Roof Restoration · Six guides · Sheet 3
Condensation · ABCB handbook and NCC Part 10.8Condensation under a metal roof
The Australian Building Codes Board’s handbook calls condensation under a metal deck roof probable: outdoor air circulating through the corrugations can cool to several degrees below the outdoor air temperature, and water forms on the sheet’s underside. Sarking can catch it and drain it to the gutter, and in climate zones 6, 7 and 8 the building code also asks for a ventilated roof space.
General information, not building advice. The ABCB’s Condensation in Buildings Handbook and the NCC’s condensation provisions are the places to check.
Where the water comes from, and where it goes
- 1Outside air in the corrugations. The handbook says this condensation comes from outdoor air circulating through the roofing corrugations, and its risk depends almost entirely on the outdoor climate.
- 2Drops on the underside. Metal sheeting has a non-absorbent underside, so the water gathers and drips. Clay and concrete tiles, with some thermal mass and absorbent undersides, are less likely to drip.
- 3Sarking to the gutter. Sarking acts as a water control layer that collects the drips and drains them to the guttering, so they do not fall on the ceiling or the insulation.
- 4A ventilated roof space. In climate zones 6, 7 and 8 the code asks for a roof space above the insulation that is ventilated to outdoor air.
- 5Moist air from inside. Unsealed downlights in particular can give water vapour a short cut through the ceiling, the handbook warns.
“All roofs have risk”
The handbook’s authors reviewed climate data for 160 Australian locations. Roof cladding condensation could occur in 95 of them without allowing for night sky cooling at all. Taking 2 degrees off the average minimum temperature, as an allowance for that cooling, raised the count to 140, which the handbook reads as showing that “all roofs have risk to experience some condensation”.
It suggests a quick test for a place: compare the average outdoor dew point with the average minimum temperature for each month, and the months at risk stand out.
Roof space ventilation, edition by edition
The code’s roof ventilation rules apply in climate zones 6, 7 and 8: the mild temperate, cool temperate and alpine zones, which take in Melbourne and Ballarat, most of Tasmania, and the alpine areas of Victoria, NSW and Tasmania. The rules changed between editions, and which one applies depends on the state: NSW and Tasmania keep the 2022 version of the condensation part, and Victoria adopted NCC 2025 on 1 May 2026.
| Rule | NCC 2022 | NCC 2025 |
|---|---|---|
| Roof space height | At least 20 mm | At least 18 mm, between the insulation and the underside of the roof or control layer |
| Pitch under 10° | 25,000 mm² per metre at each of two opposing ends | 20,000 mm² per metre, at the eaves or another low point, or for a gable roof at each of its two opposite ends, with other figures for skillion roofs and roofs under 10 m² |
| Pitch 10° to under 15° | 25,000 mm² per metre at the eaves and 5,000 at high level | 7,000 mm² per metre at the eaves or low level and 5,000 at high level or the ridge |
| Pitch 15° to under 75° | 7,000 mm² per metre at the eaves and 5,000 at high level, plus 18,000 more at the eaves for a cathedral ceiling | 7,000 mm² per metre at the eaves or low level and 5,000 at high level or the ridge |
| Not required for | Concrete roofs, structural insulated panels, and roofs subject to Bushfire Attack Level FZ | Concrete roofs, insulated sandwich panels, roofs subject to Bushfire Attack Level FZ, and tiled roofs without a control layer above the insulation |
NCC 2025’s explanatory notes add that the openings created by a metal sheet’s own profile can count towards the requirement, if they connect directly to the roof space and are not blocked by sarking or insulation, and that for pitches over 10 degrees the low openings must be larger in total than the high ones, so the roof space draws in outdoor air rather than air from the rooms below. Roofs whose insulation runs parallel to the roof plane have their own clause, 10.8.4.
Keeping the house’s own moisture out of the roof
The code asks that exhaust from a kitchen, range hood, bathroom, sanitary compartment (a room with a toilet) or laundry discharges to outdoor air, directly or through a shaft or duct, and that a venting clothes dryer does the same. The handbook explains the aim as reducing the moist air that enters a home, including the roof cavity, and notes that the 2022 rules no longer let a laundry’s exhaust discharge into a roof space.
Its checklist for people living with condensation problems adds points that bear on the roof:
- Recirculating range hoods do not capture or remove the water vapour from cooking; they return it to the room.
- When putting in a ducted exhaust, make sure the outlet does not discharge under the eaves or overhanging floors, where the vapour could find its way into other parts of the building.
- Be cautious about adding downlights, exhaust fans and other openings through a ceiling that had none, and use sealed light fittings where they cannot be avoided.
- Before altering or adding to a house, find out whether its roof space was deliberately ventilated or made as airtight as possible, and consider carrying proven arrangements through to the new work.
- Use condensing clothes dryers, or flue them to the outside.