Roof underlayment is a critical secondary waterproofing layer installed directly on top of the roof deck and beneath the primary roofing material such as shingles or metal panels. Its main purpose is to provide an additional barrier against moisture infiltration, especially in cases where wind-driven rain or ice may penetrate the outer roofing layer. In climates like Vancouver, where heavy rainfall is common, underlayment plays a vital role in protecting the structural integrity of the roof. It also acts as a temporary weather shield during construction. Modern underlayments may be synthetic or felt-based, with synthetic options offering improved durability, tear resistance, and longer service life.
