Lesson 2: Hurricane Wind Mitigation Measures

Lesson Overview

Hurricane winds exert significant force, or loads, on structures in their path.  Hurricane wind loads can cause terrific damage.  Most mitigation actions to reduce the risk of damage to structures from hurricane winds involve reinforcing or strengthening the building.

In this lesson you will learn:

• Mitigation methods for protecting a building’s load path, roof, and breach points
• How safe rooms protect people and property from injury during a hurricane
• Mitigation techniques for outbuildings and trees near structures

At the end of this lesson you will:

• Recognize the purpose and procedure for hardening a structure against wind damage using a continuous load path
• Identify the design, materials, and procedures that will mitigate wind damage to roofs.
• Identify mitigation measures to secure structural breach points (windows and doors) against wind damage
• Identify benefits and limitations of residential safe rooms
• Identify mitigation techniques to reduce wind damages to outbuildings
• Identify techniques to prevent damage to structures from wind-damaged trees

 

Continuous load Path

In order to resist loads and reduce the risk of structural damage from these winds it is necessary to reinforce or “harden” the building. 

Hardening a building requires reinforcing the connections between the main parts of a structure, creating a continuous load path.

When all of the connections are properly reinforced, hurricane-caused loads can be effectively transferred from the affected area of the building to the ground.
The important connections are:

• Roof to upper floor walls
• Upper walls to lower walls
• Lower walls to main floor
• Main floor to foundation

Metal brackets and straps are used to strengthen these connections.  To be effective, these brackets and straps should be attached at the studs and rafters, not to the plywood sheathing.  The entire structure can be bolted to its foundation using anchor bolts along the foundation sill.

There are many types of connection brackets available.  The size and quantity of connectors is determined according to design loads and local building codes and requirements.

 

Roof Design and Construction

Roof failures are a common cause of major damage to buildings and their contents in the high winds of a hurricane.  If a roof is breached, winds can more easily damage the rest of a structures continuous load path.

There are mitigation measures that can be taken to reduce potential roof damage. 

A hip roof is sloped on all sides, resulting in a lower profile and more wind-resistant design.  Gable roofs are sloped on two sides; the high profile ends of a gable roof present large surface areas to the full force of the wind. In new construction and/or roof replacement in high-wind hazard areas, installing a hip roof increases the strength of the roof framing and its resistance to wind.

The framing at the ends of a gable roof can be braced to resist the wind and reduce the risk of roof failure.
Gable bracing often consists of 2x4s placed in an X pattern at both ends of the attic. They are secured from the top center of the end gable to the bottom of the brace of the fourth truss, and from the bottom center of the gable to the peak of the roof.

Roof sheathing, also called roof decking, refers to the boards or plywood nailed to the roof rafters or trusses. Roof sheathing can fail during a high-wind event if not properly installed.

Nails or screws designed for roofing applications, anchored in the underlying rafter or truss according to building codes, enhance the stability of the roof sheathing.

The shingles are the first part of a roof to experience damage from high winds.  Choosing the right shingle and installing it properly is the best way to reduce the risk of wind damage.

While shingles are available in different wind speed and durability ratings, the rating systems are not standard among manufacturers. 
In general, roof shingles with longer warranties provide greater durability. 

Local building codes may provide specific installation requirements such as use of nails or screws, number of nails per shingle, or degree of overlap for shingles, but two conventional installation guidelines are:

• Roofing nails perform better than staples or adhesives in high winds.
• Stainless steel nails are recommended when building within 3,000 feet of salt water.

 

Breach Points

Exterior doors and windows are the weakest parts of a structure’s outer shell.  If they are broken or blown in during a hurricane, high winds can enter a structure exerting pressure on the roof and walls resulting in serious damage. 

Once the structure is breached, wind, debris, and rain can damage the interior of the structure as well as any person inside. 

A garage door is the largest potential breach point of a structure. Winds entering through a failed garage door can blow out doors, windows, walls, and the roof of the garage.

Reinforcing a garage door with metal bars and brackets helps protect the garage and its contents as well as the entire structure by increasing its resistance to high-wind pressure.

Double entry doors pose a serious risk.  Double doors of any structure can be secured with latches at both the top and bottom of the doors to prevent them from being blown in by high winds.

Windows are particularly vulnerable components on most structures. 

High winds and wind-blown debris can break unprotected windows and then enter a building.  Once inside, wind and debris can cause more damage.

Installed impact-resistant glass is intended to:

• Avoid damage to the building
• Reduce the likelihood that people will be hurt by broken glass and debris
• Prevent water entry and further damage through broken windows

Impact-resistant glass is laminated with a thin film to keep the glass from shattering.  It is also sealed to its frame with structural adhesive.

Functional shutters are made of impact-resistant materials, usually wood or composite material.  They are hinged to close over the window and include a latch mechanism to hold them closed and secure against the window in high winds.

Temporary plywood covers also provide effective protection. 

These covers are secured by screws or lag bolts long enough to penetrate the wall studs around the window, not just the siding or wall covering.

Residential Safe Rooms

A residential safe room built within a secure structure can provide protection to individuals and their families from the high winds and flying debris associated with hurricanes.

• Properly constructed residential safe rooms built to the standards of FEMA 320: Taking Shelter from the Storm, provide near absolute protection that reduces the risk of injury or death caused by the dangerous forces of extreme winds.
• It is often possible to convert existing interior rooms into safe rooms.

Safe rooms do not offer protection from flood waters; flood evacuation instructions should always be followed.

Other Mitigation Opportunities

There are other actions to help protect people, their homes, and other structures from the high winds of a hurricane.

Trees and unattached secondary buildings, blown by high winds, can be damaged or cause damage to structures during a hurricane. Steps can be taken to minimize this possibility.

Anchoring storage sheds and other outbuildings helps prevent them from becoming flying debris.

Anchor straps similar to those used in manufactured homes can be used to anchor these buildings.

Removing all trees close enough to fall on the building reduces the risk of damage to a structure.

The distance between the building and any tree should be greater than the height of the tree when it reaches full growth.

Keeping trees pruned so wind can blow through the branches instead of the branches acting like a giant sail can help trees stay rooted.

This completes this lesson.  In this lesson you learned:

• Mitigation methods for protecting a building’s roof, load path, and breach points from the risk of wind damage.
• Other mitigation techniques for reducing the risk of wind damage to outbuildings and trees near structures.