Houses rarely just keel over and die
Burn down, get smashed by tornadoes, rot beyond repair—yes. But simply topple over—hardly ever. It's not just because they have hundreds of studs, joists and sheets of plywood joined by thousands of nails. No, it's because all those parts are joined in a particularly strong, rigid shape.
You might presume this shape is a square, since most houses and construction materials are square. Nope. The shape that braces your house, garage and deck to keep them from keeling over is a triangle. And once you understand how triangles, diagonals and bracing work, you'll understand why your house is built the way it is—and how to keep your house strong as you add on to and remodel it.
Figures A and B: Unbraced vs. Braced Square
Figure A: An unbraced square is as flexible and collapsible as an accordion; not exactly what we want in our houses.
Figure B: A square with a diagonal brace becomes rigid and solid; the same principle keeps our walls and houses standing sturdy and strong.
The triangle—ain't nothin stronger
The easiest way to understand the strength of a triangle is to perform a simple experiment. Join four drinking straws into a square using straight pins. When you grab this square by opposite corners and squeeze, you'll notice how easily you can change its shape (Fig. A). Carpenters, engineers and architects (and now you) call this movement racking. Now add another straw (it'll have to be a longer one) connecting two opposite corners (Fig. B). Notice how it stiffens things up? It prevents racking. You've just created triangles and diagonal bracing to make a sturdier structure.
The walls of your house employ the same concept to prevent racking. While it's true that in a worst-case scenario a house can actually rack and collapse from strong winds, old age, hurricanes, impact or tremors, that's a rare occurrence. Bracing is primarily designed to keep things sturdy and square during construction and prevent smaller movements later on—those that crack drywall, make nails pop, cause windows and doors to bind, and open ugly gaps.
Four kinds of wall braces
Bracing is usually installed as the exterior walls are built. After the studs and plates are nailed together, the wall is squared and bracing is nailed in place. There are four common types of bracing:
Diagonal wood braces (Fig. C), usually 1x4s, create stability (and triangles) when connected to vertical studs and horizontal nailing plates. These 1x4 wood braces—usually one on each end of each exterior wall—are recessed and nailed into notches. You might be thinking that a 1x4 isn't very strong, and in some ways you're right; on a good day you could crack one of these in half over your knee. But it doesn't have to resist downward or sideways pressure, only “pulling” and “pushing” along its length. And as long as that 1x4 is nailed securely so it doesn't flex or bow, it helps resist racking.
Metal braces (Fig. D) nailed diagonally across studs and plates help counteract racking. You're most likely to find them in homes less than 25 years old. They may be T-shaped, L-shaped or flat. Metal braces are strong under tension (when forces are trying to, in effect, stretch them lengthwise), but are weak in compression (such forces can actually kink, buckle and weaken the brace). Metal braces are therefore strongest installed in a “V” or “X” pattern. That way, no matter which direction force comes from, one wall brace will be in tension to help prevent racking.
Plywood sheathing (Fig. E) may not look like a triangular brace, but it secretly is. Well-nailed pieces of plywood along the vertical studs and the horizontal plates form two legs of the triangle; countless little and big diagonals and triangles are formed by the nailing pattern on the face of the plywood in between. Oriented strand board works the same miracles. Plywood-sheathed walls that are used to resist racking are sometimes called shear panels or shear walls. They resist racking by taking force directed to the end of a wall and (just like a triangle) transferring it down to the immovable bottom of the wall.
Walls containing patio doors, an overhead garage door or many windows are often sheathed in plywood, even if the rest of the walls are covered with foam board or other sheathing. That's because the carpenter, architect, engineer or building inspector didn't feel that a long enough, or correctly angled, diagonal brace could be installed around all the openings to help make the wall rigid. Plywood, with all its little triangles, does a better job. And, of course, plywood on exterior walls performs double duty: It braces your walls and sheathes them, so they're ready for siding. Some builders sheathe both sides of all corners of all walls out of habit. It's a good habit.
Well-nailed siding, sheathing boards and other materials (Fig. D) also help prevent racking. Plywood siding (such as T1-11, which has decorative vertical grooves) helps prevent racking when nailed profusely. Typically, that would be at least every 6 in. along edges and 12 in. along studs on the face with 8d galvanized nails. Exterior insulating sheathing can also help prevent racking but must be secured with even more closely spaced nails. Manufacturers can supply you with the nailing schedules recommended to resist racking.
