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.
These are two different layers doing two different jobs, and they are not alternatives to each other. The plywood or OSB deck is the structural surface. The underlayment is the water barrier laid on top of it. Every shingle roof has both.
| Layer | Job | Typical material |
|---|---|---|
| Rafters or trusses | Carry the load | Timber, engineered truss |
| Sheathing / deck | Structural surface, nail base, diaphragm | 4x8 OSB or plywood |
| Ice barrier (eaves, valleys) | Self-sealing water barrier where ice forms | Self-adhering membrane |
| Underlayment | Secondary water barrier over the whole deck | Synthetic or asphalt felt |
| Shingles | Primary weather surface | Asphalt, metal, tile |
They are different layers doing different jobs. Plywood or OSB is the structural deck and the nail base; underlayment is the water barrier laid on top of it. Every shingle roof has both.
Yes. A new deck is not waterproof — plywood soaks up water and swells. Underlayment keeps the building dry between tear-off and the finished roof and backs up the shingles afterwards.
No. It is a membrane and needs continuous support from a deck beneath it. It is not a structural layer.
It is cheaper per square, but it tears at fasteners, wrinkles when damp — which then telegraphs through the shingles — and is rated for days of exposure rather than weeks.