A steel or aluminium sheet roof on a workshop, warehouse or livestock barn reacts quickly to the weather outside. On a clear night the sheets cool fast, and warm air inside the building carries moisture upward to meet them. Where it touches the cold underside, the moisture turns to water. Droplets form, grow and fall, sometimes over stock, machinery or a work bench, and often on mornings when it has not rained for days.
Condensation is a moisture problem that shows up as a roof problem. Searching for a hole in the sheets will not cure it, and neither will sealant. Fixing it means dealing with how much moisture the building makes, where that moisture goes and what the underside of the roof is like. This post explains the causes in plain terms for units and farm buildings around Taunton, shows how to tell condensation from a leak, and sets out the fixes in the order that usually works: ventilation to BS 5250 principles, anti-condensation linings, insulated sheets and, on older roofs, a check for asbestos first. It sits within our commercial roofing advice.
How condensation forms under metal sheets
Air carries water vapour, and warmer air carries more of it than colder air. When warm, damp air meets a surface cold enough to chill it below a certain point, some vapour changes to liquid. A bare metal sheet is a good conductor and has no insulation, so its underside follows the outside temperature closely. On a still, cold night it can be much colder than the air inside the building, and that is when it drips.
Where does the moisture come from?
From the building's own activity. People and animals breathe it out. Cattle and sheep give off a lot. Washing down yards and floors, drying processes, wet crops and stored produce, vehicles driven in from the rain and some types of heater all add to it. A unit with nobody in it can still produce moisture through damp concrete or stored goods.
Why does it seem worse in some buildings?
Tight buildings trap moisture, and open-sided ones lose it. A modern unit with sealed doors and no vents holds humidity in. A traditional barn with gaps at the eaves and under the ridge lets some escape. Fitting new cladding or closing a gap without thinking about air movement can turn a building that was dry into one that drips.
The remedy always begins with the same two questions: how much moisture is produced, and where can it go? Our answer on the leaking industrial roof explains how the two problems get confused.
Is it condensation or a leak?
The difference matters because the repairs are opposite. A leak needs the entry point found and sealed. Condensation needs the moisture handled.
What does condensation look like?
Droplets cover a broad stretch of the underside, not one trail. It is worst on cold mornings and after a clear night, and it happens whether or not it has rained. It may appear on purlins, pipes and the top of a wall as well as on the sheets. The drips often stop by midday as the roof warms, then return next morning.
What does a leak look like?
A leak tracks the weather, turning up after rain, particularly wind-driven rain, and it shows up in a line or a patch below a lap, a fixing, a rooflight or a flashing. Staining and rust trails mark the path. A leak persists while the roof is wet, whatever the temperature.
Of course, both can occur together, and a roof with condensation is more likely to be rusty at the laps, which then does leak. Write down the date, the weather and the time whenever you see water. Patterns in those notes usually settle the question before anyone goes up.
Can you test it?
On a dry, cold morning, look for droplets on the underside of the sheets and on other cold surfaces. On a wet day, look for a point of entry. Do not climb up to find out. Many metal roofs have rooflights or older sheets that will not take weight, and HSE guidance regards the whole of an asbestos cement roof as fragile. A survey from safe access finds the fault without the risk.
Ventilation and BS 5250
Moving moist air out of a building is the first line of defence. BS 5250 is a code of practice for the control of moisture in buildings. The 2021 edition is the current one, and BSI lists it as under review. It does not hand out one answer for every shed. It asks that a designer looks at how much moisture a building makes and provides for it to leave.
What does ventilation look like on an industrial roof?
Openings at the eaves and the ridge, so that air enters low and leaves high; ridge vents or turbine-type ventilators; roof-mounted extract fans in damp processes; and, in livestock buildings, open ridges or space boarding at the sides. The openings must be sized for the building and positioned so that air actually moves through, not around, the damp area.
Do fans help?
Where a process produces a lot of moisture, extract fans take it out at the source, and that works better than trying to move air through gaps. A fan fixed to a roof is a new penetration, so the sheet around it must be flashed properly. Ventilation is specified for the building and its process, with openings and extract points sized to suit.
Do not seal every gap in the name of draught-proofing. Closing eaves openings or blocking ridge vents can stop the building breathing and make the problem worse. If you are re-cladding, agree the ventilation before the first sheet goes on.
Ventilation alone may not be enough on a bare sheet roof in a cold spell. The roof then needs something between the cold metal and the damp air, which is the next section.
Anti-condensation linings and insulated sheets
Even with good ventilation, a bare metal sheet can run cold enough to drip. Two measures address the surface directly.
