Why does the plank thickness affect the cutting angle?

It does not — the two angles alpha and beta come purely from the leg distances A and B. What thickness does change is the Ca and Cb trim length marked in from the outer corner at each end: a thicker plank needs a bigger trim so the brace's inner edge still lines up correctly once it is cut.

What size timber is used for a knee brace in Gibraltar?

There is no single mandated section for a knee brace — it is usually cut from whatever graded structural softwood matches the framing it reinforces. Sizing a brace is typically a carpenter's judgement call rather than a span-table requirement, since the geometry here is a simple trigonometric fit, not a load-rated structural calculation.

Angle Cut Calculator — Knee Brace Angles and Plank Length | Gibraltar

Angle Cut Calculator: Knee Brace Angles and Plank Length

A knee brace is the short diagonal plank fitted into the inside corner of a right-angle frame joint — bracing a stud to a sill plate, or any two members meeting at 90 degrees — to stop the joint racking under load. This calculator takes the plank's thickness and the two leg distances either side of the corner and returns the outer plank length, the two cutting angles, and the small trim length to mark off each end so the brace seats tight against both faces.

How to use the angle cut calculator

Enter the plank thickness — how thick the brace stock is, in your chosen unit.
Enter leg A and leg B — the two distances from the corner to where each end of the brace meets the framing.
Read off the outer plank length (the brace's full diagonal length), the two cutting angles alpha and beta, and the Ca/Cb trim lengths to mark at each end.

The angle cut formula, explained

The brace's outer length is the hypotenuse of the right triangle formed by the two legs: C = sqrt(A^2 + B^2). The angle at the leg-A end is alpha = arccos(A / C), and because the two legs meet at a right angle, the angle at the other end is simply its complement: beta = 90 - alpha.

The plank has thickness, though, so a cut made exactly at the corner point would leave the brace's outer edge too long. Each end is trimmed back by Ca = t / tan(alpha) at the leg-A end and Cb = t / tan(beta) at the leg-B end, where t is the plank thickness — these are the two distances to mark in from the outer corner point before making the angled cut.

Worked example: a 4 cm thick plank braced across a 60 cm and a 90 cm leg. C = sqrt(60^2 + 90^2) = sqrt(11700) ≈ 108.2 cm. alpha = arccos(60 / 108.2) ≈ 56.3 degrees, beta = 90 - 56.3 = 33.7 degrees. Ca = 4 / tan(56.3°) ≈ 2.7 cm, Cb = 4 / tan(33.7°) = 6.0 cm.

Pro tips

Measure both legs from the same corner point you plan to cut to, on the same faces of the framing the brace will actually sit against — an inconsistent reference point is the most common reason a brace comes up short on one end.

In Gibraltar, having a carpenter cut and fit a diagonal knee brace like the worked example above typically costs around £75-£270 in labour, depending on how many braces the job needs and local trade rates.