it happen in dry, cold weather? Is the water spread widely across the underside rather than tracking from one point? Does it stop when the building warms up? Yes to all three and you are looking at condensation.

It is worth being sure before you spend money, because chasing a non-existent leak around a roof is a frustrating way to spend a week.

Specifying for it

If you are re-roofing a building that has had a condensation problem, say so when you specify. It changes the sensible answer, and it is much cheaper to design for than to retrofit. Talk to us about the building and what it is used for, and we will point you at the build-up that suits it.