FabLayout

Pipe roof penetration calculator

For a round pipe through a sloped roof or over a ridge: enter the pipe diameter, roof slope, pipe height and plate margin. You get the pipe cut and a flat flashing plate with its elliptical hole.

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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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680.1 mm302.0 mm
Flat pattern: cut on the bold outline, bend on the dashed blue lines, the thin lines are marking lines

Seam

931.3mm

Exact geometry

Cut length
In the app
Cut curve 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. Draw the pipe stretchout

    Draw the top edge of the pipe as a straight line as long as its circumference, divide it into equal parts and square a line down from each point.

  2. Mark the roof cut

    On each line, mark its length from the top edge and join the marks. On a single slope the seam line is the shortest.

  3. Lay out the plate

    Draw the plate rectangle, the hole plus the plate margin on every side, centred on a cross of two lines: one up the slope, one across it.

  4. Draw the elliptical hole

    Mark the ellipse on the same centre, with the long axis up the slope. For a ridge, also draw the fold line across the middle of the plate.

Good to know

  • FabLayout runs the same code as Trueflat; the result is exact geometry.
  • Source: No published source. Method: the pipe is cut by the roof plane (two planes for a ridge) and the plate is drawn in the roof plane, where the hole is an ellipse; checked by two independent implementations in Trueflat.

Method and formula

The pipe stands upright through a roof plane. Its height H is measured from the point where the pipe axis meets the roof up to the top of the pipe.

  • Single slope: the roof cuts the line at angle θ around the pipe at r × tan α × cos θ from the axis point, where r is half the diameter and α the roof slope. The shortest side is H − r × tan α.
  • Ridge: the pipe sits over the ridge and the cut is −r × tan α × |cos θ|, so the pipe has a notch on each side.
  • Plate: the roof plate is flat, so the hole is an ellipse with half-axes r ÷ cos α up the slope and r across. The plate reaches the margin past the hole on every side; on a ridge it is folded along the ridge line.

Roof slopes from 5° to 60° are accepted. The pattern is exact. Enter measurements to the middle of the sheet.

Worked example

Pipe 100 mm, slope 30°, height 150 mm, margin 100 mm, single slope: the cut sits 28.87 mm above the axis point on the uphill line, at the axis point on the side lines and 28.87 mm below it on the downhill line. The shortest pipe side is 121.13 mm. The hole has half-axes of 57.74 mm up the slope and 50 mm across, and the plate reaches 157.74 mm up and down the slope and 150 mm to each side.

Questions

How is this different from the pipe miter cut page?

The miter cut page gives only a pipe cut by one plane. This page also gives the flashing plate with its hole, handles a pipe over a ridge, and measures the height from where the pipe axis meets the roof.

What does the ridge option do?

It places the pipe over the ridge, so the roof slopes down on both sides. The pipe gets a cut on each side and the plate gets a fold line along the ridge.

Why is the hole in the plate an ellipse?

The plate lies on the slope, so a round pipe crosses it at an angle. Across the slope the hole is the pipe diameter; up the slope it is longer by 1 ÷ cos of the slope.

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