Condensation forms on a metal roof when warm, moist air from inside the building meets the underside of a cold roof sheet and drops below dew point. The roof loses heat to the night sky, so the steel ends up colder than the air around it. The fix combines vapour control, a thermal buffer under the sheet, and roof space ventilation.
In short
- Water dripping on a clear morning with no rain is condensation, not a leak.
- Three levers control it: stop the moisture at the source, block it reaching the steel, keep the steel warmer.
- Ventilation is the fourth, and NCC 2022 makes it mandatory in climate zones 6, 7 and 8.
- The damage you cannot see is corrosion starting on the underside of the sheet.
Most people find it when water drips onto a ceiling on a clear morning with no rain for days. The roof is not leaking. It is sweating.
Why it happens
Air holds water vapour. How much it can hold depends on temperature, and warm air holds more than cold air. Cool that air and it reaches a point where it cannot carry the moisture any longer. That temperature is the dew point. Touch a surface colder than the dew point and the excess water leaves the air and forms on the surface. It is the same thing that happens to a cold drink on a summer afternoon, at the scale of a roof.
A metal roof is very good at getting cold. BlueScope describes the cycle in its technical bulletin on roof condensation: the roof cools at night by radiating heat to the sky, warm moist air from the building interior enters the roof cavity, and the water vapour in that air condenses on the underside of the roof sheet. On a clear, still night a metal roof can end up colder than the outside air, which is why condensation appears on nights when nothing else is wet.
The moisture comes from ordinary use of the building. Showering, cooking, washing and drying clothes, dishwashers, unflued gas heaters, and simply having people in the room. In a commercial building add wash bays, commercial kitchens, indoor plant and anything with a wet process. A single household can put several litres of water into the air every day.
Two things have made this more common. Buildings are sealed tighter than they used to be, and they are insulated harder. Both are good for energy use. Both also mean the moisture the building generates has fewer ways out. The Australian Steel Institute names this directly: increased insulation levels and more tightly sealed buildings have created a greater need to manage internal moisture, and failure to let water vapour escape is the primary cause of moisture problems in roof systems.
Which buildings are most at risk
| Situation | Why it raises the risk |
|---|---|
| Cool climate zones | Bigger temperature difference across the roof for longer periods of the year |
| Low pitch metal roofs | Less air movement in the cavity, and condensate has to drain rather than run off |
| Uninsulated or thinly insulated sheeting | The steel surface facing the building stays cold |
| Exhaust fans venting into the roof space | Puts humid air exactly where you do not want it |
| Tightly sealed new builds | Moisture generated inside has nowhere to go |
| Wet processes in commercial buildings | Much higher moisture load than a house |
| Dark roof colours | Higher surface temperatures by day, larger swings at night |
What it looks like on site
| What you see | What it usually means | First thing to check |
|---|---|---|
| Dripping inside on a cold, clear morning | Condensation on the underside of the sheeting | Where the exhaust fans discharge |
| Damp ceiling patches that dry out by afternoon | Overnight condensation, not a leak | Roof space ventilation at eave and ridge |
| Rust on the underside of sheets and on screws | Moisture sitting against the steel | Whether insulation is wet or touching the sheet |
| Heavy or sagging insulation, stained facing | The blanket has been getting wet | Whether it overhangs into the gutter |
| Musty smell or mould high on walls | Persistent high humidity in the roof space | Ventilation and internal moisture sources |
The most expensive version is the one you cannot see. If insulation gets wet and stays in contact with the underside of the sheet, corrosion starts from the inside. BlueScope's warning is worth repeating: under those conditions corrosion of the roof sheets is generally not detectable until the material has perforated.
Four levers that control it
1. Cut the moisture at the source
Exhaust fans and range hoods must discharge to outdoor air, not into the roof cavity. NCC 2022 Housing Provisions clause 10.8.2 requires exhaust from kitchens, bathrooms, sanitary compartments and laundries to discharge directly or by shaft or duct to outdoor air, and applies the same rule to vented clothes dryers. Venting a bathroom fan into the roof space is one of the most common causes of a wet roof, and one of the cheapest to fix.
