Ventilation and Condensation Under Metal Roofs

Metal sheets are impermeable and cool quickly at night by radiating heat to the sky. Warm, moist air from inside the building can then condense on the underside of the metal or on the deck beneath. For some metals, trapped moisture causes underside corrosion that can perforate the sheet from below while the visible surface looks sound.

Metals most affected

• Zinc: underside moisture produces white, chalky corrosion. Zinc roofs are built with a ventilated space or open skip sheathing, a drainage mat, or a factory polymer backside coating, typically 30 to 60 micrometres thick.
• Lead: UK guidance calls for a through-ventilated void of at least 50 mm (2 in) beneath lead-covered decks, and at least 25 mm (1 in) behind vertical work, with no stagnant air pockets.
• Coated steel and aluminium: less sensitive, but condensation can still drip into buildings, rot timber decks or corrode unpainted backs, particularly over unheated or humid spaces such as livestock buildings.

Warm and cold roofs

In a "cold" (ventilated) roof the insulation sits below a ventilated air space, and outside air carries away moisture. In a "warm" (unventilated) roof the insulation is above the deck or directly below the metal; it relies on an effective vapour control layer on the warm side and careful detailing. Warm roofs lack the drying capacity of ventilated designs, so specifications for zinc and lead are particularly strict about them.

Good practice

• Provide continuous vapour control on the warm side of insulation.
• Provide eave-to-ridge ventilation paths where the design requires them.
• Use underlayments and drainage layers approved by the metal manufacturer.
• Avoid installing metal over wet decks or insulation.