Can you ice and water shield a whole roof, and should you?
Short answer. Yes, you can cover an entire roof deck with ice and water shield, and some low-slope roofs and high-wind specifications call for it. On a typical Georgia shingle roof it is usually more than you need: it costs much more than synthetic underlayment, turns the deck into a vapor barrier, and makes the next tear-off harder, so most roofs use it only where water concentrates.
When full coverage makes sense
- Low slopes. Shingles between 2:12 and 4:12 need extra protection, and a full membrane is one way to provide it.
- Sealed roof deck programs. The IBHS FORTIFIED Roof standard requires a sealed roof deck, which can be achieved with a full self-adhered membrane or with taped deck seams under underlayment.
- Complex roofs. Many valleys, dormers and walls can leave little area for standard underlayment anyway.
- Metal roofs. Some metal systems call for full coverage with a high-temperature membrane.
The downsides
- Cost. Self-adhered membrane costs much more per square than synthetic underlayment.
- Moisture. The membrane does not let water vapor pass. With a vented attic below, the deck can still dry downward, but poor ventilation and a fully sealed deck are a bad combination. See roof ventilation basics.
- Future tear-off. It bonds to the wood, so the next replacement often means more decking replaced.
The usual middle ground
Most Georgia roofs use self-adhered membrane in valleys, around chimneys, skylights and pipes, at roof-to-wall joints and on low-slope sections, with quality synthetic underlayment over the rest. Some homeowners add the eaves as well. The pieces are compared in synthetic underlayment vs felt and ice and water shield in Georgia.