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Designing for Extreme Weather

Designing for Extreme Weather
In this home, living spaces are raised above what's called "brea
First, the bad news. Hurricane force winds bringing missile like debris shooting through the air. Storm surges and flash floods that carry away houses and cars. Extreme weather -- like the recent downpours causing flooding in Texas and elsewhere -- threaten property and life.

Now for some good news: Building codes and construction techniques have made great advances over the last few decades. While previously there wasn’t much in the codes that addressed extreme weather conditions, and code enforcement was lax, 
building codes now take the possibilities of extreme weather conditions and the need to protect life and property very seriously. But rather than just building to code, we should look at how we can exceed code to keep ourselves and our property as safe as possible. For example, take a look at this high wind test by the Insurance Institute for Business and Home Safety.

The house on the left was built to code while the house on the right was built to exceed code at a nominal (approximately 1%) increase in construction cost (see the video 
of the test here; image courtesy IIBHS). For my money, I’d rather be living in the house on the right. No doubt anyone would. But what does it take to get the house on the right? It takes the use of impact resistant windows and doors, and the use of additional ties, straps and fasteners. Also, for homes built in areas prone to flooding and storm surges, raising the house above the Base Floor Elevation is often mandatory.

Impact resistant windows and doors
The chances are that objects will go hurtling through the air during extreme weather. The higher wind speeds can cause anything lose to behave as if it were shot from a cannon. So a tree limb that’s lying on the ground can, all of a sudden, become a projectile that goes crashing through a window. Windows and doors that have been designed to be impact resistant are built to withstand the force of these projectiles. Just remember that not all impact resistant windows and doors are created equally. Specifically, each is designed to withstand a certain design pressure (the amount of force that the window or door is designed to withstand). Factors that contribute to a design pressure rating include:
  • The type of building proposed. For example, single family houses are Category II buildings while buildings that contain essential services, such as hospitals, are Category IV buildings. The more essential the building, the stricter the requirement for keeping the building operating.
  • The type of exposure the building has. For example, a house located on a stretch of unobstructed water would be classified as having Exposure D while a house surrounded by many other houses such as in a suburb would be classified as having Exposure B. Impact resistant windows and doors will have to be stronger for Exposure D as there is the chance of a windblown projectile traveling a greater distance and striking a window with more force.
  • The locally mandated wind speed. While there are maps that provide the wind speeds, it’s best to contact the local building official to ensure that the correct wind speed is used. The higher the wind speed the more likelihood of windblown debris becoming projectiles.
And don’t forget that the use of these types of windows and doors can keep you safer from break-ins. 

Additional ties, straps, and fasteners
These components of a building structure serve to connect each piece to the adjacent piece from the foundation up to the roof. Doing this results in a stronger and stiffer structure that can only be pulled up from its foundation, like a tenacious plant that is firmly rooted in the ground. In the past we relied on simply nailing one structural member to the next. So, for example, a 2x4 wall stud was toe-nailed into the sill plate. We now know that these nailed connections just weren’t strong enough to survive during extreme winds. What has developed, and now is code-mandated, are straps such as this


that securely fasten the sill to the foundation; like these that fasten the wall stud to the sill plate;


 and like these that securely fasten the wall frame to the roof frame.

The number and location of ties, straps and fasteners is typically governed by the local building code (images courtesy Simpson Strong-Tie). What is important to note is that the building code is a minimum standard that must be adhered to. As can be seen in the video, a house that’s built “to code” may not be strong enough to withstand some of what nature throws at us. A better option is to build what the IIBHS terms the "Fortified Home." The Fortified Home Technical Requirements for New Homes Hurricane Standards publication provides information and details for how any new home can be designed and built to better withstand hurricane forces.

Building above the Base Flood Elevation
If you plan on building a new home in a coastal area or within the boundaries of a flood area, you’ll want to mitigate the risks of water damaging the structure and endangering the inhabitants.  This can be done by raising the lowest occupied level of the home to be above the Base Flood Elevation, which is the “100 year flood” elevation. However, as with other standards, building above the BFE should be the minimum. Certainly building somewhat above the BFE may be necessary to mitigate future higher flood elevations. Building above the BFE can involve a home that’s still relatively close to the ground, such as Plan 536-3 by architect Bruce Tolar shown here

or a home with the main living level raised considerably above the ground, such as Plan 443-12 by Gulf Coast Cottages, shown at the top of this post. The 
ground floor is a series of posts framing the carport and
supporting the upper floor. The main living level is on the second floor as shown here.

In the case of houses designed for coastal areas prone to storm surges, the ground floor rooms are best constructed to float away without adversely affecting the main house structure. Any doors, such as for garages, are designed to “blow out” rather than try to dam up the flooding waters. In a sense, the ground floor areas become invisible to floods, letting the waters go where they will.

In summary, if you’re looking to build a home in an area prone to flooding, storm surges or high winds, you may even want to consider building a Fortified Home. While it may cost a few extra dollars to do so, the long term peace of mind and savings on things like insurance will likely make it worthwhile.

To browse a collection of Beach style house plans click here.

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