Braces and Knee Braces

Why braces

Mortise-and-tenon joints between posts and beams act like hinges under sideways load: a rectangle of posts and beams will lean over, or rack, under wind. A short diagonal brace from post to beam turns the rectangle’s corner into a triangle, and triangles don’t change shape ¹.

Braces go wherever the frame needs lateral stiffness: in bents (post to tie beam), in walls (post to plate or girt), and sometimes in the roof.

Compression, in pairs

Braces are best at pushing. A brace pushed into its seats bears on solid end grain and shoulders; a brace being pulled relies on its pegs, which resist withdrawal much less. So frames use braces in opposing pairs: wind from the west pushes the east-leaning braces into compression, wind from the east pushes the others. Each brace only has to carry load one way.

Laying out a brace

Brace geometry is set by its run (along the beam) and rise (down the post), measured from the inside corner where post and beam meet ²:

• Equal run and rise give a 45-degree brace, the most common. A 24 × 24 brace is typical in small frames; 36 × 36 in larger ones.
• The brace’s length along its centerline, between the post face and beam face, is run × √2 for a 45-degree brace: 24 × 1.414 ≈ 33.9 inches. For unequal runs, use the Pythagorean theorem: √(run² + rise²).
• Use the framing square to lay out the angled shoulders: set the square’s two arms to the run and rise (for example 12 and 12) on the brace stock and scribe along it.

Longer braces are stiffer at the corner but get in the way of headroom, windows, and doors; designers balance them.

The brace joint

Braces usually join with angled, shouldered tenons into shallow mortises in the post and beam, pegged:

• The brace mortises are cut at an angle at one end, so the brace’s end seats into them, and are shallower than main mortises, perhaps 3 inches deep.
• The tenon is cut with an angled shoulder that bears fully on the post or beam face.
• A single peg in each end holds the brace in place; the shoulders carry the load.
• Brace tenons are usually not drawbored hard; the brace must not push the post and beam apart.

Cut braces a hair long rather than short: a brace that’s too short does nothing until the frame racks enough to close the gap.

Curved and lapped braces

Not every brace is straight or tenoned. In Norwegian grindbygg, braces are lapped into the sides of the post and beam and pegged, and they’re often made from naturally curved birch or root knees, whose grain follows the curve and makes them very strong ³. Curved braces also turn up in English and Japanese frames, where they’re as much an aesthetic choice as a structural one.

Assembly

Brace joints are assembled at the same time as the post and beam they connect, usually with the bent lying flat: tenons into post mortise and beam mortise, then the post-beam joint closed up with clamps or a come-along, then pegs driven. Brace length and angle also square the bent: if all braces are cut accurately, the bent comes together square.

What to take from this

Braces turn post-and-beam rectangles into triangles to stop racking, and they work best in compression, so they’re used in opposing pairs. A 45-degree brace has equal run and rise, for example 24 × 24 inches, about 33.9 inches long between faces. Cut angled, shouldered tenons into shallow mortises, peg each end, cut braces slightly long rather than short, and remember grindbygg’s lapped, curved alternative.