Overview
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Water appearing in a ceiling under a metal roof is frequently condensation rather than a leak. A tile roof breathes between its tiles, while a sheet roof is sealed, so on a cold night the underside of the steel becomes the coldest surface in the building and moisture rising out of the house condenses on it and drips back down. The distinguishing sign is timing: condensation appears on clear cold mornings, whereas a leak follows rain. It is prevented by an insulation blanket or sarking laid under the sheet, which keeps the steel's underside above the dew point, and it is the single item most often missing from a conversion quote.
Key points
- Condensation appears on clear cold mornings, a leak follows rain
- A tile roof breathes between tiles, a sheet roof does not
- The underside of the steel is the coldest surface in the building at night
- Sarking or an insulation blanket under the sheet is the fix, and the prevention
- It is the item most often left out of a tile-to-metal conversion quote
- Sealing the roof harder makes it worse, not better
Why a sheet roof does something a tile roof does not
A tiled roof has thousands of small gaps. Air moves between the tiles constantly, so moisture rising out of the house passes through the roof and away without anybody noticing.
A sheet roof does not have those gaps. It is a continuous sealed surface, which is exactly what you want it to be, and it means the moisture that used to leave through the roof now has nowhere to go.
On a clear cold night the steel radiates heat to the sky and its underside drops below the temperature of the air in the roof space. Moisture in that air condenses on it, exactly as it does on a cold glass.
How to tell it apart from a leak
The two look identical from below and they are distinguished almost entirely by when they happen.
- Timing
- A leak follows rain. Condensation appears after a clear cold night, often with no rain for days, and is worst in winter.
- Pattern
- A leak concentrates at a point: a penetration, a valley, a wall junction. Condensation is diffuse and can appear anywhere across the underside of the roof.
- Volume
- A leak in heavy rain produces a lot of water quickly. Condensation produces small amounts repeatedly.
- What is above it
- Trace a leak upward and there is usually something there. Trace condensation upward and there is just sheet.
Where the moisture is coming from
The water condensing on the roof came out of the house, which is why the problem varies so much between two identical roofs.
Showers, cooking, drying clothes indoors and unflued gas heating all put moisture into the air. So does a bathroom or kitchen exhaust fan that vents into the roof space rather than outside, which is common and is the single worst offender.
That is worth checking first, because a fan discharging into the roof cavity will defeat any amount of insulation under the sheet.
What actually prevents it
The fix is to keep the underside of the steel above the dew point, and to give any moisture that does get up there a way out.
- Insulation blanket or sarking under the sheet
- A layer between the steel and the roof space, laid as the roof goes on. It keeps the underside warmer and it catches what does form. This is why it is part of the roof rather than an accessory to it.
- Ventilation of the roof space
- Air movement carries moisture out rather than letting it sit against the steel. Ridge vents and eave vents do this.
- Vent exhaust fans outside
- Not into the cavity. A bathroom fan discharging into the roof space is pumping moisture directly at the coldest surface in the building.
- Ceiling insulation, properly laid
- It reduces how much warm moist air reaches the roof space in the first place.
What does not fix it
The instinct on finding water in a ceiling is to seal the roof harder, and with condensation that makes it worse.
Sealing a roof space more tightly traps the moisture that is already up there with nowhere to go. The problem is not that water is getting in, it is that water is not getting out.
Repainting the ceiling is also not a fix, for the same reason painting over a leak stain is not. It removes the evidence while the mechanism carries on.
If you already have it
On an existing roof, retrofitting a blanket under the sheet means lifting the roof, which is realistically a job for the next replacement rather than a standalone fix.
What can be done sooner is everything else: redirect any exhaust fans venting into the cavity, improve roof space ventilation, and check that ceiling insulation is present and not compressed.
That combination resolves a large share of cases, and it costs a fraction of a roof. It is worth doing before anybody talks about replacing anything.
If this sounds like your roof
We will come and measure it.
Ring and describe what it is doing. If it is a repair rather than a replacement, we will say so.
(02) 9072 1993