Sometimes you'll encounter older homes with the exterior sheathed entirely with diagonal 1x8 boards. Now those are walls that'll resist racking. Even well-nailed drywall and plasterboard help prevent racking when used in conjunction with other types of braces.
There's no universally agreed upon rule as to how many and what type of braces and bracing should be used. FHA regulations dictate that a wall should be able to withstand a “horizontal racking force of 5,200 pounds,” but what mere mortal can translate that into real life? The wall length and height, the workmanship and the type of lumber, and the likelihood of earthquake, hurricane and strong winds all affect how much bracing a wall needs. Many architects specify that two types of bracing (such as 1x4 braces and well-nailed plywood) be used. Your local building inspector has the final say. And remember, for any brace to do its job, the wall must be securely anchored to the wall or floor below.
Figure C: 1x4 Braces
Properly installed 1x4 braces should run at a 45-degree angle from the top corner to the bottom plate at both ends of the wall. Install intermediary braces if a wall is more than 25 ft. long. To install one, first make sure the wall is square, lay the 1x4 across the framed wall, then use a pencil to trace the edges of the 1x4 onto the studs and plates. Set your circular saw 3/4 in. deep and make six to eight closely spaced cuts between each set of lines. Use your hammer to whack out the pieces. Nestle the 1x4 into the notches, then secure it to each stud and plate with a pair of 8d nails. Finally, cut the 1x4 even with the top and bottom plates.
Figure D: Metal Braces
Metal braces are strongest installed in pairs. To install the T-shaped brace shown, place it upside down at a 45-degree angle across the wall, and trace one edge on top of the studs and plates. Use a carbide-tipped saw blade to cut a 3/4-in. deep kerf along your traced line. Tap the leg of the T-brace into the kerf and nail each arm of the T-brace into the studs (two 8d nails per stud and two 16d nails per plate). Use a tin snips to cut the brace, letting it extend 2 in. past the wall top and bottom. Hammer over these 2-in. ears and nail them to the top and bottom plates (see inset drawing) for greatest strength.
Triangles in other places
Walls aren't the only part of a house braced with triangles. Decks (Fig. F) are often stiffened up with diagonal braces between the vertical support posts and the horizontal beam. Another type of triangular bracing is the between-joist blocking shown, which stiffens the floor but doesn't help stabilize the overall structure.
Roofs, especially those with trusses, are solidified with diagonal braces running from the gable end peaks down to the bottom members of the trusses. Even structures as common as wooden gates are held rigid by triangles. Go ahead—take a walk and check out the neighbors' gates. You should see a board or adjustable threaded rod running diagonally from one corner of the gate to another. Oh boy, it's another triangle hard at work.
Figure E: Plywood Bracing
Plywood bracing is strongest when installed vertically with all four edges nailed to solid wood or blocking. At a minimum, install 8d common nails every 6 in. along each edge and every 12 in. along the face.
How does this affect you—and the house you constantly tinker with?
What it means is this: If you're cutting a big hole in a wall to install a big window and you run across a diagonal 1x4 or metal T-brace, you can't just lop it off. Although it may not be holding anything “up” in the strictest sense, it sure is holding things “sideways.” You'll need to add a wood or a diagonal brace somewhere else or sheathe the corners with plywood. If your walls are sheathed with rigid foam board insulation (like many homes in the energy-conscious 1970s), your walls absolutely depend on diagonal braces or plywood-sheathed corners for rigidity.
For the same reason, you can't just knock out the diagonal braces on your deck support posts because you keep bumping your head on them. Remove them, and eventually your deck will get “wiggly” and, in a worst-case scenario, rack and collapse. It happened to a deck in Minneapolis (with 15 gyrating dancers on it during a raucous, retro Donna Summers–theme disco party). The posts didn't break, but they sure did rack, leaning severely to one side, then spilling everyone to the ground 10 ft. below. Likewise, you can't just whack out the diagonal braces in your attic or garage trusses to fit more Christmas decorations up there. You'll promote racking.
And as you add patio doors, additions, decks and gazebos to your house, think (and build) “triangle.” Your home will be stronger and last longer.
Figure F: Deck Bracing
Firm up a deck with one (or both) of these types of bracing. The 45-degree post-to-beam braces take the sway out of vertical posts. The continuous angled between-joist blocking holds the deck square and rigid, which in turn prevents vertical post movement.