METAL ROOF CONDENSATION / SWEATING

ROOF CONDENSATION

ROOF LEAK or CONDENSATION?

Is it a leak due to rain or is it a condensation issue? It’s not always easy to tell. Once you’ve properly eliminated all areas where rainwater can enter, you’re most likely left with a sweating metal roof / metal roof condensation.

Water Barriers / Drainage

If your roof lacks properly installed building blanket (insulation & foil), or sarking which acts as a barrier and allow for proper water drainage, then at times you’ll notice what appears to be a roof leak. Roof ventilation can also assist with allowing the trap warmer air to escape.

Trapped damp/ humid air in your roof space will cause condensation to develop on the underside of your roof sheets. This will start to happen mainly when the air outside is much cooler than the air inside. The water air inside your roof space will rise until it meets the underside of your roof sheet — the roof sheet being much cooler will cause the air to cool and then the sweating occurs / condensation.

Prevent Condensation Problems

We use BlueScope Steel for our metal roof and custom flashing. We’ve included the below information from their website.

Condensation forms under/on roof sheeting when the sheeting becomes colder than the air in contact with it. Water vapour carried in the air then condenses on the colder surface of the roof sheet. To minimise the risk of water damage to ceilings or corrosion related to condensation under steel roof sheeting, a vapour barrier should be provided under the sheeting to prevent contact between the moist air in the building and the roof sheeting.

Reflective foil laminates (sarking) which are commonly used to provide heat insulation under roof sheeting can serve a dual function as a vapour barrier, by thoroughly sealing the foil overlaps with moisture impervious adhesive tape. 

The laps should be about 100 mm and kept in close contact when positioning the laminate so the tape can be readily applied. 

The laminate must be allowed to drape between the roof supports so that the cold temperature of the roof sheeting will not be transmitted to the laminate by contact. If this were to happen condensation could form on the underside of the laminate and may drip into the ceiling space.

Ventilation of the ceiling cavity may assist in the reduction of condensation however it has been shown that ventilation will not completely eliminate the condensation cycle. In certain marine situations ventilation may also carry salts into the roof cavity which will, in the presence of condensation, lead to accelerated corrosion attack.

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