Straight-line winds can also cause damage to property and carry a risk of casualties.

Straight-line winds rushing outward from a severe thunderstorm can be similar to the winds of a hurricane.

In this lesson you will learn about:

• Straight-line wind damage to structures and measures that mitigate or reduce the risk of these damages in future wind events
• Other mitigation activities outside of the structure that can lessen damage to property and improve life safety, including measures for outbuildings, and trees

At the end of this lesson you will:

• Recognize wind damage caused by straight-line winds
• Recall the purpose and procedure for hardening a structure using a continuous load path
• Identify mitigation measures to reduce risk of damage from straight-line winds to the roof, windows, and doors
• Recognize opportunities to reduce damage from straight-line winds by anchoring outbuildings and pruning or eliminating trees near structures

Straight-line winds are any wind not associated with the rotation of a tornado. Straight-line, high-velocity winds can exert significant force, or “loads,” on structures. Straight-line winds may also be called a derecho, thunderstorm outflow, or microburst. Straight-line winds may:

• Tear off all or part of a roof
• Break windows
• Damage siding
• Cause a structure to collapse completely
• Blow down trees and vegetation

Many wind mitigation activities involve enhancing a structure’s building performance to resist these loads.

Mitigation actions to reduce the risk of damage to structures from these winds requires connecting and reinforcing the connections between the main parts of a structure to create a continuous load path.

When all of the connections are properly reinforced, wind loads can be effectively transferred from the top of the structure to its foundation in the ground. The important connections in this path are:

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

Metal brackets and straps, often called “hurricane 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.

When all of the connections are properly reinforced in this manner, a building is less likely to collapse.

Roof failures are a common cause of major damage to buildings and their contents from high winds. There are mitigation measures that can be taken to reduce potential roof damage.

These mitigation measures include:

• Designing Wind-resistant Roofs
• Reinforcing Gable Roof Ends
• Securing Sheathing Properly
• Installing Shingles Properly

In new construction or roof replacement in high-wind hazard areas, installing a hip roof increases the strength of the roof framing and its wind performance.

A hip roof is more resistant to wind damage than a gable-style roof because hip roofs are sloped on all sides. The slopes of a hip roof do not resist winds, while the ends of a gable roof present large obstacles to the full force of the wind.

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 provide specific installation requirements, 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.

Exterior doors and windows are the weakest parts of a structure’s outer shell. If they are broken or blown in, high winds can enter a structure and create internal pressures which act 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 or injure any person inside.

Breach point mitigation measures include:

• Reinforcing Garage Doors
• Securing Double-entry Doors
• Installing Impact-resistant Glass

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. Impact-resistant glass is intended to:

• Prevent 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.

Other actions can be taken to help protect people, their homes, and other structures from the high winds. If a structure has any unattached buildings or trees near it, there may be opportunities to mitigate possible damage resulting both to and from these objects.

Mitigation measures include:

• Anchoring outbuildings
• Pruning and thinning trees

• How straight-line winds cause damage
• How a continuous load path can mitigate straight-line loads
• How specific components of a building are affected by straight-line winds and mitigation measures to reduce or prevent damage to property and people
• Other mitigation opportunities outside of the structure that can lessen damage and improve life safety