Tornado Roof Damage

Tornadoes produce some of the strongest winds on Earth over a narrow path. Roofs are usually the first part of a house to fail, and damage to them is one of the main indicators used to estimate tornado wind speeds.

The Enhanced Fujita scale

In the United States, tornadoes are rated after the event on the Enhanced Fujita (EF) scale, which entered operational use on February 1, 2007. It uses 28 damage indicators, including one- and two-family homes, to estimate three-second gust speeds:

• EF0: 105-137 km/h (65-85 mph). Some shingles blown off.
• EF1: 138-177 km/h (86-110 mph). Much of the roof covering may be stripped.
• EF2: 178-217 km/h (111-135 mph). Entire roof structures may be removed.
• EF3: 218-266 km/h (136-165 mph). Well-built houses lose most walls.
• EF4: 267-322 km/h (166-200 mph).
• EF5: 323 km/h (201 mph) or more.

Other countries use adapted versions of this scale or their own tornado intensity scales.

A typical failure sequence

FEMA's Mitigation Assessment Team studied homes after the April 2011 outbreak in Alabama and the May 2011 Joplin, Missouri tornado. It described a progressive sequence:

1. Loss of roof covering and siding, allowing water in.
2. Breach of windows or garage doors, which pressurizes the interior.
3. Uplift of roof decking, concentrated at corners, overhangs, hips and ridges.
4. Loss of the roof structure where roof-to-wall connections fail.
5. Collapse of walls that lose their top support.

Loose shingles and decking also become windborne debris; in one Tuscaloosa building a shingle passed through gypsum board on both sides of an interior wall.

Lessons for roofs

The FEMA team found that decking often detached because nails missed framing, were spaced too far apart, or were undersized, and that roof-to-wall connections were the weak link most often responsible for roof loss. Metal connectors outperformed toenails when installed correctly.

Because most tornadoes are rated EF0 to EF2, engineers argue that wind-resistant details already used on hurricane coasts (ring-shank deck nails, connectors at every rafter, stronger garage doors) can prevent much tornado damage inland. Homes cannot practically be designed to resist the direct hit of a violent EF4 or EF5 tornado, which is why storm shelters and safe rooms are recommended for life safety.