Don’t Let the Wind Destroy your Southern Home!
As we clean up from hurricane Issac, I thought it would be a good idea to look at how wind affects buildings, and how to keep it from destroying your home. This is especially important in the South, because the vast majority of hurricanes hit the US from Texas to North Carolina. I’ll try to do this without getting too dry or technical.
The wind attacks a building in two ways - Lateral Pressure, and Uplift. Lateral Pressure tries to push the building along the path of the wind, and the foundation keeps it from moving. But the wind keeps pushing and will destroy the building if not resisted properly.
Lateral Pressure is resisted by Bracing turned perpendicular to the wind. Since we don’t know which direction the wind will blow from, we must provide bracing in both directions.
Now let’s look at Uplift. It is pretty hard for most people to comprehend how the wind can pick up a building. If the roof is sloped, the wind blowing over the roof will push hard on the windward side, and then pull away from the leeward side creating low pressure. This is the same principle that allows airplanes to fly, but on a much larger scale.
High winds often create more lift than an average house weighs! Uplift must be resisted by a Continuous Load Path from the roof to the foundation.
One common myth is that high winds only occur in “hurricane zones” along the coast. While the expected wind speeds increase dramatically close to the coast, the International Building Code (IBC) sets design wind speeds of 90 mph for the interior of the US. And not building your home to this standard can have dire consequences for your home, your possessions, and possibly for you and your family!
In designing homes, I have found the least expensive way to resist the wind is by attaching structural sheathing – plywood or oriented strand board – completely over the outside of the stud framing, from the bottom of the sole plate (the board that is bolted to the foundation) to the roof bearing plate at the top of the wall (where the roof rafters are tied down with hurricane clips). This method resists both Lateral Pressure and Uplift. The IBC sets minimum lengths of fully-sheathed wall and specifications for the wall that can be counted for Lateral Bracing. Sometimes there are conditions that require more complex bracing and/or load path systems, but this method generally works in wind zones up to 110 mph – which gets you within 50-100 miles of the coast.
Of course there are a lot of calculations required to make sure your home will stand up to the winds. This is why you should have your home designed by an Architect or Structural Engineer – who are qualified to do these calculations - rather than by a Home Designer – or worse, simply purchasing stock plans. In fact, most Home Plans have a disclaimer saying that you should have the plans reviewed by an Architect or Engineer, so why not just go to a qualified professional for your plans in the first place?
Comments(2)