Underlayment vs Plywood

How Each Roof Layer Protects Your Home

Many homeowners confuse roof underlayment with plywood sheathing, but they perform completely different functions. Plywood (or OSB) is the structural deck that spans the rafters and carries snow, wind, and worker loads. Underlayment is the secondary moisture barrier installed on top of the plywood, beneath the shingles, to catch any water that gets past the shingles. This article explains the full roof assembly layer-by-layer and links to our Plywood and Underlayment calculators.