Fundamentals of Resilient Design #2: Designing Homes for More Intense Storms

Posted by on Aug 25, 2012

Route 4 washout near Killington, Vermont from Tropical Storm Irene in August 2011. Photo Lars Gange and Mansfield Heliflight.

Anybody who was in Vermont one year ago this week and witnessed the raging floodwaters of Hurricane Irene and the havoc they wreaked, understands the vulnerabilities we face from intense storms and flooding. In the Northeast, there was a 67% increase in heavy rainfall events (defined as the heaviest 1% of all rainfall events) from 1958 to 2007, according to the multi-agency U.S. Global Change Research Program.

Climate scientists tell us to get used to it.

As the planet warms over the coming decades, precipitation will increase overall—due to greater evaporation from bodies of water and, thus, more water vapor in the atmosphere—though there will be significant regional variation. Even in areas that see a drop in precipitation (an expected trend in much of the western U.S., for example), the rain that does fall is expected to increasingly fall in deluges. So, we need to prepare for more Hurricane Irenes and the resultant flooding.

Along with more intense rainfall events, we can expect stronger winds. Some of these winds will be from tropical storms whose intensity is magnified as their air masses pass over a warmer South Atlantic and Gulf of Mexico. Others of the winds will be more localized thunderstorms that result from air masses over land warming and rising into intense cumulonimbus clouds.

In coastal areas, sea level rise will play into this, as coastal storms increasingly cause storm surges—the situation that befell New Orleans in 2005 with Hurricane Katrina. Even a few inches of sea level rise dramatically increases vulnerability to storm surges. How quickly sea levels will rise depends on the ice mass on Greenland, and scientists are far more worried about this than they were just a few short years ago.

Resilient design

The bottom line is that we need to change the way we build our houses and design our communities to protect us from such storms. This is an important part of resilient design—which is the subject of this and the series of blogs that will follow.

There are a lot of strategies that come into play with storm-resistant construction. A few of these are described below:

Building to stringent hurricane standards

In designing homes that will hold up well in strong winds, we should borrow heavily from he Miami-Dade County Building Code, which includes a wide range of provisions, including hurricane tie-down strapping or clips that provide a continuous structural connection from foundation slab to roof, minimum 2×6 framing in exterior walls, minimum 19/32-inch plywood roof sheathing with 6-inch nail spacing at panel edges and 4-inch at gable ends, and hurricane-rated shingles. Other wind-resistant strategies include hip roofs that deflect winds, avoidance of deeply overhung entryways, and outward-opening doors that are held more tightly closed in heavy wind.

Control of wind-driven rain

Adequate overhangs are key to keeping rain away from homes. I like at least 24-inch-deep overhangs, in some cases even more. Gutters are advisable to capture rainwater, with downspouts and horizontal lines to carry collected water away from the house.

Basement drainage should include continuous perimeter drains that extend to the bottom of the footings. Foundation walls or the slab edge (with slab-on-grade construction) should be coated with a dampproofing layer, covered with a foundation drainage board, and backfilled with free-draining crushed stone. The perimeter drains should drain to daylight if topography permits. And the ground surface should slope away from the house.

Walls should be designed with rainscreen detailing that allows trapped moisture to escape and provides a pressure break for wind-driven rain, while allowing any moisture that does penetrate the siding to drain to the base of the wall or evaporate and escape. With an adequate overhang, the siding won’t even get wet with most storms.

Control of stormwater

More intense rainfall events will mean greater volumes of runoff, or stormwater flow. This means we should be installing larger-diameter storm sewers and culverts. It also means that we need to do everything we can to provide for infiltration of rainwater where it falls—so that we don’t need to transport that water at all.

The latter can be accomplished by minimizing impermeable surfaces, providing infiltration trenches (filled with crushed stone) between adjacent impermeable surfaces such as sidewalk and driveway, installing permeable pavement, avoiding curbs along driveways and roads, and providing grassy swales and infiltration basins that allow runoff to spread out and soak into the ground.

Along any waterways, we need to consider erosion risk. As we saw so clearly with Tropical Storm Irene in Vermont, intense rainfall events—particularly if the ground starts out nearly saturated—can result in streams and rivers quickly overflowing their banks. We need to provide for this in our placement of homes and outbuildings. The right plantings can help to stabilize banks and resist erosion.

Safe rooms

In parts of the country where tornados and hurricanes are most common, we should consider installing “safe rooms” to provide a safe haven in an extreme wind event. These are small bunkers in our homes or garages, anchored to concrete slabs and typically built of reinforced masonry block with steel-beam roofs. Such rooms are typically not connected to the walls of the house, and they should be equipped with battery-powered or wind-up radios, flashlights, etc.

The Federal Emergency Management Agency (FEMA) offers excellent guidance on the construction of safe rooms.

In this resilient design series, I’m covering how to improve the resilience of our homes and communities, including strategies that help us function well in the aftermath of a major storm or event that results in lost power, interruptions in heating fuel, or water shortages. We’ll see that resilient design is a life-safety issue that is critical for the security and wellbeing of families in a future of climate uncertainty and ever-present risk of terrorism.

Alex Wilson is the founder of the Resilient Design Institute in Brattleboro, Vermont, a nonprofit organization committed to advancing practical solutions that can be employed by communities, businesses, and individuals to adapt and thrive amid the accelerating social, ecological, and climatological change being experienced today. He is also the founder of BuildingGreen, Inc. in Brattleboro and executive editor of Environmental Building News.

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