Why metal industrial roofs sweat, and how to stop it
Updated 23 October 2026 · SEO Dons Editorial
Why does condensation form on a metal roof?
The Met Office describes the dew point as the temperature at which the air cools enough for its water vapour to condense into droplets. MCRMA's glossary defines condensation as water that collects as droplets on a cold surface when humid air is in contact with it. A metal roof can meet both conditions. MCRMA describes a single skin system as an uninsulated profiled metal sheet fixed directly to the purlins, so little separates the sheet's underside from the outside temperature, while the air below it carries the building's moisture.
MCRMA's archived 2004 roofing guide adds two useful details. On the impermeable inner face of a metal-faced roof, condensation is much more likely than mould, and it does not occur until the surface relative humidity reaches 100%. A fine mist of under 10 grams per square metre is common and disperses as temperatures rise. Water only runs once at least 70 grams per square metre has built up on a sloping surface, and drips once 150 grams per square metre has built up on a horizontal one. So a damp-looking sheet is normal, and dripping signals a real and repeated build-up.
What makes metal roofs sweat more?
| Factor | What the sources say |
|---|---|
| Clear, cold nights | The Met Office says that after sunset surfaces lose heat by radiating it to space, cloud acts like a blanket, and on clear nights objects close to the surface cool quickly. A roof is open to the sky in the same way. LRWA's 2020 guide notes that BS 6229 asks for clear sky radiative cooling to be allowed for in condensation assessments of flat roofs. |
| Humid processes | MCRMA names swimming pools, laundries and some food processing plant as higher risk, along with colder external climates. |
| Over-pressurised buildings | MCRMA says buildings run at an over pressure are at especial risk because it can drive warm moist air into the structure. |
| Rooflights | MCRMA says rooflights have significantly worse thermal performance than the insulated cladding around them, so condensation is more likely there, and occasional misting is to be expected. |
| Little or degraded insulation | The uninsulated single skin is the clearest case. Thin or deteriorated insulation moves a roof towards the same position. |
How can you tell condensation from a leak?
None of the sources checked sets out a field test for telling the two apart, so these are practical checks to put to a surveyor rather than a standard method.
| Clue | Points to condensation | Points to a leak |
|---|---|---|
| Timing | Follows cold, clear nights or humid shifts, and eases as the day warms | Follows rainfall, sometimes hours later |
| Pattern | Fine droplets over wide areas of the underside, heaviest on coldest surfaces | Concentrated at laps, fasteners, rooflight edges, valleys and gutters |
| Dry weather | Still occurs | Stops, unless water is ponded |
Both can happen together, and the industrial roof refurbishment page covers the places where profiled roofs typically let water in. Do not send staff onto the roof to look without a safe system of work. HSE says all roofs should be treated as fragile until a competent person has confirmed they are not, and the fragile roofs and rooflights guide sets out the owner's duties.
What can you put on or under a metal roof to stop it sweating?
| Fix | What it does | Limits |
|---|---|---|
| Humidity and ventilation control | MCRMA says the internal moisture content should be assessed and, if appropriate, controlled with the correct ventilation or air conditioning | Treats the source, so it comes first when a process adds moisture |
| Fleece-backed sheets | A polyester fleece on the sheet's reverse side holds moisture in pockets and releases it as the air warms | One supplier says it cannot completely stop condensation, needs adequate ventilation to dry, and is recommended for roofs pitched above 10 degrees |
| Sealed vapour control layer | MCRMA says it must be continuous with all laps sealed, including at abutments, rooflights, penetrations and gutters | A liner with open laps is not a vapour control layer |
| Insulated over-roof | MCRMA's over-roofing guide says insulation between the old and new roof reduces condensation and helps meet local authority requirements | Needs a structural check, and a condensation forecast in the design |
| Breather membrane | MCRMA says it is only needed in high-humidity double skin roofs, with an effective vapour control layer and eaves-to-ridge ventilation | Not needed in most normal applications |
| New insulated panels | MCRMA says factory composite panels have no voids above the insulation, so no rib ventilation or breather membrane is needed | Replaces the roof, see the composite panels guide |
So what goes under a metal roof depends on its construction. A double skin roof has a liner, a spacer system and an outer sheet with a mineral fibre quilt, and its liner carries the vapour control job. A fleece sits on the sheet itself. Because the Cladco page says its fleece is applied during roll-forming at its works, it is something to specify with new sheets, so ask a supplier whether a fleece can be fitted to an existing roof.
When does a fix trigger Building Regulations work?
MCRMA says an over-roof is likely to need Building Regulations compliance and a planning application, that a competent person such as a structural engineer should confirm the structure can carry the weight, and that the calculations should include a condensation forecast. On energy, the Regulations define renovation of a thermal element as providing a new layer in it or replacing an existing layer, and Approved Document L lists a new layer through cladding as renovation. If more than 50% of the roof's surface is renovated, or more than 25% of the building envelope (a major renovation), the Regulations require the whole element to meet the energy requirement so far as that is technically, functionally and economically feasible. The 2026 edition of Approved Document L takes effect on 24 March 2027 in England for building work not connected with higher-risk buildings.
That upgrade brings its own condensation question, because insulation changes where the dew point falls. The condensation and BS 5250 guide explains the calculation, and the over-sheeting page covers the method. If the roof, rooflights and gutters are all ageing, the industrial roof refurbishment page compares the routes. To compare contractors, send an enquiry through the form on this site.
Sources
- How does frost form? (fact sheet), Met Office, checked 22 October 2026.
- Why frost can still occur in late spring, Met Office, checked 22 October 2026.
- Glossary, Metal Cladding and Roofing Manufacturers Association (MCRMA), checked 22 October 2026.
- Technical Paper 6: Profiled metal roofing design guide (2004, archived), MCRMA, checked 22 October 2026.
- GD38 Over-roofing guidance document, MCRMA, checked 22 October 2026.
- Design guide for specifiers 2020 (liquid applied waterproofing), Liquid Roofing and Waterproofing Association (LRWA), checked 22 October 2026.
- Dripstop anti-condensation fleece, Cladco, supplier's own product page, checked 22 October 2026.
- Roof work, Health and Safety Executive, checked 22 October 2026.
- The Building Regulations 2010, regulation 2 (interpretation), legislation.gov.uk, checked 22 October 2026.
- The Building Regulations 2010, regulation 23, legislation.gov.uk, checked 22 October 2026.
- The Building Regulations 2010, regulation 35, legislation.gov.uk, checked 22 October 2026.
- Approved Document L, Volume 2 (2021 edition incorporating 2023 amendments), GOV.UK, checked 22 October 2026.
- Approved Document L, Volume 2 (2026 edition), GOV.UK, checked 22 October 2026.
Common questions
Is condensation under a metal roof harmful?
A fine mist is normal and disperses. MCRMA says sufficient condensation could cause corrosion of metal components and wetting of insulation, and the condensate could run into the building, staining finishes or dripping onto equipment or processes. Repeated dripping is the sign to act on.
Will over-roofing stop the dripping?
It can reduce it. MCRMA says insulation between the old roof and the new one reduces condensation, but if the existing roof is damaged or leaking, a membrane should be used so that condensation in the cavity cannot enter the building through gaps. The design should include a condensation forecast.
Why do rooflights drip more than the sheets?
MCRMA says rooflights have significantly worse thermal performance than the surrounding insulated cladding, so condensation is more likely at them. Misting from time to time generally dissipates without harm, but MCRMA adds that if condensation is unacceptable, rooflights should be avoided completely.