FabLayout

China cap flat pattern calculator

A rain cap for a flue or stack: a flat cone over a short collar. Enter the cap diameter, stack diameter, cap rise and stack length; you get the cap sector and the collar band.

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Enter your measurements

Your dimensions are measured to the

Export from the Trueflat app

1:1 PDF and DXF files of this flat pattern are made in Trueflat, ready for print, plasma, laser or waterjet. It works offline on site.

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1,374.4 mm563.2 mm
Flat pattern: cut on the bold outline, bend on the dashed blue lines, the thin lines are marking lines

Seam

864.0mm

Exact geometry

Slant length
In the app
Arc length
In the app
Sheet size
In the app

Every result is in Trueflat

The free web version shows the main result and the drawing. Trueflat gives you every mark and measurement, and exports a cut sheet. It is free to download.

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How to lay it out

  1. Swing the cap arc

    Set your trammel to the slant length √((D ÷ 2)² + H²) and swing an arc from a centre point.

  2. Mark the cap angle

    Draw one radius line and mark the sector angle from it, or step the cap circumference along the arc. Draw the second radius line.

  3. Lay out the collar

    Draw a rectangle one stack circumference long and one stack length high.

  4. Add allowances

    Add lap or lock allowance on both seams, and on the collar edge that meets the stack.

Good to know

  • FabLayout runs the same code as Trueflat; the result is exact geometry.
  • Source: Cap laid out as a right cone, Kittredge, The New Metal Worker Pattern Book (1917), Problem 122, pp. 249–250; the collar is a plain cylinder band. No published china cap problem was checked page by page

Method and formula

The cap is a right cone with its point up, and the collar is a plain cylinder. Both unroll exactly. The stays that hold the cap above the collar are flat bars and are not part of the pattern.

  • Slant length L = √((D ÷ 2)² + H²), where H is the cap rise.
  • Sector angle 360° × (D ÷ 2) ÷ L. A low cap has a large angle, often close to a full circle.
  • Collar π × D2 long and Ls high. Set the stack length to 0 if you only need the cap.

The cap must overhang the stack: its diameter is held between 1.2 and 4 times the stack diameter, and its rise between D ÷ 20 and D, so rain runs off. Twice the stack diameter is a common choice. Thickness is taken to the middle of the sheet only.

Worked example

Cap 600 mm, stack 300 mm, rise 400 mm, collar 200 mm: L = 500 mm and the sector angle is 216°. The cap arc is 1884.96 mm, which is π × 600. The cap slope is 53.13° and it overhangs 150 mm on each side. The collar is a 942.48 × 200 mm rectangle.

Questions

How big should the cap be?

A common rule is a cap about twice the stack diameter. The calculator accepts 1.2 to 4 times: smaller does not keep rain out, larger is a cover rather than a cap. The rise must be between 1/20 of the cap diameter and the full diameter.

Why is the sector angle so large?

A china cap is a shallow cone. The lower the rise, the closer the slant length gets to the cap radius, and the sector angle approaches 360°. A steep, pointed top is better made on the plain cone page.

Does the pattern include the stays?

No. The stays are flat bars bent to hold the cap above the collar; cut them to length from your own drawing.

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