Rust streaks on the underside of the sheeting, black spotting on the ceiling, sagging wet batts and stains that appear on dry mornings are four symptoms of the same problem. The roof is not leaking, it is condensing. Reading which symptom you have, and where, tells you where the moisture is coming from and how urgently it needs dealing with.
In short
- A leak follows rain. Condensation follows a cold clear night, which is the first test.
- Rust on the underside of the sheeting and around the screws means water has been sitting there repeatedly.
- Wet insulation does not recover. It stays wet, loses R-value and holds moisture against the steel.
- The moisture almost always comes from inside the building, not from outside.
- Fixing it means cutting the moisture source, controlling vapour, and ventilating, in that order.
Most people meet this problem as damage rather than as physics. A brown ring on the plasterboard. Screws weeping rust. A batt that has slumped off the ceiling. The temptation is to treat what you can see, seal the roof, repaint the ceiling, replace the batt, and the same thing comes back the next winter.
This guide is about reading the evidence. For the underlying mechanism and how to design it out on a new roof, see condensation in metal roofs.
First test: leak or condensation
Everything downstream depends on getting this right, and it is usually settled in one question: when does the water appear?
| A leak | Condensation | |
|---|---|---|
| Timing | During or shortly after rain | Clear still nights, worst in winter, often no rain for days |
| Where it shows | Traceable to a penetration, a lap, a valley or a fastener | General, spread across the underside of the sheeting |
| Pattern | One path, repeated in the same spot | Many small drips, or an even film |
| What is wet | A defined run from the entry point | The whole cold surface, including screws and battens |
| Wind | Worse in driving rain | Worse when still, because still nights radiate best |
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 inside the building reaches the roof cavity, and the vapour in that air condenses on the underside of the sheet. On a clear, still night the roof can end up colder than the outside air, which is exactly why the water turns up when nothing else is wet.
If it is a leak, stop here and get the roof looked at. If the timing points to condensation, the rest of this applies.
What each symptom is telling you
| What you can see | What it usually means | How urgent |
|---|---|---|
| Beads or a film on the underside of the sheeting on a cold morning | Active condensation, caught early | Deal with it before it soaks anything |
| Rust streaks running down from the screws | Water has been sitting there repeatedly, over seasons | The fasteners and sheeting are being consumed. Act now |
| Rust on the underside of the sheeting away from any penetration | Condensation, not a leak. A leak would rust a path, not an area | Act now |
| Sagging or slumped batts | The insulation has taken on water and lost its structure | The R-value is already gone. Replace, do not dry |
| Black spotting on the ceiling, usually near the edges | Persistent surface moisture and poor air movement | Fix the moisture, not just the surface |
| Stains on plasterboard with no rain | Water has passed through the insulation and reached the lining | Advanced. The layers above are already wet |
| Damp, musty smell in the roof space | Moisture is sitting in there long enough to matter | Investigate before it becomes visible damage |
| Corroding batten or purlin fixings | The cold surface is wetting the structure, not just the lining | Get it assessed structurally |
Rust from the inside
Roofs are engineered to shed water off the top. The underside is a different environment, and it is not designed to sit wet.
Condensation puts a film of water on the coldest surfaces, which is the sheeting itself and the metal fasteners passing through it. That water sits for hours at a time and evaporates slowly, because the roof space has poor air movement at night. Repeat that a few hundred times across a few winters and the result is corrosion on a roof that has never leaked a drop.
Two details make it worse. Screws are a bridge between the cold outside and the warm inside, so they run colder than the sheet around them, which is why the streaks start at the fasteners. And any insulation sitting hard against the underside of the sheeting holds the water in contact with the steel instead of letting it evaporate.
This is why BlueScope's guidance for a separate reflective membrane is to drape it between the battens with air space above and below, rather than pulling it tight to the sheeting. Trapped water against steel is the thing to avoid.
Mould and black spotting
Mould is a moisture problem wearing a different costume. It needs a damp surface to grow on, so where you find it tells you where the surface is staying wet.
In a roof context it usually turns up in the same places: the ceiling near external walls and corners, where the lining runs coldest and air moves least, and on the underside of linings below an uninsulated or poorly ventilated roof space. Cleaning it treats the appearance. Unless the surface stops being damp, it comes back.
The practical sequence is the same as for everything else here: find the moisture, cut it, then deal with the surface. If the mould is extensive, or anyone in the building has a health concern about it, get a specialist assessment rather than working from a blog post.
Wet insulation, and why it stays wet
This is the part people underestimate. Insulation that gets wet in a roof space does not behave like a towel on a line.
- It loses its R-value. Bulk insulation works by holding still air. Fill that space with water and the thermal performance collapses while the water is there.
- It does not dry. A roof space at night is cold, still and humid. There is very little drying energy available, so a wet batt in July is still a wet batt in August.
- It holds water against the steel. Wet material in contact with the sheeting keeps the corrosion cycle running long after the night that caused it.
- It slumps. Once a batt has taken on water and lost its loft it does not spring back, so you get gaps at the edges as well as a wet middle.
Wet batts are replaced, not dried. And there is no point replacing them until the reason they got wet has been dealt with, because the new ones will do the same thing.
Where the moisture comes from
Almost always from inside. A household puts several litres of water into the air every day through showering, cooking, washing, drying clothes indoors, unflued gas heating and simply breathing. In a commercial building add wash bays, kitchens and any wet process.
