The home pictured in the photo below is a perfect example of what can happen to a home without proper gable end bracing. I would have to guess that the high winds blew the bottom of this gable end into the attic area. Then when the wind direction changed as Andrew when though the area it was blown out off of its support wall. Without the wall holding it up the gable end fell to the ground. Next various sections of the support wall failed. Notice how most all of the roof trusses stayed in place and how only two large pieces of roof sheathing are missing! This home was very close to total destruction, thankfully Andrew was a fast moving hurricane and left the area before any more damage was done.
Pictured below is what is known as gable end bracing. As you can see from the picture you need to place two 2x4s in a "X" pattern running from the gable end to the fourth truss. This will help keep that gable end up in high winds, by transferring the pressure into all the trusses.
Spreading the pressure out over several structural members decreases the load and preventing any one member from failing. This is not strictly gable end bracing, as this helps brace the trusses as well. If an interior wall were located below the fourth truss, this would be great. If there is a wall located here it would help transfer the load to the foundation, which would be the best situation. So, to do this brace you need more of the 3" screws. Placing two in each truss/brace intersection. You need to have a good connection, with the screws, to the gable end. If you can't find a real good member to connect to, attach a small length of 2"x 4" say 10" or 12" long to the gable end then screw the long brace into the side of it.
I now want to get into true gable end bracing. I know this diagram is a little more difficult to follow but it's vital that you understand what's going on here. When hurricane force winds pound on your home, things start to shake rattle and roll. Because the gable end has a large surface area it really takes a beating.
The contractor who built your home did a fine job, it would stand up to normal weather conditions, but 120-mph winds are not normal. Recent storms have shown that the gable end pops loose, normally at the bottom and then collapses. Shortly after this the roof is blown away, the home it totally destroyed. What we are doing in this diagram is tying the bottom of the gable end to the to the rest of the trusses and the gable end support wall. The top of the gable end is also connected, but is not shown in this drawing.
The gable end is really just another roof truss except it is located on the end. In many ways it should match the adjacent roof trusses. Because of its similarity to the other roof trusses it should be a simple task to tie it to them. Every situation may be a little different but this plan should work pretty well for most everyone.
What is done is a block of good quality 2"x4" stock is cut to fit tightly between the gable end bottom cord and the bottom cord of the first roof truss. This block is called the support block. On the gable end side you should be able to connect it to the ceiling nailer. A 7/16 inch plywood block or shim may be needed here to keep all the parts level. On the truss side connect the support block on its end as shown. Once this is done you can connect your longer 2"x4" lateral brace to the gable end, the support block and on across the bottom cords of the roof trusses.
More is said about lateral bracing in the truss bracing page. These su pport blocks need to be spaced every four feet on center or as close to this as possible. It is not necessary to run lateral bracing all the way across your attic spaced every four feet. However every support block must have a lateral bracing block nailed to it even if it is only eight feet long. If this is done properly the wind will not be able to push the gable end into the attic or pull it out from the gable end support wall.
I placed this last drawing here to help you visualize what I've been talking about. The gable end braces need to connect both the top and bottom cords of the trusses to the gable ends. It is very important that your gable ends are tied into the rest of the roofing system, make this a high priority item.
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