A structural engineer will be needed to make those decisions. That does not mean, however, that protection against an EF-2 tornado need be only half as strong.

Note that FEMA's P-320 design guidelines make no mention of the varying storm strengths and thus appear to be designed to protect against an EF-5 tornado (which has twice the wind speed of an EF-2). You will need to design the walls, door and ceiling to be able to stand up to high wind pressure, as well as resist penetration or crushing from flying or falling debris.The room must be anchored securely to ensure that it does not lift or overturn in high winds or a tornado.The room should not have windows, but if it does, they should be very small (too small for a burglar to crawl through) and constructed of Plexiglass to prevent shattering.Concrete walls are a great choice, but if you want to adapt an existing wooden-walled room, you can reinforce the insides of the walls with steel sheathing. The room must be constructed out of material that can withstand high winds as well as heavy debris that may be flying around, for instance in a tornado situation.If you live in an area that has never experienced an EF-3 tornado, there is questionable need to have an EF-4 rated shelter, for instance. There are 5 levels of tornadoes and 5 levels of hurricanes, for instance, each demanding different levels of protection. The safe room's construction and design must be reinforced and built to withstand storms and threats of attack be sure you understand these factors as you plan and build the safe room.

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