Tiles, slates & materials

What does roofing underlay actually do?

Short answer: Underlay provides a secondary barrier beneath tiles or slates, helps manage wind-driven rain and interacts with ventilation. It is not intended to replace the primary covering.

What this means in practice

The product must suit the roof pitch, exposure, fixing zone, headlap, supporting structure and appearance of the building. Substituting a similar-looking tile or slate without checking the manufacturer’s fixing specification can create wind-uplift, weight and weathering problems.

What to check next

  • Choose a compatible underlay
  • Detail laps, penetrations and eaves correctly
  • Do not assume ‘breathable’ removes every ventilation need

When should you ask a roofer to inspect it?

Ask for the exact manufacturer and product in the quotation, not a generic description such as ‘grey tile’. A survey is important before changing covering type, reusing battens or mixing old and new materials on an exposed North East roof.

Safety first: do not climb onto a roof without suitable access, training and protection. Keep clear of loose coverings, unstable ceilings and water near electrical fittings.

Technical content reviewJ & L Welch Roofing & Paving Services Ltd · Over 20 years of combined roofing experienceReviewed 28 August 2026 · How our answers are produced

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