Diagonal bracing: Essential for sturdy structures

November 11, 2011

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. 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, Ind.

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, but I’ll do my best to give you the basics so you don’t fail at any of your projects.

Diagonal bracing is a structural component of just about any building. It provides lateral stability, preventing the collapse of walls, decks, roofs and many other structural elements.

Let’s talk about what happens to a structure when you don’t have diagonal bracing in place. Imagine building an eight-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 that you could put weight on top of it.

But here’s the scary test. Get on a stepladder at one end of the wall. Push on the end of the top plate in a motion parallel with the wall. You’ll quickly discover you can collapse the wall onto itself with as little effort as it takes to close an accordion door. In seconds, you can have the eight-foot wall folded up on itself and only inches high on the floor.

Now imagine what would happen if you built a house with no or inferior diagonal bracing, and a severe windstorm blew 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 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 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.

Insufficient diagonal bracing causes many decks to collapse. I’ve seen decks whose outer support beam just rests on wood posts. If a group of people on a 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 rather than relying on nails.

You can also install a flat 2-by-6 or 2-by-8 on the underside of the floor joists to give the decking great diagonal support. Drive no fewer 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 a 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 if they have no 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 an engineer at the truss company if you have questions.

Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site at www.askthebuilder.com. ©2009 Tribune Media Services

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