Why a Metal Roof Needs an Air Gap

Warm air holds more moisture than cold air. When warm, humid air from inside a house reaches a cold surface, the moisture it was carrying stops being vapour and becomes water. In a roof, the cold surface is the underside of the deck or the panel, and this happens on winter nights, quietly, above your ceiling.

Managing that is most of what roof ventilation is for. It is also the part of a roof that is easiest to do badly without anyone noticing for years.

What the gap is

On a properly built metal roof there is a deliberate space between the underlayment and the panel, created by strapping: battens running up the slope, then a second set across them that the panels fix to.

That space does three jobs at once.

• It lets the roof dry. Air moving through the gap carries away moisture that would otherwise sit against timber.
• It gives water somewhere to go. Anything that gets past the panel drains down the underlayment and out at the eaves rather than pooling.
• It sets out the fixings. The horizontal strapping is spaced to the panel's own module, so every fastener lands in solid material at the right point.

The other half: attic ventilation

The gap under the panels is not the same thing as ventilating the roof space, and you need both.

Attic ventilation is a path: air in low at the soffits, up the underside of the roof, out high at the ridge. It only works if the whole path is open. The most common failure is insulation pushed into the eaves during an upgrade, blocking the intake completely. The vents are all still there. Nothing is coming in.

This is why soffit condition matters more than its appearance suggests, and why a roof project is the sensible moment to deal with it.

What happens when it is missing

Nothing, for a while. That is the difficulty with it.

Then, over several winters: damp sheathing, dark staining on the underside of the deck, insulation that has been damp often enough to stop performing, and in bad cases mould and softened timber. From inside the house you may see nothing at all until a ceiling stain appears, and by then the cause is years old.

None of that is a failure of the metal. A roof with no ventilation path will do the same thing under shingles. It is a failure of the assembly.

How to tell whether yours is real

Ask these, and listen for specifics rather than reassurance.

• Where does air enter this roof, and where does it leave?
• Is there strapping between the underlayment and the panels, and how is it set out?
• Are the soffit intakes clear, and will you check them?
• What happens at the ridge?

If you have an older house that has had its attic insulation topped up at some point, the intake question is the one to press on. It is a very common and very fixable problem, and the roof is the right time.

The full assembly is laid out step by step on our installation page. To talk about your own roof, get in touch.