2. Stop vapour reaching the steel
A vapour barrier under the sheeting keeps internal moisture below the cold surface. The Australian Steel Institute is explicit that the foil on a blanket and foil system must be impermeable, so internal water vapour cannot reach the roof sheet.
Where a separate reflective membrane is used, BlueScope recommends it should have a vapour barrier classification to AS/NZS 4200.1:2017, should not degrade or shrink at the high temperatures under dark roofing, and should have reflective properties on both sides. In cooler climates it recommends sealing the overlaps with appropriate tape so the membrane works as a vapour control layer rather than just a thermal one.
3. Keep the cold surface away from the moist air
The more insulation sits between the roof sheet and the roof space, the warmer the surface facing the building stays. BlueScope makes the point directly: adding a blanket layer between the reflective membrane and the roof sheeting insulates the membrane from the cold sheet and further reduces condensation risk. This is why blanket and foil systems perform better in cold climates than a bare membrane.
Air gaps matter as well. A reflective membrane should be draped between battens to give at least 20 mm of air space above and below it.
4. Ventilate the roof space
Vapour that does get into the roof needs a way out. The Australian Steel Institute notes that practices restricting roof space ventilation can trap internal water vapour and cause moisture problems, and shows the standard arrangement: ventilation at the eave or fascia, with matching ventilation at or near the ridge. Air enters low and leaves high, which is what makes it move at all.
NCC 2022 made this a requirement in the colder zones. Housing Provisions clause 10.8.3 sets roof space ventilation for climate zones 6, 7 and 8, with opening areas that vary by pitch, for example 25,000 mm² per metre at each of two opposing ends for a roof under 10 degrees, and 7,000 mm² per metre at the eaves plus 5,000 mm² per metre at high level for a roof between 15 and 75 degrees.
What the NCC asks for
| Requirement | Where | What it says |
|---|---|---|
| Membrane vapour permeance in external walls | Housing Provisions 10.8.1 | Climate zones 4 and 5 need at least a Class 3 membrane (not less than 0.143 µg/N.s). Climate zones 6, 7 and 8 need Class 4 (not less than 1.14 µg/N.s). |
| Exhaust discharge | Housing Provisions 10.8.2 | Kitchen, bathroom, sanitary and laundry exhaust, and vented dryers, must discharge to outdoor air. |
| Roof space ventilation | Housing Provisions 10.8.3 | Applies in climate zones 6, 7 and 8. Opening areas set by roof pitch, with a minimum roof space height of 20 mm. |
| Thermal break | Housing Provisions 13.2.3(7) | Metal sheeting fixed directly to metal purlins, rafters or battens, with no ceiling lining or a lining fixed to those members, needs a thermal break of at least R0.2. |
The vapour permeance rule at 10.8.1 is written for external walls, not roofs. It gets quoted loosely as a roof requirement, which it is not. Roof condensation is handled through the ventilation and construction requirements above, plus the general performance requirement.
Detailing that decides the outcome
- Trim insulation clear of the gutter. Blanket that overhangs the last batten into a gutter acts as a wick and draws water through a large area of insulation. Keep it back from the drip edge.
- Watch low pitch roofs. BlueScope notes that low pitch systems relying on drainage to remove condensation need special attention to anything that ponds water or blocks that drainage, including battens, membrane sag, penetrations and fasteners.
- Tropical roofs are detailed differently. In warm humid places such as Darwin and Cairns, where the outside air is warm and moist and the inside is air conditioned, the vapour drive reverses. BlueScope notes the membrane may be placed with the reflective surface towards the roof sheeting, which typically means a membrane both above and beneath the blanket.
- Seal the laps. Taped overlaps turn a thermal membrane into a working vapour control layer.
- Do not block the eaves. Insulation pushed hard into the eave line closes off the ventilation path the roof depends on. Leave the air path open.
- Mind the penetrations. Flues, vents and skylights break the vapour layer. Seal them back to the membrane rather than leaving a hole around every pipe.
