Wind-Driven Rain Demonstration

In August 2011, IBHS conducted a first-of-its-kind full-house examination of how wind-driven water penetrates common types of openings in residential roof systems. The study was modeled on real-world, post-event damage assessments in areas where winds were strong enough to blow off the roof cover, but not strong enough to tear off roof sheathing or decking.

A full-scale, 1,300-square-foot duplex was designed and constructed for the demonstration and quantitative testing. Sheathing joints on one half of the roof deck were sealed, while joints on the other half were not sealed. Both halves of the roof were then covered with simple felt paper underlayment prior to installing asphalt shingles. The roof system included gable ends fitted with vents and one-foot wide soffits at the eaves. The roof sheathing stopped short of the peak along the primary ridge, so it was possible to install a ridge vent during one set of tests. A series of quantitative tests was conducted before the scheduled hurricane demonstration.

Demo Specs

Objectives for IBHS’ first wind-driven water research project included:

  • Quantifying the relative volume of water penetration through different roof openings
  • Cataloguing types of water penetration damage to different parts of a house
  • Demonstrating effective individual damage mitigation techniques, such as sealing the roof deck
  • Illustrating why sealed roof decks are core components of the IBHS FORTIFIED program requirements for hurricane-prone regions

Demo Highlights

At 50 mph, water entered the attic at a rate of about 1.3 inches per hour per square foot of open soffit area (simulating loss of soffit material). Most water was deposited within the first 10 feet of the attic space adjacent to the open soffit. By comparison, only 6 percent of the water quantity entering in the open soffit test accumulated during a test conducted at the same wind speed when the soffit was covered with typical perforated soffit vent covering (simulating soffit material remaining in place).

At 70 mph, water entered the attic at a rate of about 2.9 inches per hour per square foot of open soffit area. By comparison, only 25 percent of water entering in the open soffit test accumulated during the same test when the soffit was covered. With the soffit cover in place, the 70 mph wind produced about 9 times more water accumulation in the attic than the 50 mph test for the same conditions.


Additional Resources