Underlayment & Ice-and-Water Shield

Underlayment is the water-resistant layer installed directly on the roof deck, beneath the shingles, tile, or metal panels that people actually see. Ice-and-water shield is a self-adhering membrane used in the areas most prone to water backup — eaves, valleys, and roof penetrations. Together they form the roof's last line of defense if wind-driven rain or ice ever gets past the surface layer, which is why they typically come up during a full roof replacement, a roof-over, or new construction.

What Goes Into the Work

Deck check first — Wood moisture meters confirm the deck underneath is sound before any underlayment goes down over it.
Accurate layout — Laser distance meters, levels, and chalk lines keep courses straight so laps and overlaps land where they should.
Fastening that holds — Cap nailers set synthetic underlayment with capped nails that grip and resist tearing in wind.
Clean detail cuts — Hook blade utility knives trim membrane at valleys, penetrations, and edges without gouging the layer beneath.

Where It Fits With Other Work

Full replacement or overlay — Underlayment and ice-and-water shield are installed as part of full roof replacement, roof-over, and new construction roofing.
Flashing and valleys — This work pairs directly with drip edge, flashing, and valley installation along the same roof planes.
Deck repair — Where moisture readings show rot, deck repair happens before new underlayment goes on top.