Underlayment
What is the purpose of underlayment beneath a standing seam metal roof?
Underlayment is the secondary weather barrier between the metal panels and the roof deck. If wind-driven rain or condensation bypasses the panel seams, the underlayment prevents moisture from reaching the plywood or OSB substrate. This layer also prevents direct metal-to-wood contact that can cause corrosion.
Should residential standing seam use synthetic or felt underlayment?
Synthetic underlayment is the current standard for metal roofing in California. It resists UV exposure during the installation window, has higher tear strength than felt, and provides a lower moisture vapor transmission rate. Felt paper degrades faster under UV and tears more easily during clip installation.
How does underlayment interact with ice and water shield?
Ice and water shield is applied at critical areas like eaves, valleys, and penetrations. Synthetic underlayment covers the remaining field area. The two materials overlap at transition points to create a continuous moisture barrier across the entire roof deck.