Roof Underlayment Explained

Aug 14

Roof underlayment sits between the roof deck and the finished roofing material. You rarely see it after installation, but it provides a secondary layer of protection while the roof is being built and when wind driven water gets past the surface covering.

Not every underlayment does the same job. A good roofing quote should name the products, explain where they go, and show how they work with flashing and shingles.

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What underlayment protects

Shingles form the primary water shedding surface. Underlayment protects the deck during construction and adds a backup drainage layer. Self adhered membrane also seals around fasteners and protects vulnerable areas where water can move under shingles.

It does not replace flashing

Underlayment cannot correct poorly installed chimney, valley, skylight, or wall flashing. A complete roof system uses the right underlayment, flashing, drip edge, ventilation, and roofing material together.

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Compare the main underlayment types

MaterialWhat it does wellWhat to confirm Asphalt saturated feltTraditional secondary layer with familiar installation methodsWeight, fastening, exposure limits, and compatibility Synthetic underlaymentLightweight, tear resistant, and easier to walk during installationManufacturer, approved fastening, and exposure rating Self adhered membraneSeals around fasteners and protects high risk water entry areasExact coverage at eaves, valleys, penetrations, and transitions

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Why Ontario roofs need membrane at vulnerable areas

Snow can melt above the eaves and refreeze at the colder roof edge. Water may then move under the shingles. Self adhered membrane helps protect the deck at the eaves and other vulnerable locations.

Other high risk areas

Valleys that collect runoff from two roof slopes
Skylights, chimneys, and plumbing vents
Low slope transitions
Wall and roof intersections

The exact design should follow the roof, product instructions, and applicable requirements. Avoid a quote that simply says “underlayment included” without naming the material or coverage.

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Ask how the deck will be prepared

Underlayment needs a sound, clean deck. During tear off, the contractor should inspect for rot, delamination, gaps, and damaged sheathing. The quote should explain how unforeseen deck replacement is priced and documented.

Our Oakville roof installation process includes a deck assessment before the new system is covered.

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Read the roofing quote carefully

A complete scope should identify the underlayment manufacturer or type, membrane locations, flashing work, drip edge, ventilation, deck repair allowance, and warranty terms.

Questions to ask

Which underlayment goes on the main roof deck?
Where will self adhered membrane be installed?
How will valleys and penetrations be flashed?
What happens if damaged decking is uncovered?
Does the complete assembly meet the shingle warranty requirements?

Compare this detail when reviewing a roofing estimate in Oakville. A low total with vague materials may exclude important parts of the system.

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Frequently asked questions

Is roof underlayment waterproof?

Some self adhered membranes are designed as waterproof protection in specific areas. Standard felt and synthetic products act as secondary water shedding layers, not finished roofs.

Is synthetic underlayment better than felt?

Synthetic products are often lighter and more tear resistant, but the correct choice depends on the roof covering, design, installation, and manufacturer requirements.