What is an anti-condensation lining?
A coating or backing on the underside of the sheet that absorbs moisture when it forms and releases it again when the air warms and dries. It does not stop condensation forming. It holds the water until conditions change, so it does not fall onto what is below. Some sheets come with this backing from the factory, and a similar treatment can be added to existing sheets from beneath. It works within limits: in a building that makes a great deal of moisture, a lining can saturate and drip anyway.
What do insulated sheets and panels do?
An insulated roof panel has a core between two metal skins, so the inner face stays much closer to room temperature and the surface that meets the damp air is no longer cold. A built-up system, with a liner tray, insulation and an outer sheet, does the same. The risk with a retrofit is moisture getting into the insulation at laps and fixings, so details at joints and penetrations need care. Our answer on insulated roof panels explains what to ask.
The choice depends on the building. A lining is a lighter, cheaper treatment for a modest problem. Panels or a built-up roof are the answer when the building is to be heated, when people work under the roof, or when the roof is being replaced anyway. Sheets that sit on a purlin frame also need checking for load, since insulated systems can weigh more than the bare sheets they replace.
Whatever you choose, ask for the details at the eaves, ridge and rooflights in writing. Condensation problems usually come back at the junctions, not on the open sheets.
Barns and farm buildings: special cases
Livestock, stored hay and grain, and wash-down water make farm buildings among the most humid roofs we visit. A cattle shed in winter can produce a great deal of vapour, and a closed shed will drip onto the animals and the feed.
How should a livestock building be ventilated?
Generously. The usual approach is plenty of air inlet low down at the sides and a continuous open ridge or ridge outlet above, so that warm, damp air rises and leaves. A shed shut up tight to keep it warm will rain on its occupants. The opening sizes depend on the stock and the building, and the farm's vet or adviser may have views as well as the roofer.
What if the roof is old asbestos cement?
Do not drill it, cut it or fix anything to it to improve ventilation. Check what the sheets are first. If they may be asbestos cement, drilling for a vent or lining turns a moisture problem into an exposure one. A survey settles the material before anyone picks up a drill.
Does condensation damage the building?
Over time, yes. Dripping water rusts purlins and fixings, wets insulation and feed, and speeds corrosion at cut edges. The answer on coating a rusty metal roof deals with the aftermath. Treat the cause as well as the rust, or you will be recoating again in a few years.
Damp air stays around for longer where a roof sits on the Levels or in the hills, so ventilation matters most there.
A sensible order of fixes
Work through the fixes in an order that starts cheap and reversible and moves on only if needed.
Step by step
First, reduce the moisture at source. Stop unflued heaters adding vapour, vent dryers and wash areas outside, cover stored wet materials and fix leaks so that damp does not rise from the floor. Second, improve ventilation, with eaves and ridge openings or fans where processes demand them. Third, add an anti-condensation lining if the sheets still drip in cold spells. Fourth, if the roof has little service life left or the building is to be heated, replace it with an insulated system. Fifth, deal with the damage: treat the rust and re-seal the laps.
When should you call a roofer?
When you cannot tell condensation from a leak, when sheets are rusting or sagging, when you plan to change the use of the building or when ventilation needs new openings in the roof. The roofer should ask how the building is used, look at the underside as well as the top, and show you photos of your roof, not stock pictures.
What will it cost?
That depends on the area, the access, the type of sheet and the system chosen, and the survey fixes it.
For the next step, send us the building type, what happens in it and where the drips fall, and we will arrange a survey that looks at ventilation, linings and the sheets together.
Drips under metal sheets
Will sealant stop condensation on a metal roof?
No. Condensation forms on the underside from moist air inside the building, so sealing the top changes nothing. It can even trap moisture at laps. Look at ventilation, moisture sources and the underside of the sheets first, and have a survey tell you whether any laps or fixings also leak. Sealing the wrong place wastes money and hides the real fault.
Do I need roof vents on a steel-framed unit?
Often, yes, especially where the building makes moisture or is closed up tight. BS 5250, which covers moisture control in buildings, points to getting damp air out. The type and size of the openings depend on the building and what happens inside it, so a designer or roofer should work them out for your shed.
Can I drill an old cement sheet roof to add vents?
Not until you know what it is made of. Old corrugated cement sheets may contain asbestos, HSE advises assuming it in anything from before 2000, and drilling can release fibre. Get a survey, and cut openings only in a way the survey allows, using a method that keeps dust to a minimum.
More on this subject: Commercial roofing in Taunton