Two changes have made this worse. Buildings are sealed tighter than they used to be, and they are insulated harder. Both are good for energy bills. Both also mean the moisture a building generates has fewer ways out, so more of it ends up finding the coldest surface in the building, which is the roof.
The code addresses the same chain in a few places, and it is worth knowing which rule sits where.
| Requirement | Where | What it covers |
|---|---|---|
| Exhaust discharge | NCC 2022 Housing Provisions 10.8.2 | Kitchen, bathroom, sanitary and laundry exhaust, and vented clothes 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 |
| Membrane vapour permeance | Housing Provisions 10.8.1 | Written for external walls, not roofs. Climate zones 4 and 5 need at least Class 3, not less than 0.143 µg/N.s. Zones 6, 7 and 8 need Class 4, not less than 1.14 µg/N.s |
| 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 on those same members, needs at least R0.2 |
That third row catches people out regularly. The vapour permeance numbers are quoted all over the internet as though they apply to roofs. In NCC 2022 they sit in the external wall provision. Read the clause against your own build up rather than trusting a summary, including this one.
What to do, in order
Order matters. Doing step four without steps one and two is how people spend money twice.
- Confirm it is condensation. Use the timing test above. If it tracks rain, it is a roof repair job.
- Cut the moisture at the source. Check that every exhaust fan and the clothes dryer actually discharge to outdoor air, not into the roof space. This is both the cheapest step and the most commonly failed one.
- Remove what is already wet. Slumped or soaked batts come out. They will not recover and they keep water against the steel.
- Deal with the vapour path. On an existing roof this usually means the membrane layer and how it is installed, including whether laps are taped. On a re-roof it is designed in from the start.
- Ventilate the roof space properly. Ventilation needs intake and exhaust. Eave vents let air in low, a roof ventilator lets it out high. One without the other moves very little air.
- Reinstate the insulation. Last, not first, and only once the reason it got wet is gone.
What does not fix it
- Sealing the roof. The water is forming on the inside. Sealant on the outside changes nothing.
- Drying the batts. They do not recover their loft or their R-value, and a roof space has no drying energy to offer.
- A roof ventilator on its own. Without intake at the eaves it has nowhere to draw air from, so it moves far less than the box suggests.
- Repainting the ceiling. It hides the stain until the next cold snap.
- More insulation, on its own. Insulation changes where the cold surface is. It does not remove the moisture, and badly placed it makes the sheeting colder.
- Waiting for summer. The corrosion carries on between events, and next winter starts from a worse position.
We supply it, and we install batts, wall wrap and sarking
AWBM supplies vapour control membranes, roofing blanket, thermal break, eave vents and roof ventilators across Australia. Our install work is batts, wall wrap and sarking. Blanket and ventilators we supply for your roofer to fit. Tell us what you are seeing and we will work out what it needs.
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Frequently asked questions
How do I tell if my roof is leaking or sweating?
Check the timing. A leak appears during or just after rain and traces back to one point. Condensation appears on clear, still, cold nights, often with no rain for days, and shows up across the whole underside of the sheeting rather than in one path.
Can condensation rust a metal roof from the inside?
Yes. Repeated films of water on the underside of the sheeting and on the fasteners drive corrosion on a roof that has never leaked. Rust streaks running down from screws are a common early sign.
Does wet insulation dry out on its own?
Not usefully. A roof space is cold, still and humid, so there is very little drying energy. A soaked batt loses its loft, does not recover its R-value, and holds water against the steel. Replace it rather than trying to dry it.
Where is the moisture coming from?
Almost always from inside the building. Showers, cooking, washing, indoor clothes drying and unflued gas heating all release water vapour, and a tightly sealed building gives it fewer ways out.
Will a whirlybird stop my roof sweating?
Not on its own. Ventilation needs an intake as well as an exhaust, so eave vents low and a ventilator high work as a pair. A ventilator with no intake path moves very little air, and ventilation is only one of the levers anyway.
Why is there mould on my ceiling near the walls?
Those edges run coldest and have the least air movement, so they stay damp longest. Cleaning treats the surface. Unless the moisture source and the ventilation are dealt with, it returns.
Does the NCC require roof space ventilation?
NCC 2022 Housing Provisions clause 10.8.3 sets roof space ventilation requirements in climate zones 6, 7 and 8, with opening areas that vary by roof pitch and a minimum roof space height of 20 mm. Check the clause against your own building.
Do the NCC vapour permeance numbers apply to my roof?
Clause 10.8.1 in NCC 2022 is written for external walls, not roofs, even though the numbers get quoted for roofs constantly. Read the provision against your actual build up and confirm it with your designer or certifier.
Can AWBM fix a roof that is already sweating?
We supply the membranes, blanket, eave vents and roof ventilators the fix usually needs, and we install batts, wall wrap and sarking. The roof side, blanket and ventilators, goes to your roofer. Send photos and your postcode and we will tell you what we think it needs before quoting.
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
- NCC 2022 Housing Provisions, Part 10.8 Condensation management, clauses 10.8.1, 10.8.2 and 10.8.3
- NCC 2022 Housing Provisions, clause 13.2.3(7) thermal break
- AS/NZS 4200.1:2017 and AS 4200.2:2017, Pliable building membranes and underlays
- Product data from the manufacturers of the products listed above
General information, not a structural or health assessment. Corrosion affecting roof structure should be assessed by a suitably qualified person, and extensive mould or any health concern about it warrants a specialist rather than a guide like this one. NCC requirements vary by climate zone and building class, so confirm the provisions that apply to your building.