Diagonal Brace Tips
DEAR TIM: I have some building projects facing me. One's a tall deck, another is framing a garden shed, and then I need to help a friend with a room addition. I'm wondering about diagonal bracing. I don't understand it and need to know why it's important. Is there a short cut? Can you give me the executive summary so I don't have any problems on these jobs? Randy P., Kokomo, IN
DEAR RANDY: You'll be getting the executive summary all right. Books have been written about diagonal bracing. Talk to any structural engineer and I'm sure he'll tell you that entire college courses are offered on the topic. It's a very complex topic, but I'll do my best to give you the basics so you don't have a failure on any of your projects.
Diagonal bracing is a structural component of just about any building. It provides lateral stability preventing collapse of a wall, deck, roof, etc.
Let's talk about what happens when you don't have diagonal bracing in place so you get a better understanding. Imagine if you were to build a wall using 2x4s 16 inches on center and you make it 8 feet tall. If you stand the wall up and nail the bottom plate to the floor to hold just that in place, the wall might seem strong if you put weight on the top. Don't you stand on the wall to test this. It will collapse.
But here's the scary test. Get on a stepladder at one end of the wall. Push on the end of the top plate as if you're trying to move the 2x4 plate forward, not side to side so as to make the wall tip over. You'll quickly discover you can collapse the wall down onto itself with little effort like you'd close an accordion door. In seconds you can have the 8-foot tall wall folded up on itself and only inches high on the floor.
Now imagine what would happen if you built a home with no or inferior diagonal bracing and a severe windstorm blows against the house. Or imagine the violent side-to-side shaking that happens when the shear waves of an earthquake hit a house. Can you see how the house could easily collapse? When pro carpenters build a house, they install different types of bracing. One might be a metal diagonal bracing from the lower corner of a wall up to the top plate.
Plywood or oriented strand board (OSB) properly nailed will also provide excellent diagonal bracing. You typically only have to put one full sheet of plywood at each corner of a wall to provide the needed stability.
Deck collapses can also be traced to a lack of sufficient diagonal bracing. I've seen decks built where the outer support beam just rests on wood posts. If a group of people up on the deck start dancing and a harmonic motion builds up, the entire deck can collapse as the deck starts to shift sideways. Diagonal bracing prevents this side-to-side movement.
There are any number of ways to install diagonal bracing for a deck. One way is to put diagonal braces that connect the deck beams to the vertical posts. These braces are typically cut at a 45-degree angle. It's really important to bolt these instead of relying on nails.
You can also install a flat 2x6 or 2x8 on the underside of the floor joists to give the decking great diagonal support. Drive no less than two 16d galvanized nails through the brace at each floor joist. Be sure the nails have the proper coating to match the treated lumber you're using to prevent corrosion.
Diagonal bracing is really important if you're working with roof trusses on that upcoming room addition. Many a carpenter has been killed or seriously injured when roof trusses suddenly collapse as they're erected. Wind can easily push them over without bracing.
Large truss roofs often come with detailed drawings that show bracing that needs to be installed in the webs of the trusses. Always be sure to reference any drawings or talk with the engineer at the truss company if you have any questions.
Be aware there are very strict building code guidelines with respect to diagonal bracing. The code almost always dictates the type of nail, length, shape of the head, special coatings, etc. The reason is simple: Diagonal bracing is mission critical to the structural stability of a building.
I've always found it best to install diagonal bracing for walls while the wall is built flat on a floor surface. With the wall down on the ground, it's easy to square it up. With the wall square on the ground, you can temporarily toenail the bottom and top plate so the wall doesn't move while you nail on the plywood or OSB at the corners.
You can also nail on the rest of the wall sheathing if you like. When you tilt up the wall, it's already square and you can move on to the next wall.