Hurricane Design


There is no such thing as “hurricane-proof” design because experience has shown that the things we have not yet seen are greater than what we have seen, but there are ways to build that dramatically improve our prospects. Originally, before there were hurricane experts, if someone was lucky enough to survive a hurricane but their house was not, when they crawled out of its wreckage and saw a neighbor’s house still standing, they said “I’m going to rebuild like that.” Europeans brought European ways of building to the Americas, but it didn’t take too many decades of acquiring the hard-won wisdom of survival to adapt their buildings to a region frequented by heat, humidity, and hurricanes. Today, it seems that the engineers have relieved us of the burden of building wisely, because with enough concrete and steel, a building can survive a storm of shocking strength… but who wants to live in a bunker?

Today’s hurricane experts can explain why lovable Caribbean Rim Architecture is so durable in a storm. And in every pattern of that resistance, we can see that it’s done more with wisdom than with brute structural force. Let’s look at some of those patterns:

Roof Shape

hipped silver roofs gleam against clear tropical blue sky at midday as they top white-painted shops at Mahogany Bay Village on Belize's Ambergris Caye
Tweet: Build hip roofs on all but the smallest or strongest buildings to be wind-strong so each side supports its neighbors. http://ctt.ec/ea10y+

Build hip roofs on all but the smallest or strongest buildings to be wind-strong so each side supports its neighbors.

Triangles are keys to structural strength because a three-sided shape is not easily broken. The short sides of hip roofs are usually perfect triangles, and in any case, each side of a hip roof leans back against and supports the neighboring sides of the roof. Because of this, you might call the hip a “good-neighbor roof.”

The smallest of roofs don’t need to be hipped because their roof areas are so small that they are naturally strong. And the heaviest of buildings are built so strong that their roofs need not be hipped either, but if you look around the tropics, you’ll see that the majority of the middle-size roofs tend to be hipped.

Roof Pitch

Ambergris Caye moonrise over Mahogany Bay Village with twin cottage windows glowing orange from the second level as the rest of the village street sleeps in Belize
Tweet: Pitch wind-strong roofs 8/12 to 9/12 - steep enough to resist uplift but shallow enough to resist overturning. http://ctt.ec/t25Ye+

Pitch wind-strong roofs 8/12 to 9/12 because this is steep enough to resist uplift but shallow enough to resist overturning.

Flat and low-slope roofs normally fail in high winds due to a force known as “uplift.” Simply put, the wind sucks the roof up off the building. A really steep roof, on the other hand, is so tall that the horizontal force of the wind simply turns the roof (and the building, if it’s well-attached) over on its side. There is one common exception to this rule of thumb: buildings with half-stories built into the roof can be steeper, up to 12/12, because the interior walls that are normally built in such a half-story reinforce the roof, as do the dormer walls built to bring light and air into the upper rooms.

Eave Overhang

evening sun paints scroll-cut rafter tails golden at Mahogany Bay Village, rising from the sands of Ambergris Caye in Belize
Tweet: Overhang wind-strong eaves less than you would inland eaves so there’s less for hurricane winds to grab. http://ctt.ec/aexrh+

Overhang wind-strong eaves less than you would inland eaves so there’s less for hurricane winds to grab.

Most structural collapses in a hurricane begin at the roof, I’m told. If the winds take hold of the eaves and begin to peel them back, the roof decking can be lost quickly. That roof decking acts with the rafters as a giant beam, supporting the top of the wall. With the decking gone, the top-story walls succumb quickly to the winds and complete structural collapse often follows. This is why shorter overhangs are so common in the tropics. Open rafter tails like these are OK, so long as they’re unusually short (as these are) and even closed classical eaves should overhang less than they would far from the coast.

Sacrificial Eaves

front porch of newly-built whitewashed cottage sits against balmy blue tropical sky at Mahogany Bay Village
Tweet: When long eaves are necessary in the tropics, design them to blow off, leaving the main roof intact. http://ctt.ec/cieTC+

When long eaves are necessary in the tropics, design them to blow off, leaving the main roof intact.

The largest sacrificial eaves are porch roofs like this one, because even if the porch is entirely destroyed, the main building can remain intact so long as the roof decking is not continuous from the sacrificial eave to the main roof. The more common sacrificial eaves of the Caribbean Rim, however, are those that project out two to four feet from the face of the building and that are supported by brackets designed to let the roof go when the storm gets too strong for the brackets to hold it secure.

Brackets

enormous white wood brackets reach up to hold deep sheltering eaves firmly in their embrace against cotton puff-ball clouds set against the blue sky of Rosemary Beach on Florida's Design Coast
Tweet: Long eaves on wind-strong buildings can be supported by heavy brackets. http://ctt.ec/8WKlc+

Long eaves on wind-strong buildings can be supported by heavy brackets.

There’s another option to designing long eaves to be sacrificial: they can also be supported by ultra-strong brackets such as the ones shown here. Heavy brackets work best when they support a heavy cross-beam that in turn supports the rafters, like the one shown here. Because spans are short and most of the structural stresses are pure tension or compression, square sections often work best for components of the brackets. Again, this building is an ideal example.