Fixes that do not work
Three things get tried first and none of them solve it on their own.
Adding a dehumidifier inside. It treats the symptom in the room and does nothing for the air already in the roof space.
Sealing the roof space tighter. The instinct is to stop the moist air getting in. In practice, sealing a roof cavity without addressing the sources traps whatever is already there, and the ASI is direct that restricting ventilation makes moisture problems more likely, not less.
Painting the underside of the sheets. A coating does not change the surface temperature, so the air still reaches dew point against it. It also hides the corrosion you need to be able to see.
Is it condensation or a leak
A leak follows the weather. Condensation follows the temperature. If water appears after clear cold nights and there has been no rain, if it is worst in winter, if the drips are spread evenly across the underside of the sheeting rather than tracking from one point, and if the building runs showers or a wet process, condensation is the likely cause. A leak usually stains the same spot every time, tracks from a penetration or a lap, and appears during or shortly after rain.
Fixing an existing roof
Order of work, cheapest first:
- Duct every exhaust fan and range hood to outside. This alone solves a surprising number of cases.
- Open up roof space ventilation at eave and ridge so moist air can leave.
- Check the roof space for blocked eaves, missing sarking laps and insulation sitting hard against the sheet.
- Look for wet insulation near gutters and trim it back clear of the drip edge.
- If the sheets are coming off anyway, add blanket and foil during the reroof. That is the only practical time to put insulation under the sheeting.
Dealing with a roof that sweats? We stock the vapour control and thermal layers this comes down to: membranes and sarking from Ametalin, Fletcher Insulation and Proctor Group, roofing blankets for under the sheeting, and thermal break products.
Browse building membranes and roofing blankets, or send us the details and we will work through the build up with you.
Frequently asked questions
Why does my metal roof sweat?
The roof radiates heat to the sky at night and becomes colder than the air inside the roof space. Warm moist air from the building reaches that cold steel, drops below dew point, and condenses on the underside of the sheets.
Why is water dripping from the inside of my metal roof with no rain?
That is the signature of condensation rather than a leak. It appears on clear cold mornings, spreads evenly across the sheeting, and is worse in winter and in buildings that generate a lot of moisture.
Does insulation stop roof condensation?
Insulation reduces the risk by keeping the surface facing the building warmer and by blocking vapour from reaching the steel. It does not remove moisture from the building, so it works with ventilation and exhaust ducting, not instead of them.
Does the NCC require roof space ventilation?
In climate zones 6, 7 and 8 it does. NCC 2022 Housing Provisions clause 10.8.3 sets minimum ventilation opening areas that change with roof pitch.
Can condensation rust a Colorbond roof from the inside?
Yes. Where wet insulation stays in contact with the underside of a steel sheet, corrosion can start from the underside and is generally not detected until the sheet perforates.
Should the sarking be sealed or breathable?
It depends on the climate and where it sits. Under a metal roof an impermeable foil keeps internal vapour away from the sheet. In external walls, NCC 2022 requires a minimum vapour permeance in climate zones 4 to 8. Different layers, different jobs, so check the specification.
Will a whirlybird fix condensation?
It helps only if there is somewhere for replacement air to enter. Ventilation works as a path, low at the eaves and high at the ridge. An extractor with no inlet moves very little air.
Is condensation covered by warranty?
Usually not, because it is a design and use issue rather than a product fault. That is the argument for getting the vapour layer, the insulation and the ventilation right at build time.
Sources
- BlueScope Corrosion Technical Bulletin CTB-11, Roof Condensation and Insulation, October 2022
- Australian Steel Institute, Building with Steel Factsheet #2, Vapour Barriers in Metal Roofing
- Australian Steel Institute, Building with Steel Factsheet #3, Blanket and Foil insulation in Metal Roofing
- NCC 2022 Housing Provisions, Part 10.8 Condensation management and Part 13.2 Building fabric
- AS/NZS 4200.1:2017 and AS 4200.2:2017, Pliable building membranes and underlays
General information only. Condensation is a design problem specific to each building, its climate zone and its use. Get advice on your own roof before committing to a fix.