Roofing

# What New Homeowners Should Know About Roof Underlayment & Ventilation

November 18, 2025·7 min read·By IronWrap Team

If you ask ten roofers what makes a roof last, you'll get ten answers about panels, fasteners, and warranty. Almost none will lead with the two things that actually decide longevity: underlayment and ventilation. They're invisible from the curb, but they're 80% of why one roof lasts 20 years and an identical-looking roof lasts 50.

## Underlayment: the layer between deck and roof

Underlayment sits on top of the structural deck and underneath the metal panels (or shingles). It's the secondary water barrier, the thermal break, and — on metal roofs — the slip plane that lets panels expand and contract without grinding against the deck.

### Felt vs synthetic vs self-adhered

- 15-lb felt: cheap, absorbs water, tears easily, falls apart in UV. Avoid except for low-budget asphalt re-roofs.
- Synthetic underlayment: woven polyolefin, lightweight, UV-stable for weeks of exposure, doesn't wrinkle. The right baseline for most metal installs.
- Self-adhered (peel-and-stick): rubberized asphalt or butyl, sticks directly to the deck, seals around fasteners. Use at eaves, valleys, penetrations, and anywhere ice-damming is a risk.

On a metal roof in Alberta, the right spec is high-temp peel-and-stick at eaves and valleys plus heat-reflective synthetic across the field. That gets you ice-dam protection where it matters and a thermal break across the whole assembly.

## Ventilation: why your attic needs to breathe

An attic without proper ventilation is a moisture accumulator. Warm humid air from inside the house rises into the attic, hits the cold underside of the roof deck, and condenses into water that soaks insulation and rots framing. Most leaks that homeowners blame on the roof are actually condensation problems caused by inadequate ventilation.

### How proper ventilation works

A balanced attic vent system uses intake vents at the soffit (low) and exhaust vents at the ridge (high). Cold dry air enters through the soffits, warms slightly, picks up moisture from the attic, and exits through the ridge — pulling moisture out before it can condense. The ratio is typically 1 sq ft of net free area per 300 sq ft of attic, split 50/50 between intake and exhaust.

## Common mistakes

- Soffit vents that have been blanketed-over by attic insulation. The intake side is blocked and the system stops working.
- Mixing vent types — adding gable vents to a ridge-and-soffit system disrupts the airflow pattern and reduces overall flow.
- Bathroom and dryer vents discharging into the attic instead of through a roof cap. This dumps warm humid air directly into the space you're trying to keep dry.

## What to ask your roofer

Before you sign a quote, ask: 'What's the underlayment spec?' and 'What's the ventilation calculation for my attic?' If the roofer can't answer specifically — gives you a brand or a part number for one and a math result for the other — you're talking to the wrong contractor. The roof lasts as long as those two systems hold up. Everything else is finish.

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