Underlayment is the protective layer installed directly on the roof decking, underneath the shingles. It acts as a second line of defense against water, so if wind drives rain under the shingles or a shingle is lost in a storm, the underlayment keeps the deck and the home below it dry. Modern roofs use either a felt paper underlayment or a tougher synthetic underlayment, with synthetic now the common choice for its strength and water resistance.
Think of a roof as having layers that each do a job. The shingles you see are the first defense and take the sun, rain, and wind. Underneath them sits the underlayment, a full-coverage sheet rolled out over the wood deck before the shingles go on. It is the backup that quietly protects your home when the top layer is breached.
That backup matters more than people expect. Wind-driven rain can push water sideways under shingle edges, debris can lift shingles, and during a storm a shingle or two can tear away entirely. In each case the underlayment is what stands between that water and the wood deck. Without it, a small breach in the shingles could mean a wet, rotting deck and a leak inside.
For a homeowner, underlayment is a good example of why the cheapest bid can be a false economy. It is hidden under the shingles, so a corner-cutting roofer can skimp on it or skip an upgrade and you would never see it from the ground. A quality replacement uses a proper synthetic underlayment across the whole roof because it is part of what makes the new roof last its full life.
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