Roofing Material

corrugated metal roofs of Mahogany Bay Village glow white as they bounce the sun's heat back to the tropical sky on Ambergris Caye in Belize
Tweet: Properly-attached metal roofing is the best material for a hurricane zone. http://ctt.ec/Deuqb+

Properly-attached metal roofing is the best material for a hurricane zone.

Metal roofing does other good things as well, including being the most frugal roofing you can use in places where the summers get hot, but for now, let’s talk about its windstorm benefits. For decades, people thought that clay tile roofs were the most desirable in a storm, but quite the opposite is true. When a clay tile roof begins to fail, each clay tile becomes a potential missle hurled at over 100 miles per hour at its neighbors. But if you attach a metal roof properly, it endures a storm like no other.

Shutters

Mahogany Bay Village's tropical windows are built of sturdy mahogany louvers with glass transoms above
Tweet: Shutter windows in some way so that most if not all of the opening is protected with wood. http://ctt.ec/2a_0H+

Shutter windows in some way so that most if not all of the opening is protected with wood.

This window has a screened opening closed by strong hardwood shutters, and the small transom panes above are strong enough to endure substantial impacts. But windows in most places don’t meet either criteria, so they should be shuttered before a storm. We have worked for years with Schooner Bay in the Bahamas, which endured the worst of Hurricane Irene, yet sustained essentially no damage. As a matter of fact, not a single pane of glass was broken. This is because windows were protected with shutters which were shut in advance of the storm, and the shutters, not the glass, endured the impacts of wind-borne debris.

Wall Construction

rusted steel reinforcing bars hang grotesquely out of the crumbling remains of a highway bridge in the Bahamas that endured too much salt spray
Tweet: Choose carefully between wood walls and masonry walls in a hurricane zone; each has its strengths. http://ctt.ec/d1T5V+

Choose carefully between wood walls and masonry walls in a hurricane zone; each has its strengths.

Choosing concrete over wood seems like a no-brainer in a hurricane zone, but this isn’t necessarily true. Look around the Caribbean Rim. What you’ll see is a gumbo of architecture that is part masonry, part wood. The wealthiest citizens often built of masonry, but most buildings in town were wood-framed, or some combination of wood and masonry. This is because most people couldn’t afford a full masonry house. But because everyone wanted their home to survive the storm, measures evolved to allow both wood and masonry buildings to weather the storm. But things change. Today, we build the strongest buildings of reinforced concrete… but while it’s amazingly strong, reinforced concrete is not so durable in salty air, as the reinforcing bars rust and the concrete spalls away. Here’s a highway bridge that’s less than ten years old, yet it has already been condemned due to failure of the reinforced concrete in salt spray.

Building Elevation

whitewashed front porch with weathered mahogany trimmings sits in the tropical breeze at the edge of Mahogany Bay Village on Ambergris Caye in Belize
Tweet: Build above the floods in a hurricane zone so the storm surge does little or no damage. http://ctt.ec/0aN18+

Build above the floods in a hurricane zone so the storm surge does little or no damage.

Originally, people had no idea how high the floods would surge in a storm, so they either built tropical buildings several feet above the ground or they built the first floor so it could flood, dry out, and be OK. Science has given us a better idea what is most likely to flood today, allowing governing bodies to establish flood elevations above which habitable buildings must be built. So because of science and modern record-keeping, this building can sit with its main floor only three feet above grade, but that's still two feet higher than the highest storm surge ever recorded in this place. 

Strong Piers

field of mahogany piers sit ready to accept cottage foundations at Mahogany Bay Village on Ambergris Caye in Belize
Tweet: If piers are masonry build them thick, or if wood, drive them deep so that they resist storm surges. http://ctt.ec/nFeu5+

If piers are masonry build them thick, or if wood, drive them deep so that they resist storm surges.

This is a field of piers driven deep into the earth for building foundations. Each can take quite a lick from water-borne debris in a storm surge, because wood resists bending really well, especially when you’re trying to bend a short, thick section like the tops of these piers. And the wood species here is Cabbage Bark, one of the hardest woods on the planet. Masonry is different. It’s strong against vertical loads, but very weak in bending, so if you’re building with masonry foundations, they need to be unusually thick in order to resist the impacts during a storm surge.

Wall Base

white masonry walls of Alys Beach on Florida's Design Coast sit on subtle masonry base, trimmed with palms sitting in tall terra cotta pots
Tweet: Express the base of a masonry wall differently from the rest so it can be refinished more frequently. http://ctt.ec/bef33+

Express the base of a masonry wall differently from the rest so it can be refinished more frequently.

See the break in the wall a foot or so above the ground in this image? It’s not there just for style. Instead, it’s a common feature that occurs on masonry walls because the bottom of the wall takes more abuse than higher on the wall. Think everything from lawnmowers to kids kicking balls. If you have to refinish the entire wall every time the base gets scuffed up, that gets really expensive, so masonry buildings have traditionally been built with a base anywhere between one and six feet tall that can be refinished on a different schedule than the rest of the wall.

Most of these images are sourced from our work at Mahogany Bay Village on Ambergris Caye in Belize, which is doing beautiful wood work, as you can see. The Brackets image is from Rosemary Beach, whereas the Wall Base image is from Alys Beach, two places that are building unusually good masonry buildings today in the tropics.

~Steve Mouzon



© Studio Sky 2014