Roofing Underlayment Types

Asphalt-Saturated Felt

Asphalt-saturated felt, commonly called tar paper or felt paper, is the oldest type of roofing underlayment still in use. It is made from a base mat of organic fibers or fiberglass saturated with asphalt. The two standard weights are #15 (approximately 8 pounds per 100 square feet) and #30 (approximately 20 pounds per 100 square feet). The numbers originally referred to the weight per square but now represent a nominal designation. #15 felt is a single layer used in standard applications. #30 felt is thicker, more tear-resistant, and required by some shingle manufacturers for warranty compliance. Felt absorbs water, which can cause wrinkling and buckling if left exposed to rain before the shingles are installed. Its service life when exposed to UV is limited to days, not weeks.

Synthetic Underlayment

Synthetic underlayment is made from woven or non-woven polypropylene. It weighs significantly less than felt (typically 2 to 4 pounds per 100 square feet), is far more tear-resistant, and does not absorb water. Synthetic products lie flat on the deck, resist wrinkling, and provide better traction for installers walking on the roof. Most synthetic underlayments can withstand UV exposure for 30 to 180 days depending on the product, giving the roofing crew more flexibility if the shingle installation is delayed. Synthetic underlayment has largely replaced #30 felt as the preferred full-roof underlayment for residential roofing. It costs more per roll than felt but covers more area per roll due to its thinner profile and comes in wider, longer rolls that speed up installation.

Self-Adhering Membrane

Self-adhering membrane, most commonly known by the brand name Ice and Water Shield, is a rubberized asphalt sheet with a polyethylene film on one side and a peel-off release liner on the other. It sticks directly to the roof deck and seals around nail penetrations, forming a watertight barrier. The IRC requires self-adhering membrane at eaves in cold-climate regions where ice dams form. It is also recommended (and often required by manufacturers) in valleys, around skylights, at roof-to-wall transitions, and at any area prone to water backup. Self-adhering membrane is the most expensive underlayment type and is typically not used as a full-roof covering due to cost. It is instead applied strategically in high-risk zones, with felt or synthetic covering the remainder of the deck.

Choosing the Right Underlayment

The choice depends on climate, budget, shingle manufacturer requirements, and local code. In cold climates, self-adhering membrane at the eaves extending at least 24 inches past the interior wall line is code-required, with synthetic or #30 felt covering the field. In moderate climates, synthetic underlayment across the entire deck offers the best protection and workability. In budget-sensitive projects, #30 felt meets minimum code requirements in most jurisdictions. Regardless of type, the underlayment must be installed per the shingle manufacturer's specifications to maintain the warranty. For insurance purposes, documenting the underlayment type and condition during a tearoff provides evidence of whether the roof assembly was properly constructed and whether water infiltration was caused by the storm event or a pre-existing deficiency.