interlayment
a felt, metal, or membrane sheet material used between courses of steep-slope roofing to improve the weather and water-shedding characteristics of the primary roof covering during times of wind-driven precipitation.
interlayment: the safety net between your roof and water damage
Interlayment is a sheet material installed horizontally between successive layers of steep-slope roofing (slopes of 4:12 or greater), positioned under the primary shingles or tiles. Its job is straightforward: catch water that gets driven under the top layer during heavy wind-driven rain or snow melt, and funnel it safely down and off the roof before it reaches the deck.
The material comes in three main varieties. Felt, typically 15 lb or 30 lb asphalt-saturated paper, is the traditional choice and remains common on asphalt shingle roofs. Synthetic membranes, usually non-woven polypropylene or polyester, offer better tear resistance and longer lifespan. Metal interlayment, less common, appears in some high-wind or metal roofing applications. Each type is stapled or mechanically fastened to the roof deck or to the course below it, overlapped by at least 2 inches per industry standards.
Where it sits in the system
Interlayment occupies the second line of defense in the roof assembly. The primary covering (shingles, tiles, or metal panels) sheds most water. Interlayment catches the remainder, passing it toward the eaves rather than allowing it to pool or back up under flashing and into the structure. This distinction matters: interlayment is not a vapor barrier and does not fully waterproof the roof. It is a secondary weather barrier that extends the tolerance of the primary layer during extreme conditions, particularly during installation wind events or with marginal slopes.
Installation quality determines effectiveness. Tears, wrinkles, and gaps defeat the purpose. Improper overlap or fastening too close to edges creates channels for water to migrate sideways. On low-slope roofs (under 4:12), interlayment becomes progressively less effective and is often omitted in favor of fully bonded or adhered systems. Building codes and manufacturer specifications govern where it is required; many jurisdictions mandate it in high-wind zones or on slopes over 6:12.
The term derives from its literal position: a material layered in between courses. It is sometimes called underlayment in industry speech, though underlayment more broadly refers to any weather barrier beneath the primary roofing. Interlayment specifically describes the intermediate layer in a multi-course steep-slope assembly.