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Surviving the Storm, Energy efficient framing practices for hurricane and tornado country
comments (1) April 30th, 2011 in Blogs
Michael Chandler, member
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Optimum Value Engineering (OVE) and some custom details combine to make an energy-efficient system that will weather the storm
Photo: Pencil drawings by Michael Chandler
Looking at the pictures and video of the recent tornados leaves a feeling of hopelessness. What can we do as builders to defend against that kind of destructive force? A closer look at how the damage really happens shows a few lessons that we can use to serve our customers better in the event of violent weather.
The damage may be more widespread than you think
The video and news reports we see show only the extreme conditions. Far more common are damages to people and property at the edge of the storm that are far less catastrophic and even preventable. We joke that tornados are somehow magnetically attracted to trailer parks but 15 of the 24 deaths from the April 16 tornados here in NC were in mobile homes. They are very vulnerable to being crushed by falling trees. A fellow builder lost three children when a tree came crashing through his trailer while he was doing a side job that Saturday afternoon, his wife was home with the kids and survived. Another man was crushed when a tree came down on his car, while parked in his driveway. Having good shelter and knowing when to take cover can make all the difference.
Finding Refuge
If you are at risk from a tornado or from the “microbursts” (80-120 mph vertical winds) that accompany hurricanes don’t wait in a trailer or in your car. Find refuge away from the windows in a stronger building that doesn’t have a lot tall trees around it and wait it out there. If you are driving and a tornado is going to overtake you get out of the car and lie down in a ditch or depression so the debris will blow over you and the wind won’t have something to push around. Teach your kids to listen for the “freight train” sound of a tornado or microburst and take cover in a safe area of your home under the staircase, in a windowless powder room or a storm hardened closet. I recently waited out a microburst in my hardened powder room with my wife, child and a family friend. Four people clutching cell phones in a darkened 3x5 room while listening to the trees come down on the hillside above us.
Building a safe haven
We often build safe rooms into master bedroom closets by lining the walls and ceiling with plywood before hanging the sheetrock. Typically we will include a 32x32 tempered awning window for keeping track of conditions outside. A connection to the alarm system, and some form of bolt on the door allow it to protect against burglars and home invasions. Builders have been slow to adopt 24” OC 2x6 optimum value framing due to concerns about the strength of the structure. We have adopted several framing techniques that work with Optimum Value Engineering (OVE) to improve storm survivability while also maintaining good energy efficiency and stiffness.
Read on to see some of our standard strategies.
1. Strengthen the impact zone where the trees hit the top of the wall
We nail the headers to the top plate with cripples as needed between the header and the top of the windows and doors. We still use double top plates which allows us to place 2x blocking on the outside of the upper top plate on the top edge of the sheathing so the impact of a tree on the overhang is transferred by the blocking to the shear nailing on the sheathing rather than just crushing the top edge of the sheathing and coming on into the room. Of course we fully sheath the exterior walls with OSB and use more than code minimum hurricane clips and we still “right size headers” to double 2x6’s when less than 36” as allowed by the engineer.
2. Plan "break-aways" to minimize water damage.
If hurricanes and tornados didn’t involve so much rain the recovery would be much easier. Trees on the roof can let in a lot of water and the structural damage is often much less expensive to repair than the water damage. We break the roof sheathing over the squash block on the outside of the wall so that when the trees land on the house the damage is largely held to the roof overhangs and the water is kept out of the attic. We also use 2x treated subfascia and nail galvanized roof edging through the sheathing into the subfascia to help keep the wind from getting a grip on the roof edge. Likewise we flash our chimney pipe to the roof as if it didn’t have a chase and build a separately flashed chase above the roof that can break clear if hit by a tree without opening a huge hole in the attic.
3. Don’t assume that gravity is going to hold the bottom plate to the subfloor
During the recent storms in NC one house had four windows blown out by wind bourn debris. That was enough to pressurize the house and pop all the down-wind walls off the subfloor and into the yard totaling the home. Rather than simply nailing the bottom plate to the subfloor with 16D nails we hold the sheathing up a foot off the bottom plate and add a strip of OSB or treated ply that spans from the mud sill to the studs. We also use real anchor bolts rather than the expensive straps and put them closer than required by code. Code will allow us to skip the blocking if we run all the sheathing horizontally as shown but we usually block this and run 8’ sheathing vertically above this point. On the second floor the sheathing is run long so that when the wall is tilted up the bottom of the sheathing can be nailed to the upper top plate and rim joist below (with a ¼” gap for shrinkage of the rim joist.) Blow-outs are more commonly caused by failed garage doors on attached garages so be sure to lock the garage door with the slide bolt locks on the inside and lock the door between the garage and the house with a deadbolt and pick up lawn furniture and trash cans that can break windows when a storm is coming.
4. Stiffen the ridge and use real barge rafters
Even if you use trusses and ridge vents it is still a good idea to stiffen the roof sheathing at the ridge and to use real old fashioned barge rafters. It seems like barge rafters are a lost art these days but they really give a lot of solidity to the rake edges by tying them in to the next to last rafter or truss. Nailing the roof sheathing off to 2x4’s on the flat between the trusses is especially important on single story homes where the trees may hit the ridge first and it helps stiffen the roof by tying the two planes together. I had seven trees on my roof after a microburst touched down fifty feet from my home during Hurricane Fran but no water damage and very little structural damage other than a bit of shingle and gutter repair and some broken rafter tails. Nothing can make a home safe from a direct hit from a tornado but we can do a lot to minimize damages from a close call.
posted in: Blogs, green building, framing, hurricane, tornado, ove
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Comments (1)
How about a nice Ferro-cement Dome?
Posted: 7:17 am on May 2nd
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