Is ice and water shield waterproof?
What waterproof means here
Standard underlayment is water-resistant: it sheds water but can let it through at nail holes and seams under pressure. Self-adhered membranes are tested under the ASTM D1970 standard, which includes checks of how well the membrane seals around a nail. Because the adhesive bonds to the deck and closes around fasteners, water that gets under the shingles cannot follow the nails into the wood.
Where it can still let water in
- Reversed laps. If an upper piece is tucked under a lower one, water runs into the seam.
- Top edges. Water that gets above the membrane, at a wall or under siding, can run down behind it.
- Poor adhesion. Membrane applied over dust, frost or a wet deck may not bond, leaving channels.
- Gaps at penetrations. A loosely cut hole around a pipe or a missing piece at a chimney corner.
- Long sun exposure. Beyond the product's limit, UV degrades the surface and adhesive.
It blocks vapor too
Because it is waterproof, the membrane also stops water vapor. That is fine in valleys and at eaves, but covering an entire deck means the attic below must be well ventilated so moisture can escape another way. See roof ventilation basics.
Not a tarp
After a storm, membrane is not a long-term substitute for a finished roof or a properly secured tarp. More on where it belongs in ice and water shield in Georgia.