Guides · 14 September 2026 · 6 min
The two-piece mitred elbow: the cladding pattern and the seam that matters
The simplest bend in the box and the one most people get slightly wrong. The mitre angle, the curve, where the seam goes, and why the swage lives on the mitre and not the plain end.
A two-piece mitred elbow is two straight lengths of cladding cut on the same angle and turned to meet. It is the free fitting in TurboClad because it is the one every cladder should be able to prove the app on: make one by hand, print one, lay them on top of each other.
The angle
For a bend of any angle the mitre is cut at half of it. A 90-degree elbow is two pieces mitred at 45 degrees; a 45-degree elbow is two pieces at 22.5. Half the bend angle is the only angle in the job, and it is measured between the mitre face and a square cut across the pipe.
The curve
Cut a cylinder on an angle and unroll it, and the cut edge becomes a sine wave. That is all a mitred elbow pattern is. The wave runs from the throat, the shortest point, to the back, the longest, and the height of the wave is the cladding radius times the tangent of the mitre angle. On 218.3 mm cladding at 45 degrees the tangent is 1, so the back stands 109.15 mm proud of the throat. On a 45-degree elbow, mitred at 22.5, it is 109.15 times 0.414, which is 45.2 mm.
Twelve stations round the pipe give a curve you can draw fair by hand; the app plots it properly. Either way the girth along the base is pi times the cladding diameter, and the lap is added down one edge, outside that girth, never inside it.
Where the seam goes
The seam is the join in the rolled piece, and where you put it decides whether the fitting can be fastened once it is on the pipe. Numbering the stations 1 to 12 from the throat, a back fastener puts the seam at station 7, the back of the bend, which is the easy reach on a pipe in the open. A side fastener puts it at station 4, on the side, which is what you want when the bend runs against a wall or another pipe and the back is not there to get at. The pattern is the same curve either way; the seam just cuts it at a different station, and the two halves swap sides.
Lay the seam where you will actually be standing with the rivet gun. It sounds obvious. It is the mistake people make when they draw the fitting at a desk.
Swage and plain end
The mitre joint is where the two pieces meet, and that is where the swage allowance goes: a few millimetres so one piece can reach into the other and lock. The plain end, the square cut that lands on the straight cladding, gets no swage on the elbow; the straight cladding butts to it and the joint is made there. Putting a swage on both ends of a two-piece elbow is a habit carried over from ductwork and it leaves you with a fitting that is too long.
A, B, C, D
A bend on an outdoor pipe sheds water differently depending on which way it turns and which way the joint faces. The A to D marks on a bend are the four ways the same elbow can sit, and they decide which piece is the upper one at the joint so the water runs over the lap, not into it. Pick the letter that matches how the bend hangs on site, and the swage and lap land the right way round.
The mitred elbow is free in TurboClad, forever, with no account. Enter the pipe, the insulation and the angle; put the seam at 7 or 4 or anywhere you like on the dial; print it at 100 percent and check the 100 mm bar. If it fits your metal, the paid fittings are developed the same way.