Secondary Water Barrier

A secondary water barrier is a waterproofing layer installed under the main roof covering so that, if the covering is damaged or blown off, rain does not pour straight through the roof deck. It is a central idea in hurricane-resistant roofing because covering loss is common even when the roof structure survives.

Why it is needed

Investigations after Hurricane Andrew (1992) noted that wind-driven rain caused costly interior damage even on homes that kept most of their roof covering, and the report recommended a water-resistant membrane beneath the covering. Later storms reinforced the point: after Hurricane Michael (2018), FEMA documented water entering through damaged coverings, lost ridge vents and soffits, saturating insulation and causing mold.

Research by the Insurance Institute for Business & Home Safety (IBHS) illustrates the scale of the problem: one inch (25 mm) of rain falling on an exposed roof deck can let in water equivalent to about nine bathtubs.

Common approaches

• Self-adhered membrane over the whole deck. A fully adhered underlayment (often modified bitumen or a synthetic with adhesive backing) stays bonded even if the covering leaves.
• Taped deck seams plus underlayment. Self-adhering tape is applied over the joints between deck panels, then covered with a conventional underlayment. This is often called a sealed roof deck.
• Double-layer underlayment. Two overlapped layers of felt or synthetic underlayment, with specific fastening, as permitted in some codes.

Codes and programs

Newer code approaches in high-wind areas of the United States call for a secondary water barrier through either a sealed deck or enhanced underlayment, and the FORTIFIED Roof standard requires a sealed roof deck. IBHS reports that sealing the deck can reduce water intrusion by up to 95 percent in its testing.

Limits

A secondary water barrier does not stop structural failure and is not a permanent roof. It buys time to make temporary repairs and protects the interior during the storm. Its performance depends on deck preparation, adhesion in hot or cold weather, and correct lapping at valleys, eaves and penetrations.