Tips on Protecting Your
House Against Tornadoes

1. Use additional fasteners—sufficient to resist strong separation pressures—on the roof decking.

2. Use hurricane clips, or connector plates, to reinforce the roof-to-wall connection.  Use the proper nails–as specified by the manufacturer—with the hurricane clips or connector plates.

3. Use anchor bolts to secure the structure to the foundation.

4. Angle the roof a bit more.  Wind loadings decrease as the roof angle increases up to an angle of about 45 degrees.  A 45-degree roof (for the gabled variety) provided the least wind loading of all roofs tested.  However, hip and mansard roofs appear to offer better resistance to wind overall. It should also be noted that steeper roofs are less susceptible to snow loading for homes built in cold climates.

5. The use of well-anchored dormers or other irregularities in roof lines appear to break up the wind and reduce peak loadings although shingle damage potential is slightly greater near such irregularities.

6. Effective corner bracing at the junction of exterior walls and diagonal wind braces on the framing face can add appreciable strength to the exterior walls should the roof disengage.  A few connector plates along the junction of the base plate and framing stud can also help strengthen free-standing walls.  Typically plywood is used in the corners, but it must be properly nailed.

7. Venting the roof helps to lower internal pressure potential and eases the effect of separation pressures on the roof decking.  The loading on the roof, rear wall and at least two side walls is reduced.  Even though increased loading results on the up wind side of the house, the external walls should remain intact as long as the roof does (except for possible damage by missiles--airborne projectiles or debris).

8. With an attached garage facing the approaching wind, a house is more susceptible to damage than a house without an attached garage or with a garage that does not face approaching winds.  House with porches or large overhanging eaves that face the wind, and houses oriented so that gables face the approaching wind are also more vulnerable to wind than similar houses in other orientations.  However an overhanging eave is still a desirable feature in that it shields the house from the weather and it is aesthetically pleasing.

9. Shielding by trees also mitigates wind damage in residential areas and in mobile home parks.  Mutual shielding in a group of houses is a recognized but not well understood effect.  Single houses in the open (such as rural houses) and houses at the windward edge of a group feel the full effects of a windstorm.  For houses in the interior and leeward side of a group, some protection through mutual shielding may be realized, although effects of wake turbulence and missiles may increase damage.

10. Mobile homes present special problems.  Authorities agree that mobile homes should be vacated when a windstorm threatens.  To mitigate property damage, a well-engineered and properly installed tie-down system is essential.  Anchorages should be selected in light of soil conditions.  Whether for mobile homes or conventional housing, foundations should extend at least two feet into the soil, and concrete block foundations should be reinforced with vertical reinforcing steel.  For mobile homes, anchoring against tornadoes requires special consideration; consult with the manufacturer or Housing and Urban Development (HUD) guidelines.

The preceding information has been provided by structural and construction engineer members of the Ohio Society of Professional Engineers.

Special thanks to editors Don Liszkay, P.E., of Criterium Withem Liszkay Engineers, 110 North High Street, Gahanna, OH 43230, 614-418-7200, and Tom Swain, P.E., Ralph Swain Builder Company, 788 Lauraland Drive, Columbus, OH 43214, 614-451-9207