Last issue, Coastal Connection discussed the Institute for Business and Home Safety's recommendations for improvements to roofs, as presented in the IBHS report "Hurricane Ike: Nature's Force vs. Structural Strength". In this issue, we take a look at the Institute's "Tier 2" recommendations for toughening buildings: measures to protect openings and strengthen gable walls. The failure of a big window or door that faces the oncoming hurricane-force winds, IBHS notes, can be the first step in the loss of the building's entire roof system and its subsequent structural collapse. When wind punches in a window or door, the inflowing wind pressure is added to the uplift already affecting the roof, drastically increasing the total load on the roof's connections. As a result, the report says, "The failure of such a large opening can subject the walls, roof, and leeward windows and doors to the kinds of wind forces associated with a much stronger storm, perhaps one that is one to two categories stronger on the Saffir-Simpson Hurricane scale, than the hurricane actually hitting the area." For that reason, protecting windows and doors could be the improvement that saves the whole house — especially an older house with roof-to-wall connections that are weaker than required by new, modern codes.
Shutters may take a beating, but if they are properly rated to withstand hurricane-force winds, they should protect the windows from flying debris. Photo courtesy IBHS. If a homeowner is replacing existing windows, IBHS recommends considering using impact-rated windows. An equally effective but more economical strategy would be to apply a retrofit storm shutter. The best shutters, says IBHS, have been tested for compliance with Miami-Dade County (Florida) standards TAS 201 and TAS 202. These units can take a strike by a 9-pound 2x4 flying at 34 mph, without the missile penetrating the shutter or making a hole in it. Polycarbonate shutter products, while expensive, will also allow light into the building in the event of a power failure. IBHS endorsed temporary plywood window protection only as a last resort. The institute prefers plywood over OSB because of plywood's higher strength per unit thickness: "A piece of OSB must be 30% thicker to equal the impact resistance of a piece of plywood," says the report. Impact resistance is proportional to thickness, notes the report, recommending a minimum of 5/8-inch-thick plywood. If 5/8-inch sheets are too heavy, you can double up two sheets of 3/8-inch plywood and achieve the same protection as a 3/4-inch sheet.
Gable ends are a weak point in a truss roof system and are susceptible to collapse in high winds (top). The solution recommended by the Institute for Business and Home Safety involves reinforcing the gable studs, and bracing the top and bottom of the gable wall to the rafters and attic joists, respectively (above). Photos courtesy IBHS. Gable end walls are another critical vulnerability, the report says. Like windows and doors, gable ends on older homes may blow in during a storm, leading to water damage of the building or allowing the loss of the rest of the roof structure and the destruction of the house. "The good news is gable end walls can be the easiest part of a home’s structure to strengthen and should be a high priority on a retrofit list,” says the report. IBHS publishes detailed instructions on gable end reinforcement at their website. For more info, see also " Gable End Retrofits," by Richard Reynolds (Coastal Contractor, May 2008).
Also included in the IBHS Tier 2 recommendations are anchoring and connection upgrades for porches and carports. These building elements typically are built to handle gravity loads, with scant attention to uplift. But in hurricanes, these small roofs can get ripped loose — potentially taking a piece of the main roof with them, and exposing the home to water damage or even to structural failure. IBHS recommends a grouted post anchor retrofit for post bases, and the addition of steel connectors at weak points in the wall-to-roof joints. The full IBHS Hurricane Ike report, and other supporting information, is available for download from the disastersafety.org website. In an upcoming issue, we'll take a look at "Tier 3" of the report's upgrade strategy: adding framing connectors to create a continuous roof-to-foundation uplift-resisting load path.