Diagonal bracing is critical to the stability of your structure
DEAR TIM: I’m facing some building projects. 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. Can you give me the executive summary? - Randy P., Kokomo, Ind.
DEAR RANDY: Books have been written about diagonal bracing. It’s a complex topic, but I’ll give you the basics so you don’t have a failure on any of your projects.
Diagonal bracing provides lateral stability, preventing the collapse of walls, decks, roofs and other elements. Imagine building an 8-foot high wall using 2-by-4s 16 inches on center. 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 enough to put weight on top of it.
But get on a stepladder at one end of the wall and push on the end of the top plate in a motion parallel with the wall. You can collapse the wall onto itself with as little effort as it takes to close an accordion door. In seconds, the 8-foot tall wall will fold up on itself and fall to the floor.
Imagine if you built a house with no or inferior diagonal bracing, and a severe windstorm blew against it. Or imagine the side-to-side shaking when the shear waves of an earthquake hit. See how the house could easily collapse?
When professional 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, 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.
I’ve seen decks whose outer support beam just rests on wood posts. If people up on the deck start dancing and a harmonic motion builds up, the entire deck can collapse as it 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 is to put diagonal braces that connect the deck beams to the vertical posts. These are typically cut at a 45-degree angle. It’s important to bolt these rather than rely on nails.
You can also install a flat 2-by-6 or 2-by-8 on the underside of the floor joists to give decking diagonal support. Drive 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 important if you’re working with roof trusses on that room addition. Many a carpenter has been killed or seriously injured when roof trusses suddenly collapse. Large truss roofs often come with detailed drawings that show where bracing needs to be installed in the webs of the trusses. Always be sure to reference any drawings, or talk with an engineer at the truss company, if you have any questions.
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Building codes have strict guidelines with respect to diagonal bracing. The code almost always dictates the type of nail, length, shape of the head, special coatings and so forth. The reason is simple: Diagonal bracing is 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.