Felt Underlayment: What Was Standard and Why It Changed

Asphalt-saturated felt, known as 15-pound or 30-pound felt or tar paper, was the standard residential underlayment for most of the twentieth century. It is still code-compliant and still appears on lower-cost installations. Its limitations in Minnesota's climate are significant: felt absorbs moisture and loses water resistance as it ages, becomes brittle and tears easily in cold temperatures, and wrinkles when wet in ways that can affect shingle adhesion over time. A roof installed with felt underlayment in 1990 has underlayment that has been degrading for 35 years underneath shingles that may look like they have ten years left.

Synthetic Underlayment: How It Is Different

Synthetic underlayment is manufactured from engineered polymers, typically polypropylene or polyethylene, woven or spun-bonded into a sheet that is then laminated for water resistance. Synthetic does not absorb water, meaning it maintains its water-resistant properties throughout its service life. It does not become brittle or stiff in cold temperatures, which matters for fall and early spring installations in Plymouth. It is lighter than felt, reducing load on the roof system and making it easier and faster to install. And it has a significantly longer expected service life, with many products carrying warranties of 25 to 50 years versus felt's 12 to 20 years.

The premium on a typical Plymouth roof for synthetic versus felt is roughly $75 to $200. Given that underlayment is installed once and then inaccessible for the life of the roof system, the performance difference relative to the cost difference is one of the more straightforward upgrade arguments in roofing.

What Synthetic Underlayment Cannot Do

Synthetic underlayment is water-resistant, not waterproof. It sheds water under normal conditions but does not self-seal around nail penetrations the way ice and water shield does. This distinction is important in a Minnesota climate where ice dams form at the eave edge and in valleys, creating standing water conditions that can be pushed uphill beneath the shingles. In those locations, synthetic underlayment is not a sufficient barrier. Self-adhering ice and water shield is required at eaves per Minnesota code and is strongly recommended in valleys on every quality installation.

Synthetic underlayment also has lower vapor permeability than felt, meaning it allows less moisture vapor to escape through the roof assembly. This makes adequate attic ventilation more important on a roof with synthetic underlayment, because moisture that cannot escape through the underlayment must exit through the ventilation system.

Manufacturer Warranty Requirements for Synthetic

Several premium shingle manufacturers require synthetic underlayment as a condition of their enhanced warranty products. GAF's enhanced warranty tiers, available through Certified Plus contractors, specify synthetic underlayment as part of the system warranty. This means a homeowner who wants the strongest available warranty coverage on their GAF shingle installation needs both a certified contractor and synthetic underlayment to unlock it. An estimate that specifies felt underlayment while also promising enhanced warranty coverage contains a conflict worth clarifying before signing.

WHAT THIS MEANS FOR YOU

When reviewing a roofing estimate, ask for the underlayment product by name and warranty period. An estimate that says "felt underlayment" or does not specify the product at all is either specifying the lower-performing option or declining to disclose what will be used. On a new roof in Plymouth's climate, synthetic underlayment is the right specification and the cost premium over felt is small enough that it should not be the line item a contractor cuts to produce a lower bid number.

HOW PROOF BUILDERS APPROACHES THIS

Proof Builders installs synthetic underlayment as standard on Plymouth roof replacements, with ice and water shield at all eaves and valleys. The specific underlayment product is named in every written estimate. As a GAF Certified Plus contractor, we install to the system specifications required for enhanced warranty coverage, which includes synthetic underlayment as a component of the warranted assembly.