Tornadoes are one of nature’s most violent and destructive forces. The extreme winds of a tornado can cause catastrophic damage, tearing roofs off homes, knocking down walls, and sending debris flying at deadly speeds. For homeowners living in tornado-prone areas, finding ways to protect their homes from tornado damage is a major concern.
While no house is completely tornado proof, there are steps homeowners can take to make their homes more resistant to tornado damage. Properly constructed and reinforced safe rooms, strategic structural bracing, and impact-resistant windows and doors are some protective measures experts recommend.
How much wind force can residential homes withstand?
Most standard wood-frame houses are designed to withstand straight-line winds up to about 100 mph before experiencing structural damage. However, the swirling winds of a tornado can subject a home to forces exceeding 100 mph from multiple directions simultaneously.
According to FEMA, the winds in a typical tornado can reach maximum speeds between 100-200 mph. More violent tornadoes can have winds exceeding 200 mph. Forces this strong can easily destroy most residential homes not specifically designed and reinforced to withstand tornado-force winds.
Are brick homes stronger in tornadoes?
Brick homes are inherently more rigid than homes using lighter framing materials like wood. However, this increased rigidity comes with some trade-offs when subjected to extreme tornado winds.
The main risk with brick houses is that the brick facades can detach from the wall framing and collapse in a tornado, causing devastating collapse. Proper structural anchoring is required to keep brick veneers secured to the framing in high winds.
Reinforced concrete block walls threaded with steel rebar are extremely rigid and can withstand very high wind pressures. However, the expense involved makes solid block wall construction impractical for most residential construction.
What structural vulnerabilities allow tornado damage?
There are four main weaknesses in standard home construction that make them vulnerable to tornado damage:
- Roof uplift: The large surface area of a roof combined with extreme wind suction can literally rip the roof off a house from the upward force.
- Wall failures: High winds flowing around the home can push walls inwards causing collapse. Sidewalls perpendicular to the wind direction are most at risk.
- Openings: Any window, door, or garage door opening allows wind to infiltrate the home and increase internal pressures, further stressing the structure.
- Connections: Nails and hardware used for structural connections can pull loose under extreme wind forces, causing the home to break apart.
To strengthen a home against tornadoes, mitigating these failure points through proper anchoring, bracing, and reinforcement is key.
What makes a safe room effective?
A safe room provides occupants a designated space inside the home to take shelter that is hardened against wind pressure and impacts from windborne debris. The main factors that make a safe room effective include:
- Reinforced concrete or steel wall construction capable of withstanding extreme lateral wind pressures.
- Sufficient anchoring to resist uplift and overturning forces on the safe room.
- A heavy, impact-resistant door.
- No exterior windows or openings that could be penetrated by debris.
- Careful sealing against air infiltration around wall/door edges and penetrations.
FEMA guidelines recommend safe rooms built to ICC 500 standards, which provide detailed construction and testing criteria to withstand an EF-5 tornado.
What are the best ways to anchor a house?
For homes without a safe room, strengthening the structural anchoring is a key way to protect against tornado damage. Recommended anchoring techniques include:
- Foundations: Bolting sill plates to the foundation footing using anchor bolts embedded deep in the concrete.
- Roof framing: Using hurricane straps/clips to securely fasten roof rafters and trusses to the top plates and wall studs.
- Interior walls: Secure interior load-bearing walls to the ceiling/floor framing using framing anchors.
- Exterior walls: Brace unsupported spans of exterior wall framing with sheathing panels or diagonal cross-bracing.
Steel frame anchor systems that cinch the walls to the foundation also provide excellent anchoring for wood-framed homes.
What building codes help strengthen homes?
The International Residential Code (IRC) contains several provisions that strengthen homes against tornado and high wind damage when implemented properly, including:
- Detailed roof bracing and framing connection requirements.
- Horizontal and vertical framing reinforcement at wall openings.
- Required wall sheathing panel fastening and bracing specifications.
- Anchoring requirements between structural elements like roof-to-wall and wall-to-foundation.
- Standards for storm shelters and safe rooms.
Homes built under the latest IRC will have better tornado resistance than older homes meeting outdated codes.
What building materials provide impact resistance?
Windborne debris slamming into walls, windows, and doors causes severe damage during tornadoes. Using impact-resistant building materials can mitigate this vulnerability:
- Doors: Steel and fiberglass doors with polyurethane foam cores resist impacts better than standard wood doors.
- Windows: Impact-resistant glass, acrylic, and polycarbonate panels withstand breakage. Laminated glass provides good protection.
- Roofing: Class 4 impact-rated asphalt shingles resist damage from hail strikes better than Class 3 versions.
- Siding: Impact-resistant fiber cement panels and vinyl siding outperform standard vinyl and wood sidings.
For the most critical walls, shotcrete, reinforced CMU block, or poured concrete provide the best impact protection.
Are there roof shapes better suited for wind?
Certain roof shapes tend to have better performance in extreme winds when properly built:
- Hip roofs: The sloped hips have no abrupt edges for wind to catch, reducing lift forces from air suction on the roof.
- Gable roofs: The triangular end walls need extra bracing, but avoid lift forces concentrating on edges.
- Flat roofs: Require heavy deck anchoring but lack overhangs susceptible to uplift.
Roof shapes putting edges perpendicular to prevailing storm winds, like A-frames and mansards, are more vulnerable to uplift and damage. They require special reinforcement.
How effective is residential storm shelter construction?
| Shelter Type | Typical Construction | Tornado Resistance |
|---|---|---|
| Basement | Reinforced concrete walls and floor | Excellent, if no openings |
| Safe room | Reinforced CMU or concrete. Steel door. | Excellent, when built to code |
| Storm cellar | Reinforced concrete or CMU | Very good, when underground |
| Bathroom | Normal house framing materials | Marginal protection |
| Closet | Normal house framing materials | Marginal protection |
Basements, safe rooms, and storm cellars built with reinforced walls and ceilings anchored to foundations offer the best tornado protection. Bathrooms and closets in standard construction provide more protection than no shelter but are still vulnerable to major damage.
What precautions should be taken when building a new home in tornado zones?
Key tornado damage mitigation precautions to consider for new home construction in tornado-prone areas include:
- Constructing a safe room or storm shelter compliant with FEMA 320 or ICC 500 standards.
- Securing all structural connections using hurricane clips, anchors bolts, straps and wall bracing.
- Using roof framing members rated for high wind loads and installing hurricane straps.
- Reinforcing garage doors and large doors/windows from wind pressure.
- Installing laminated glass windows or window coverings to mitigate glass breakage.
- Using Class 4 impact-rated roof shingles.
- Considering a hip roof shape over more damage-prone alternatives.
- Siting the home in a sheltered location away from hills and wind tunnels if possible.
A licensed structural engineer can help design critical connections to withstand tornado-force wind loads.
Conclusion
With proper structural design, reinforcement, and material selection, homes can be made significantly more resistant to tornado damage compared to standard construction. Safe rooms offer the best life-safety protection for occupants during a tornado. However, enhancing the overall structural robustness of the home through careful attention to the roof, walls, openings, and framing connections also reduces tornado vulnerability.
While the extreme winds of an EF-4 or EF-5 tornado will destroy almost any residential structure, implementing as many damage-mitigating measures as feasible can greatly improve occupant safety and reduce repair costs. Consultation with qualified structural engineers and builders with tornado-resistant construction expertise is highly advised for new homes in tornado-prone regions.