Underlayment and leak barrier

Normal function

It provides backup water resistance when wind-driven rain gets beneath shingles and helps protect the deck while the roof covering sheds water.

What a homeowner may notice

• Water stains or damp deck after shingles appear intact.
• Exposed, torn, wrinkled, brittle, or unsealed membrane at a repair opening.
• Leak concentration at valleys, eaves, wall lines, or penetrations.
• A re-roof with missing or incompatible underlayment layers.

Professional inspection

Open the smallest practical test area near the suspected entry point, inspect laps and fasteners, compare with roof slope and roof covering, and check whether water is entering from a flashing transition instead.

Repair or replacement path

Remove enough roof covering to expose sound material, replace damaged membrane with a compatible product, maintain laps and end laps per the product instructions, then restore the roof covering and flashings.

Typical materials and equipment

Synthetic underlayment or approved leak-barrier membrane, compatible primers/tapes where specified, cap fasteners or roofing nails, replacement shingles, and metal flashing.

When to act

Do not leave exposed or torn underlayment uncorrected; it is a near-term leak risk even if the ceiling is currently dry.

What moves the price

Roof pitch, wet weather, brittle shingles, access, material compatibility, concealed deck damage, and whether the opening is part of a larger flashing rebuild.