Do you need roof underlayment over ZIP System sheathing?
What the panel is doing
Integrated-barrier sheathing is OSB with a coated face that sheds water. The panels go down with that face up, square edges gapped about 1/8 inch, and every seam covered with the matching tape. Done that way, the deck itself acts as the underlayment the residential code asks for under shingles. An untaped or poorly rolled seam is just a gap.
Why many roofers still add a layer
- Warranty terms. Shingle manufacturers often tie their stronger warranties to a full system that includes their own underlayment, starter and ridge. Read the warranty before skipping it.
- Valleys and eaves. Self-adhered ice and water membrane still belongs in valleys, along eaves where your building department or the shingle maker calls for it, and around chimneys and skylights.
- Low slopes. Shingle roofs from 2/12 up to 4/12 need double underlayment under the code. Ask the building department how they treat taped sheathing on those sections before relying on it.
- Tape damage. Framing crews walk, drag and drop things on the deck for weeks. Scuffed or lifted tape is common by the time roofers arrive.
- Footing. The coated face can be slick when wet or dusty, so crews often prefer a textured synthetic for traction on steep pitches.
What to ask your builder or roofer
Ask whether the inspector signed off on the taped deck as the underlayment, whether the shingle warranty requires anything more, and what goes in the valleys. If the house is new construction in Georgia, also ask how long the taped deck will be exposed before shingles go on, since summer sun and storms are hard on tape.
For the layer itself, compare options in synthetic underlayment vs felt, see where membrane belongs in ice and water shield in Georgia, and read about deck panels in OSB vs plywood roof decking.